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Integrated Water Management

Irrigation Water Resources

Dr. A.K. BhattacharyaEmeritus ScientistWater Technology Centre

Indian Agricultural Research Institute

New Delhi 110 012

Date of submission: November 10, 2006

Key words: Water resources, Rainfall, Groundwater, Watershed/Catchment area,

Reservoir, Barrage, Irrigation, Evapotranspiration, Crop water

requirement, Irrigation demand.


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CONTENTS

Topic Page No.

Introduction 1

The Hydrologic Cycle 2

Measurement of the components of the hydrologic cycle 3

Rainfall 3

Interception 4

Depression storage 4

Overland flow and stream flow 5

Infiltration 5

Soil moisture storage 7

Deep percolation 9

Subsurface flow 10

Evaporation and evapotranspiration 10

Water Resources of India 10

Space-Time Variability of Water Resources 14

Quantification of Water Resources 16

Crop Water Requirement 20

Measurement of evapotranspiration 21

Estimation of evapotranspiration 22

Irrigation Water Demand 24

Irrigation Water Supply 25

Irrigation from surface sources 25

Irrigation from groundwater sources 26

Formation characteristics 27

Different types of water bearing formations 27

Recording and interpretation of water table data 28

Groundwater pimping 29

Pressurized irrigation 30

Demand based and supply based irrigation 30

Summary 31

References 32

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LIST OF FIGURES

Fig. 1. Hydrologic cycle

Fig. 2. Nonrecording raingauge.

Fig. 3. Recording raingauge.

Fig. 4. Scheme of surface water resource development in a watershed.

Fig. 5. Infiltration characteristics of a clay soil.


Fig. 6. Tension vs. moisture deficit in Loamy Sand soil.

Fig. 7. Tension vs. moisture deficit in Sandy Loam soil.

Fig. 8. Tension vs. moisture deficit in Sandy Clay Loam soil.

Fig. 9. Major rivers and their tributaries.

Fig. 10. A simple hydrograph for the 97.5 km2 Nagwan watershed in Upper Damodar

Valley, Jharkhand.

Fig. 11. Surface water resource assessment through hydrograph analysis of an 84 km

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watershed.

LIST OF TABLES

Table 1 Range of average annual rainfall over different regions of Indian mainland.

Table 2 River basins of India and their surface water resources.

Table 3 Variability of monthly rainfall in monsoon India with varying annual rainfall.

Table 4. Variation of runoff percentage of rainfall in a few contiguous small

agricultural watersheds in the Tawa Command area of Madhya Pradesh.

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Irrigation Water Resources

Introduction

Water is an essential input for agriculture. Precipitation is a major renewable resource and is

the only source of all water present over the earths surface and below the surface as

groundwater. Precipitation comprises all forms of water such as rainfall, snowfall, dew, etc.,

received on the earth surface. In India, rainfall makes up the largest portion of precipitation.

Snowfall occurs over high hills and mountains in the north. Snowmelt flows down the

streams and rivers. The percentage of dew in the total precipitation is negligible and except

moistening the few millimeters of top soil, it is not useful in agriculture.

At a macro-scale, the annual precipitation volume over India is 4000 km3. The geographical

area of India is about 328.8 million hectares. Hence, the total annual rainfall is equivalent to a

depth of water of about 1.22 m or 1220 mm over the country. This amount of water, if

available at the time of need by the crop plants and if this amount of water is availableuniformly over the geographical area, necessity of irrigation would not arise for most of the

crops that are cultivated in the country. At a micro-scale, however, the aberration is large,

from an average annual rainfall of 100 mm over western Rajasthan to 11000 mm in

Mausingram in Meghalaya. Besides this tremendous spatial variation, the temporal variability

of annual rainfall is also very high; 80% occurring in 4 south-western monsoon months from

June through September and the rest 20% in the remaining 8 months distributed as winter

monsoon during January-February, pre-monsoon during March-May, and post-monsoon

during September-December. There is a distinctly prolonged dry and hot period for at least

three months continuously from March through May. Within the four monsoon months, there

are prolonged periods of rain break when the standing crop is stressed for want of water

leading to lower production and even crop failure in some cases. Thus, the apparently

comfortable quantum of average water availability in reality gives rise to serious discomfort

due to either deluge or extreme scarcity of water at different times of the year in different

regions. This is the feature of monsoon climate, which is strong over India and this feature

cannot be changed for better by human intervention. This is why development of water

resources for irrigation is essential in India for meeting the crop water demands at the time of

need ensuring a higher level of agricultural production. Of the 4000 km3 of precipitation

renewable every year, the average annual surface flow is 1869 km3 and the utilizable portion

is 1122 km3, including surface water (690 km3) and groundwater (432 km3). Much of the

water is rendered unutilizable due to evaporation losses, topographic variation, uneven

distribution in time, etc (CWC Pocket Book, 2005)The rainfall distribution over India represented as average annual rainfall of certain ranges is

summarized in Table 1.

Table 1. Range of average annual rainfall over different regions of Indian mainland*.

Range of average

annual rainfall (mm)

Regions

Less than 500 Western Rajasthan.

500 - 1000 Central Maharashtra, Chandigarh, Delhi, Eastern Rajasthan,

Haryana, Kutch, Marathwada, North interior Karnataka,

Pondicherry, Punjab, Rayalseema, Saurashtra, Tamil Nadu,

Telengana, Western plains of Uttar Pradesh.

1000 - 1500 Coastal Andhra Pradesh, Eastern Madhya Pradesh, Eastern Uttar

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Range of average

annual rainfall (mm)

Regions

Pradesh, Gangetic West Bengal, Gujarat, Himachal Pradesh, Jammu

and Kashmir, Jharkhand, Orissa, South interior Karnataka, Vidarbha,

Western Madhya Pradesh.

1500 - 2000 Uttaranchal.2000 - 2500 Manipur, Mizoram, Nagaland, Tripura.

2500 - 3000 Arunachal Pradesh, Assam, Meghalaya, Sikkim, Sub-Himalayan

West Bengal.

More than 3000 Coastal Karnataka, Goa, Kerala, Konkan.

* Adapted from Anonymous, 2006.

The Hydrologic Cycle

The rainwater is partly intercepted by the vegetation and partly evaporated before it reaches

the earth surface. The losses due to interception and evaporation are small, usually not

exceeding 5 per cent of the total. The balance 95 per cent, upon falling on the land and waterbodies is disposed in various ways. The part that falls over seas and oceans is immediately

rendered unsuitable for use due to high salinity in such water bodies. The part that falls over

the land mass is first absorbed by the soil. The soil, however, has a limited absorption

capacity. This causes generation of excess water on the land surface that starts flowing

overland in different directions depending on the land slope. Soon it cuts the soil into small

rills, which gradually widen and deepen to form small streams. These streams form

tributaries to the rivulets and rivers. Different tributaries join the river at different places and

the river widens carrying more and more water as it travels downstream.

The largest sources of surface water are the rivers. The river water is stored in reservoirs and

is utilized for irrigation, hydroelectric power generation, supplying drinking water and for

recreation purposes. If dam and reservoir construction is not feasible due to site constraints,

barrages are constructed to divert excess river water mainly during the monsoon season for

irrigation. River lift schemes are also commissioned to lift the excess river flow during the

monsoon months and irrigate regions at higher elevations. Irrigation sector is the largest

consumer of fresh water in India, to the tune of 83 per cent, among all the fresh water using

sectors (CWC, 2005-2006).

During the overland flow, river flow and from the various inland water bodies as well as from

the seas and oceans, evaporation continues returning the water to the atmosphere. Plants draw

water from the soil and transpire most of it back to the atmosphere. Seepage and percolation

losses from land surface, irrigation canals, reservoirs etc., replenish the groundwater

reservoir. Groundwater is withdrawn by pumping for various purposes, including irrigation.Once brought to the surface, groundwater also undergoes losses due to seepage, percolation,

evaporation and transpiration. The cycle of water movement from atmosphere to the earth

surface to the groundwater reservoir and back to the atmosphere is known as the Hydrologic

Cycle. Long before the development of the science of hydrology, poet Kalidasa while

describing nature in his beautiful style had put down the essence of the hydrologic cycle in

Sanskrit, which may be rendered as: The sun impregnates air with minute droplets of water

drawn from the ocean; these droplets coalesce and grow in size to form raindrops that fall on

the earth and travel through various routes to come back to the ocean. The Vedic literature

contains adequate description of all the major components of the hydrologic cycle (NIH,

1990).

The various components of the hydrologic cycle are precipitation, interception by vegetation,

temporary storages in depressions on the earth surface, overland flow, stream flow,

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infiltration, soil moisture storage, deep percolation, subsurface flow, evaporation and

evapotranspiration. For planning and development of water resources, understanding of the

hydrologic cycle and quantitative information of its various forms are the important database.

A qualitative description of the hydrologic cycle is given in the schematic diagram of Fig. 1.

Fig. 1. Hydrologic cycle (Source: http://www.uwsp.edu/geo/faculty/

ritter/images/hydrosphere/hydrocyc.jpg).

Measurement of the components of the hydrologic cycle

Rainfall

Rainfall is measured either in a non-recording raingauge or in a recording type (automatic)raingauge. A non-recording raingauge of IMD (India Meteorological Department)

specification has a 127 mm receiving funnel (catch) diameter and also for the container (Fig.

2). The depth of water collected in the container is magnified by pouring it in a pre-calibrated

measure glass to read the rainfall to the nearest 0.1 mm depth. The measure glass becomes

full with 25 mm of rainfall. If the rainwater collected in the container is more than 25 mm,

more than one measurement is needed to know the total rainfall. The observations are taken

once a day at a fixed time, usually between 8.30 AM and 9-00 AM. One gets the total rain in

a 24 hour period using this type of raingauge. IMD maintains a vast database of daily rainfall

throughout the country.

A recording type (or automatic) daily rain gauge of IMD specification has a catch diameterof 203 mm and a float chamber capacity of 323.5 mL, which is equivalent to 10 mm of water

depth over a circular area of 203 mm diameter (Fig. 3). A pen is fixed to the stem of the float

to mark on a chart wound over a clock-driven drum. The drum makes one revolution around

a vertical axis in 24 hours. As the rainfall accumulates in the float chamber, the float rises and

the pen marks the chart. When 323.5 mL of water (or 10 mm of rain) is collected in the float

chamber, the float and the pen rises to their topmost position and the water is drained by a

siphon bringing the float and the pen back to their initial lowermost position. In the recorder

chart, a no rainfall period is marked as a horizontal line, a period of low intensity rainfall is

marked as a slowly rising line and a period of intense rainfall is marked as a single or a group

of steeply rising and falling lines. The chart is changed once a day, at a fixed time, usually

between 8-30 and 9-00 AM. Further details about the IMD raingauge and its use are availablein the IMD publications (IMD, 1958) in Circular Nos. 6 and 116. There are other types of

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recording raingauges such as the weighing type and the tipping bucket type. The siphon type

is mostly used in India.

Fig. 2. Nonrecording raingauge.

Fig. 3. Recording raingauge.

Interception

Interception is that part of rainfall, which is intercepted and partially held by the vegetation. It

is a small quantity in the hydrologic cycle. It may be roughly measured by placing small cans

under the vegetation and outside. The difference in the depth of rainfall received in these two

cans is the interception loss. For representative measurement, a number of cans should be

placed both within and outside the vegetation and the average value of these should be used

for finding the difference. This is an approximate method, as some of the intercepted water

drips on the land surface once the water holding capacity of the vegetation is full. The greater

is the density of vegetation, the greater will be the interception loss. Tea bushes may intercept

up to 30 per cent of the rainfall.

Depression storage

There are innumerable depressions on the earth surface where rainwater and the overlandflow are stored temporarily. Such storages cannot be measured. For large water bodies such

as ponds and lakes, the storage volume and its changes due to evaporation and seepage losss

or addition of water from rainfall and surface runoff can be determined from the record of

water level changes and the dimension of the storage structure. Currently, a net of about 3

million hectares of agricultural land are irrigated using water stored in pond. This is about 18

per cent of the canal irrigated area and about 10 per cent of the area irrigated by groundwater

lifted from tube wells and dug wells (Anonymous, 2006). The largest pond-irrigated area is in

Tamil Nadu, followed by Andhra Pradesh, Maharashtra, Orissa, West Bengal, Karnataka and

Madhya Pradesh, where pond irrigated area is greater than 200,000 ha. Some of the least

pond irrigated areas are in the states of Tripura, Jammu and Kashmir, Haryana and Himachal

Pradesh, each with 5000 ha or less under pond irrigation.

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Overland flow and stream flow

When the intensity of rainfall exceeds the infiltration rate of the soil (See next section for

infiltration), the excess water flows over the land surface in different directions depending on

the land slope. This overland flow or the surface runoff is short-lived, as the flowing water

soon erodes the soil underneath to form small rills. These rills gradually grow in size to form

larger streams, tributaries and rivers. Measurement of the flows in streams and other higher

order natural channels is done by stream gauging. It involves measuring the flow depth, the

corresponding flow area and the mean velocity of flow. The product of the mean velocity and

flow area gives the discharge. As a standard method, stream gauging is done at the outlet of a

watershed. Watershed is a geographical area bounded by a ridge such that all surface water

flow within the watershed is directed towards a common and single outlet. A large watershed

may have many small sub-watersheds, each with a single outlet. The concepts of

watershed/sub-watershed or catchment/sub-catchment, natural streams of different orders,

dam, reservoir, irrigation command area, etc., relevant to surface water resource development

are explained inFig. 4.

Fig. 4. Scheme of surface water resource development in a watershed.

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Infiltration

Infiltration is the vertically downward entry of water through the soil surface. The infiltrated

water is held in the soil pores. When the water holding of the soil is exceeded, the excess

flows down joining the water table causing it to rise. Infiltration takes place from rainfall,

water applied for irrigation, water stored in reservoirs, water that flows in canals and streams,

etc. At a given location, infiltration varies with time. Its rate reduces as the surface and lower

soil layers become more and more wet. It reachs a constant low value after a long time.

Infiltration at different times will vary due to changes in the soil conditions, mainly its

compaction and initial moisture content.

Infiltration is represented through the relation between infiltration rate and time, between the

cumulative infiltration and elapsed time, and the basic infiltration rate, which is the low and

steady state rate of water intake at the soil surface after a prolonged standing water condition

on the field. The infiltration rate and the cumulative infiltration as functions of time are

represented graphically and also through the best fit empirical relations. Figure 5 shows the

graphical representations of instantaneous infiltration rate, the cumulative infiltration and the

basic infiltration rate of a clay soil (Bhattacharya and Michael, 2003).

Infiltr

ationrate,cm/h

CumulativeInfiltration,cm

060504030200

50

40

30

20

10

7010

Time since start of infiltration, min

Cumulative infiltration

Instantaneous infiltration rate

Basic infiltration rate,obtained by drawing thisline horizontally to theleft to meet the Y-axis

Infiltr

ationrate,cm/h

CumulativeInfiltration,cm

060504030200

50

40

30

20

10

7010

Time since start of infiltration, min

Cumulative infiltration

Instantaneous infiltration rate

Basic infiltration rate,obtained by drawing thisline horizontally to theleft to meet the Y-axis


Infiltration characteristics are determined in the field using a double ring cylinderinfiltrometer comprising two 2-3 mm thick mild steel rings, one of 40 cm and the other of 25

cm diameter approximately and each about 30 cm in height and open at both the ends, a 3

mm thick and at about 40 cm square mild steel plate and a heavy hammer, a point gage, a

stop watch and large water containers to store about 50 litres of water. In the field, the two

cylinders are driven to a depth of about 10 cm, keeping them concentric and using the steel

plate to hammer the cylinders down. Water is slowly poured in both the cylinders up to a

depth of about 5 cm below the rims. Water level drop in the inner cylinder and the

corresponding time are noted. Water surface depth readings from the upper edge of the

cylinder are taken at close time intervals till the water surface decline for two successive

readings at equal time intervals become very close to each other. In most situations, fairly

steady rate of infiltration is achieved within 2 to 3 hours of starting the test. The depth ofwater infiltrated between two successive times is obtained from the difference between the

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corresponding point gage readings. During the test, the water level in annular space between

the two cylinders is maintained close to that in the inner cylinder.

The cumulative infiltration data generated during a cylinder infiltrometer test when plotted on

a logarithmic graph paper, the scatter diagram in most cases can be represented by a best-fit

straight line. The straight line has equation of the form: I = a tb + c where I is the cumulative

infiltration in terms of depth of water in time t. Such an equation for a clay soil of Madhya

Pradesh was obtained as: I = 0.42 t0.612 + 0.6, where, I is in centimetre and t is in minutes.

Thus, if in one hour of surface irrigation, 5.75 cm of water will infiltrate into the soil. When

the cumulative infiltration equation is differentiated with respect to time, one gets the

instantaneous infiltration rate equation. For the above cumulative infiltration equation, the

instantaneous infiltration rate equation will be: i = 0.257 t - 0.388 where, i is the instantaneous

infiltration rate in centimetre per minute at time t minutes after the start of infiltration.

Knowledge of infiltration behaviour of soil is useful in selecting irrigation methods, in

assessing percolation loss from water bodies stored in ponds and in categorising soil into

various hydrologic soil groups, which is done for runoff estimation from watersheds. From

Fig. 5, taking the basic infiltration rate as 4 cm/h, the total water infiltered from a pond in oneday will be 96 cm. This is a high rate of water loss and calls for lining of the inner pond

surface to reduce the water loss. For an irrigation situation, we may consider a field where the

earlier mentioned cumulative infiltration equation applies. To this field growing wheat, if

irrigation water depth of 7.5 cm is applied over a dry soil, it will be absorbed in 96.9 minutes

or, in 1 hour and 37 minutes. This is obtained by substituting 7.5 for I and calculating t from

the cumulative infiltration equation: I = 0.42 t0.612 + 0.6. In sprinkler and drip irrigation

methods, the water discharge rate can be adjusted to deliver water matching to infiltration

rate of soil such that both surface runoff loss and wasteful deep percolation loss of water can

be avoided.

Soil moisture storageSoil is a porous mass with major mineral components of sand, silt and clay comprising the

solid portion. The grain size of sand ranges from 0.05 to 2 mm; that of silt from 0.002 to 0.05

mm and that of clay 0.002 mm and smaller. Other solids present in small quantities are the

organic matter, pebbles and crystalline salts. The voids or the pore space of the soil contains

air and soil solution, which is water containing dissolved salt of various types and in different

concentrations. The total pore space in a given soil volume is a function of the relative

proportions of sand, silt and clay and also of the degree of compaction of the soil. A fine or

heavy soil, which contains more of clay and silt and less of sand (eg. Silty Clay Loam), has a

higher porosity and hence, a higher water holding capacity. A coarse or light soil, which

contains more of sand and less of silt and clay (eg., Loamy Sand), has a lower porosity and

hence, a lower water holding capacity. A compact soil has less porosity than a loose soil.Loam is a medium soil having well balanced, though unequal proportions of sand, silt and

clay. The moisture held in the top about 1 m of soil is important for the plants, as they draw

this water through their root system.

The important soil moisture parameters are the Saturation Moisture Content (SMC), the Field

Capacity (FC) and the Permanent Wilting Point (PWP). At saturation, all the soil pores are

filled with water and the water remains at atmospheric pressure. Some of this water is drained

down under gravity. The water held in partially saturated soil at one-third atmosphere

pressure is the FC moisture content. This is also called the water holding capacity of the soil.

Evaporation and transpiration further reduces the soil moisture content and increases the

pressure with which it is held in the soil pores. At a pressure of 15 atmosphere, the soil

moisture content is termed as PWP, when the soil physically appears completely dry. Themoisture content between FC and PWP is called available water, i.e., the water that is

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available for plant use. For most crop plants, however, need of irrigation arises when 50 per

cent of the available water is depleted. Soils of different texture with different percentages of

sand, silt and clay, have different values for the above mentioned soil moisture parameters.

Soil moisture content may be determined by various methods. By gravimetric method, a

known mass of wet soil sampled from the field is oven dried at 105 0C for 24 hours and the

dry mass is determined. The difference between the dry mass and wet mass is the mass of thewater present in the wet soil sample. The ratio of the mass of water to the mass of the dry soil

multiplied by 100 is the soil moisture content in per cent dry mass basis. Gravimetric method

is most accurate but cumbersome and time consuming. Other methods are based on using

neutron moisture meter, time domain refractometer, infrared lamp, resistance block (gypsum

block), tensiometer, etc.

Tensiometer is a simple device to indicate the pressure (also called tension) at which the

moisture is held in the soil and it determines the pressure which the plant roots must exert to

draw the soil moisture. It is the soil moisture tension, which determines moisture availability

to the plants rather than the absolute value of soil moisture content. Soil moisture tension and

soil moisture deficit (from field capacity moisture content) are related but the relation isdifferent for soils of different texture (Ray and De, 1983). Such relations for three soil

textures namely, Loamy Sand, Sandy Loam and Sandy Clay Loam are given in Figs. 6, 7 and

8, respectively.

Fig. 6. Tension vs. moisture deficit in Loamy Sand soil.

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Fig. 7. Tension vs. moisture deficit in Sandy Loam soil.

Fig. 8. Tension vs. moisture deficit in Sandy Clay Loam soil.

Some typical values of important soil water parameters for a medium (Loam) soil may beSaturation Moisture Content: 35%; Field Capacity: 25% and Permanent Wilting Point: 10%,

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all in per cent dry mass basis. This means that, respectively, 35, 25 and 10 per cent of the

mass of the soil will be the mass of water corresponding to Saturation, Field Capacity and

Wilting point moisture contents. These may also be expressed in per cent volume basis by

multiplying with the dry bulk density (or dry density) of the soil. The dry density of soil,

depending on the degree of compaction may vary from 1.0 g/cm3 to 1.5 g/cm3. If the dry

density of a soil is 1.2 g/cm3

, then the earlier mentioned dry mass basis moisture contents willbe: saturation moisture content: 35x1.2 = 42%; Field Capacity moisture content: 25x1.2 =

30% and Wilting Point moisture content: 10x1.2 = 12%, all in per cent volume basis. This

means that 1 cubic meter of wet soil will contain 0.42, 0.3 and 0.12 cubic meters of water,

respectively, at Saturation, Field Capacity and Wilting Point moisture contents. All these

values will be higher for a soil that is finer than Loam (eg., Clay Loam) and lower for a soil

that is coarser than the Loam soil (eg., Sandy Loam).

Rainfall and irrigation water are expressed in terms of depth over an area. A rainfall of 10

mm that has fallen uniformly over an area of 1 hectare amounts to a water volume of 10 ha-

mm. This is equivalent to 1x10,000x10/1000 = 100 m3 of water (Note that 1 ha = 10,000 m2

and 10 mm = 10/1000 m). If irrigation water of 7.5 cm is applied to 1 ha of a cropped land inone irrigation, it is equivalent to applying 1x10,000x75/1000 = 750 m3 of water over 1 ha of

the cropped land. Wheat for good production requires on an average 30 cm of water, which

may be applied in four irrigations. If all this water is to be applied from the irrigation source,

the total water withdrawal from the source will be 3000 m3 for 1 ha of a wheat field from

sowing to harvesting. To be compatible with the conventional units of expressing rainfall or

irrigation depth, it is useful to express the soil moisture content also in terms of depth of

water. This is done by attaching to the volumetric moisture content (expressed in fraction) the

depth unit compatible with the unit in which the volume of wet soil was expressed. Thus, 1

meter cube (1m x 1m x 1m) of wet soil at saturation will contain 0.42 m (or, 42 cm) of water

if its volumetric moisture content were 42%.

For the water properties of the Loam soil discussed above, the FC being 30 cm and the PWPbeing 12 cm, the available water per meter depth of the soil is (30 12) = 18 cm. Allowing

50 per cent deficit, consequent to evaporation and evapotranspiration, irrigation will be

required when half of this 18 cm of water has been depleted. Assuming that the depletion

started when the soil was at FC, the moisture held in the soil will be (30 9) = 21 cm and

water to be added through irrigation will be 9 cm to bring back the soil again to FC. Bringing

the entire 1 m depth of soil to saturation by irrigation is wasteful of water, as the water held

between saturation and FC is soon lost as deep percolation and becomes unavailable to the

plants. In this example, a soil depth of 1 m was assumed for calculation, as for most crop

plants, the depth of active root zone is limited to 1 m. For crops with deeper or shallower

effective root zones, the calculation results will be different. Effective root zone is the depth(measured from soil surface) from where most of the requirement of water and nutrients are

extracted by the plant roots.

Soil moisture and climatic parameters play important roles in deciding a proper irrigation

interval. If the water loss rate due to evaporation and evapotranspiration in a certain region

with the above discussed soil water parameters is 5 mm/day, then 50 per cent of the available

water will be depleted in 90/5 = 18 days, when one should apply irrigation. Thus, the

irrigation interval becomes 18 days. However, applying water in precise depth and at precise

times is possible when the irrigator controls the irrigation source and the land is smooth. In

large canal irrigated areas, soils are never uniform, crops vary and so do the soil moisture

properties and water depletion rate. The variation is both in space and time. Space variation

occurs due to heterogeneity of soil, crop, etc., and time variation occurs due to inter seasonaland intra-seasonal variation of temperature, relative humidity, air velocity, radiation received

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on the earth surface, sun shine hours, etc. Accordingly, during the operation of an irrigation

system, the irrigation interval is fixed based on the major crops grown in the command area

and their water demand, the water available at the source, losses of water in transit and from

the cropland and ensuring as much equitable distribution of water as feasible among the large

number of beneficiary farmers.

Deep percolation

For agriculture, deep percolation is a loss, as the plant roots cannot extract this water. Thus,

any water that has moved beyond the crop root zone is a deep percolation loss. For the

groundwater reservoir, which is getting replenished by the deep percolated water, it is a gain.

For normally irrigated fields, deep percolation loss can be estimated by soil moisture

sampling. If there is an increase in the moisture content of the soil below the root zone, it is

termed as deep percolation loss. It may not join the water table, which may be even deeper

but such losses in each irrigation, ultimately lead to water table rise. In paddy fields where

during certain stages of plant growth the fields retain standing water, deep percolation loss

may be measured by conducting infiltration test described earlier.

Subsurface flow

The subsurface flow refers to the flow of water below the ground under saturated soil

condition. It may be estimated by DArcys equation: Q = A K I, where A is the flow area, K

is the saturated hydraulic conductivity and I is the hydraulic gradient. It can also be estimated

by applying water balance equation relating all inflows to a region, all outflows from the

region and the change in water storage in the region during a chosen time interval. On annual

basis, the subsurface inflow and outflow may balance each other. This assumption is not valid

if the water balance equation is to be used for shorter time period, say one season.

Evaporation and evapotranspiration

Evaporation is measured by filling a standard evaporation pan with water and noting downthe decline the water level at specified time intervals, usually 1 day. Evapotranspiration is

determined in lysimeters. A lysimeter is a tank made of metal (weighing type lysimeter) or

other constructional material (non-weighing lysimeter), containing soil with plants growing

over them, which are irrigated from time to time. In weighing type lysimeter, the reduction in

weight of the lysimeter after irrigation is due to evapotranspiration loss. In non-weighing type

lysimeter, the loss of water due to evapotranspiration is calculated by soil moisture sampling

from the lysimeter soil. Conducting lysimeter studies is expensive and time consuming.

Instead, one may use a suitable empirical relation from a large number of developed relations

for estimating evapotranspiration (See section on Crop Water Requirement later).

A somewhat detailed and fundamental discussion and simple calculations presented aboveabout the hydrologic cycle and its components is useful in understanding the nature of the

water resources, their development and utilization for irrigation of agricultural lands. Assured

water to the crops ensure better production. Under monsoon climate over India, the

occurrence of natural rainfall is highly erratic in time and space. Even within the monsoon,

there may be long rain breaks, which put the standing crop under water stress. Under stressed

condition, the plant roots are unable to extract nutrient from the soil. The purpose of water

resources development for irrigation is, therefore, to timely supply the water needed by the

crop plants.

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Water Resources of India

Surface water resources of the major river basins of India are given in Table 2 and natural

drainage network (River system) is depicted in Fig. 9.

Table 2. River basins of India and their surface water resources.

River

system

Basin

area

km2

Regions/states Origin Outfall Average

annual

flow

km3

Present

water

use

km3

Brahmani-

Baitarani

51882 Jharkhand, Madhya

Pradesh, Orissa

Keonjhar Bay of

Bengal

28.5 4.8

Brahmaputra-

Barak

Brahmaputra

Barak

194413

(inIndia)

41723

(in

India)

Bangla Desh,

Bhutan, India, Tibet

(China), In India:

Arunachal Pradesh,Assam, Meghalaya,

Nagaland, Sikkim,

West Bengal

Bangla Desh, India,

Mayanmar, In India:

Assam, Manipur,

Meghalaya,

Nagaland, Tripura

Tibet (China)

Manipur Hills

Bay of

Bengal

Bay of

Bengal

Together:

585.61.1

Cauvery 87900 Karnataka, Kerala,

Tamil Nadu

Talakaveri,

Western Ghats,

Karnataka

Bay of

Bengal

21.4 18.0

Ganga 1086000 Bangla Desh, China,

India, Nepal. In

India: Bihar, Delhi,

Haryana, Himachal

Pradesh, Madhya

Pradesh, Nagaland,

Rajasthan, Uttar

Pradesh, West

Bangal

Gangotri

Glacier, as

Bhagirathi.

Alaknanda joins

at Devprayag

and then known

as Ganga

Bay of

Bangal

525 37.8

Godavari 312812 Andhra Pradesh,

Karnataka, Madhya

Pradesh,

Maharashtra, Orissa

Nasik,

Maharashtra

Bay of

Bengal

110.5 41.0

Indus 1165500

321289

(in

India)

Afghanistan, India,

Pakistan, Tibet

(China). In India:

Chandigarh,

Haryana, Himachal

Pradesh, Jammu &

Kashmir, Punjab,

Rajasthan

Mansarovar,

Tibet (China)

Arabian

Sea

73.3 40.0

Krishna 258948 Andhra Pradesh,

Karnataka,Maharashtra

Mahabaleshwar,

Western Ghats

Bay of

Bengal

78.1 50.0

Mahanadi 141589 Bihar, Madhya Raipur, Madhya Bay of 66.9 17.0

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River

system

Basin

area

km2

Regions/states Origin Outfall Average

annual

flow

km3

Present

water

use

km3

Pradesh,

Maharashtra, Orissa

Pradesh Bengal

Mahi 34482 Gujarat, Madhya

Pradesh, Rajasthan

Viddhyas,

Madhya

Pradesh

Arabian

Sea

11.0 2.5

Narmada 98796 Gujarat. Madhya

Pradesh,

Maharashtra

Amarkantak,

Madhya

Pradesh

Arabian

Sea

45.6 8.0

Pennar 55213 Andhra Pradesh,

Karnataka

Chenna Kesava

Hills, Karnataka

Bay of

Bengal

6.3 5.0

Sabarmati 21674 Gujarat, Rajasthan Aravalli Hills,

Rajasthan

Arabian

Sea

3.8 1.8

Source: MoWR River Basin Maps.

Fig. 9. Major rivers and their tributaries.

As has been mentioned earlier, the entire generated surface water resources in the form of

runoff and then as stream and river flow cannot be utilized due to various constraints. Thelast column of Table 2 reflects this, as it is seen that the current utilization is far below the

average annual flow in many cases. There are several ongoing water resources development

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projects at various stages of completion. Besides, new irrigation projects are also coming up.

When all these are completed, the surface water utilization scenario will improve. It is

estimated that the irrigation potential of the utilizable water resources, when fully developed,

will be about 140 million hectares. From the pre-plan period till the year 2002, the irrigation

potential created was 116.5 million hectares through major, medium and minor irrigation

projects. It may be noted that the major and the medium projects are surface water based andthe minor projects are both surface and groundwater based. Considering groundwater alone,

the created potential is over 35 million hectares. There is, however, a gap between the

irrigation potential created and the irrigation potential utilized in actual irrigation. This gap

occurs, as it takes time to make the entire irrigation command area ready to receive water

after the creation of the water resources through construction of reservoirs and barrages.

Making the land ready for receiving irrigation implies completing the entire water

distribution network in the command area from main canals to the water courses, including

the large number of water flow control and diversion structures in them. Also, land is not

fully ready till it is properly leveled and developed for irrigation water distribution. In view of

these constraints, the irrigation potential utilized stands at about 102.6 million hectares

against the created potential of 116.5 million hectares. There is a constant endeavour tobridge this gap. In the above figures of created and utilized irrigation potentials, the

contribution of decentralized small community based water resources development effort

through the construction of ponds and water harvesting structures and the privately owned

tube wells and dug wells, which are used for irrigation water lifting are not included.

Groundwater plays a very important role in irrigation in India. Besides irrigation,

groundwater is also used for many other purposes such as supplying drinking water, to meet

industrial requirement, to augment canal water supply, for water table control, to lay

foundation of large structures when they are to be constructed in shallow water table areas,

etc. As a water resource, shallow groundwater directly contributes to meet crop water

requirement partially thereby reducing the need for irrigation. The source of groundwater isthe rainfall, which directly or through storage structures such as reservoirs, lakes, ponds and

through irrigation canals and flowing natural streams continuously infiltrate. Part of this

infiltrated water, after satisfying the soil moisture storage capacity joins the water table and

becomes a part of groundwater reservoir. Groundwater continuously moves from higher

elevation to lower elevation.The utilizable groundwater potential in a region isconsidered as

85 per centof the long-term average groundwater recharge from rainfall and other sources

such as irrigation conveyance network, subsurface flow from higher reaches, etc. Usable

groundwater potential is created bycommissioning tube wells or other types of wells, ready

to support pumping of water through a pump-prime mover (diesel engine or electric motor)

combination. Irrigation potential created so far from groundwater is over 35 million hectares.

Groundwater resources development shows a picture of tremendous variation among thevarious states, from 1.8 per cent in Jammu & Kashmir to 175 per cent in Punjab. These are

the percentage of annual replenishment from rainfall and are average values for the state.

Within a state, there may be tremendous variation in its availability. For example, the 175 per

cent exploitation in Punjab is a feature of central, eastern and northern regions where the

groundwater quality is good. In south-west Punjab, there is little exploitation of groundwater,

as the water is brackish and cannot be used in irrigation or for other purposes.

When in a region, the annual groundwater extraction is less than 65 per cent of the annual

groundwater recharge, the region is called white and further groundwater extraction is

possible. If the extraction is between 65 and 85 per cent of the recharge, the region is called

grey and further groundwater extraction must be made with caution. If the extraction isbetween 85 and 100 per cent of the recharge, the region is called black, implying that no

further withdrawal of groundwater should be made. If the extraction exceeds 100 per cent of

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recharge, the region is called overexploited. Overexploitation causes groundwater table to

continuously decline and results in increasing pumping cost, drying up of shallow wells,

exposure to brackish water aquifer that may lie at deeper depths, etc. Over exploitation takes

place where the groundwater is sweet (i.e., of good quality) and the crop water demand

cannot be fully met from the developed surface water resources and rainfall. Currently, the

annual groundwater extraction in the states of Punjab and Haryana as a whole is more thanthe annual replenishment. In Rajasthan and Uttar Pradesh, it has exceeded 80 per cent of the

annual replenishment and in many states such as Andhra Pradesh, Himachal Pradesh, Assam,

Bihar, Karnataka, Kerala, Madhya Pradesh, Maharashtra, Orissa, West Bengal and all north

eastern states the annual groundwater extraction is 50 per cent or lesser than the annual

replenishment.

Considering the country as a whole, the irrigation potential of the utilizable groundwater is

about 64 million hectares, the potential created is over 35 million hectares and the level of

groundwater development is about 55 per cent. The Central Ground Water Board monitors

the groundwater status all over the country district-wise and down to block level. Analysis of

such data reveals that even in those states where the current groundwater extraction is lessthan 50 percent for the state as a whole, there are a number of blocks where the groundwater

is over exploited. Thus, though the states of Andhra Pradesh, Maharashtra, Orissa, Karnataka

and in some others the groundwater extraction is less than 50 per cent, yet there are several

blocs in these states which are over exploited.

Declaration of utilizable groundwater potential on the basis of aquifer and water table

information of a region may sometimes be misleading, unless the declared potential has good

possibility and demand for its use in the region. Unlike surface water, which can be

transported from one basin to another, transport of groundwater from a surplus region to a

deficit region is not feasible.

Space-time Variability of Water ResourcesThe monsoon climate over India causes complete reversal (a change by 1800) in the direction

of wind flow from land to sea and from sea to land between summer and winter. The large

land mass of India surrounded by a large water body (comprising Arabian sea, Indian ocean

and Bay of Bengal) and the presence of several hill ranges acting as barriers to air flow give

rise to drastic change in the atmospheric temperature and humidity between summer and

winter. This is briefly the cause of great variation of rainfall over space and time over India.

However, from other aspects such as soil, temperature, sunshine hour, etc., which are relevant

for agriculture, cultivation of diverse crops are possible over India throughout the water. This

is in contrast to many of the countries at higher latitudes where the land may be covered with

snow for several months in a year or the countries with desert climate, which is too harsh to

support crop activity. Thus, despite having a well developed natural river and tributary

network, as shown in Fig. 9, the nature of monsoon climate make them overflow with water

for a few months and practically have no flow at all for several months, except the large

rivers and those which are also fed by snow melt. During such periods, water is the sole

constraint for agricultural production. This is irrespective of the total rainfall received in a

year, whether low or high. The nature of rainfall variability is explained through Table 3.

In Table 3, Machilipatnam, Ongole and Mysore have two distinct monsoons namely, south-

west and north-east and due to this, the variability is somewhat lower in these places. In all

other cases, from a very high rainfall region to a very low rainfall region, the variability of

monthly rainfall is very high. The inter-month variability of rainfall would be low if the

rainfall is well-distributed in the year, such as in Tokyo, Japan, where the mean annual

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rainfall is 1563 mm and the coefficient of variability of monthly rainfall is 45.5 per cent,

much lower than any of the places of India indicated in Table 3.

Like surface water resources, groundwater availability also varies over space and with time.

Within a year, the time variation is temporary. On a long term basis, however, the variation

may be substantial. Alarming water table decline due to excessive pumping and alarming rise

in water table in many irrigation command areas and also in areas with poor quality

groundwater are common in India. Space-wise, the annually replenishable groundwater varies

from as low as 1.82 billion cubic meter in the Subarnarekha river basin to as high as 171

billion cubic meter in the Ganga river basin. Correspondingly, the utilizable groundwater is

also the lowest at 1.7 billion cubic meter in the Subarnarekha river basin and the highest at

156.8 billion cubic meter in the Ganga river basin. In Rajasthan, of the total area of 34.2

million hectares, the area underlain by saline groundwater is 14.1 million hectares (41.2 per

cent). On the other hand, in Uttar Pradesh, of the total area of 29.44 million hectares, the area

underlain by saline groundwater is only 0.136 million hectares (0.46 per cent). Some other

states where considerable area overlies saline groundwater are Haryana (25.9 per cent),

Gujarat (12.4 per cent), Delhi (9.4 per cent), Punjab (6.1 per cent) and Karnataka (4.6 percent).

Table 3.Variability of monthly rainfall in monsoon India with varying annual rainfall.

Place Number of

distinct

monsoons

Annual

rainfall

mm

CV of monthly

rainfall

%

South Canara, Karnataka (Coastal) 1 3930 135

Mornai, Assam 1 3640 105

Karumady, Kerala (Coastal) 1 2967 84

Patashpur, West Bengal (Coastal) 1 1928 99

Kotdwara, Uttar Pradesh 1 1730 141

Kausani, Uttar Pradesh 1 1611 103

Uttar Kashi, Uttar Pradesh 1 1552 121

Chinsura, West Bengal 1 1454 96

Gorakhpur, Uttar Pradesh 1 1393 129

Parvatipur, Andhra Pradesh 1 1286 85

Barna, Madhya Pradesh 1 1184 168

Machilipatnam, Andhra Pradesh (Coastal) 2 1013 74

Ongole, Andhra Pradesh 2 843 99

Mysore, Karnataka 2 762 76

Kurnool, Andhra Pradesh 1 607 97

Sonepat, Haryana 1 531 125Firozpur, Punjab 1 465 125

Faridkot, Punjab 1 447 103

Bhatinda, Punjab 1 392 126

CV = Coefficient of variability = (Standard deviation/Mean) x 100.

The presence of saline groundwater in a region implies that groundwater cannot be used for

irrigation or to be used with caution and only after mixing with good quality surface water in

such a proportion that the resulting water has salt concentration that is not harmful for the

crop plants. Saline water in shallow aquifers undergoes changes in salt concentration

seasonally, being less saline during and after the monsoon season and more saline during the

dry season. Irrigation requirement is high during the dry season when surface water

availability is less. Many farmers, therefore, are compelled to draw upon groundwater for

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irrigation to save the crops. If the groundwater quality is bad, in many occasions it has been

found to destroy the crop rather than saving it.

Quantification of Water Resources

For planning and construction of irrigation reservoirs, barrages, and small water harvesting

structures and also for planning groundwater extraction to meet irrigation demand, theavailability of water resources is to be ascertained through measurement or estimation. It is to

be noted that the water resource available for use is a part of the rainfall resource that is

transformed into surface flow or groundwater recharge. It is recommended to plan water

resource development on watershed basis. For small water harvesting structures, the

watershed size may be from 500 to 1000 ha. It may be smaller also. For large reservoir based

irrigation projects, the watershed size is much larger and may be 100 km 2 or larger. When the

stream flow at the outlet of the watershed is measured with respect to time, the measurement

data is represented through a graphical relation called a hydrograph. An example of a simple

hydrograph (that has a single peak) is given in Fig. 10. In a large watershed there may be a

dry weather flow even without an immediate past rainfall, as shown during the first 18 hours

in Fig. 10. Rainfall over the watershed causes runoff, which passes through the outlet of thestream that is being gauged (flow is being measured). Runoff rate gradually increases, as

more and more of the watershed area contributes to runoff, it reaches a peak and then

declines when the rainfall ceases. After separating the base flow, the area under the

hydrograph (now called direct runoff hydrograph) represents the direct runoff, which is

generated by the causative rainfall (see the rainfall bar in Fig. 10). In Fig. 7, the direct runoff

is 32.3 mm, from a rainfall of 49.5 mm (= 15 x 198/60). The balance 17.2 mm is the sum of

interception, depression storage, infiltration, etc., which are not available as surface water

resource. In this example, runoff works out as 64.6 per cent of rainfall. The percentage of

rainfall appearing as runoff varies depending upon wetness of the watershed soil, seasonal

variation in land cover, particularly over agricultural land, the watershed size, the intensity

and duration of rainfall and the direction of rainfall movement whether from outlet towardsthe upstream or from upstream towards the outlet, as governed by the wind direction. An

example of this variation recorded for three agricultural watersheds of 1, 5 and 25 ha area in

Madhya Pradesh is given in Table 4.

The modulating (or smoothening) effect of watershed on rainfall variability can be seen in

Table 4 from the gradually reducing coefficient of variability of runoff, as the watershed area

increases.

Table 4. Variation of runoff percentage of rainfall in a few contiguous small

agricultural watersheds in the Tawa Command area of Madhya Pradesh.

1 ha watershed 5 ha watershed 25 ha watershedYear Rainfallmm Runoff

mm

Runoff,

% of

rainfall

Runoff

mm

Runoff,

% of

rainfall

Runoff

mm

Runoff,

% of

rainfall

57.0 29.9 53 22.0 39 17.2 30

61.5 44.0 72 30.6 50 29.6 48

51.3 43.8 85 28.5 56 24.5 48

16.5 12.6 76 7.3 44 2.5 15

34.0 28.5 84 14.7 43 12.4 37

13.5 6.2 46 3.1 23 2.1 16

23.5 17.8 76 12.5 53 9.2 39

13.5 9.6 71 6.8 50 4.8 3622.5 15.1 67 7.7 34 4.9 22

1997

16.5 8.4 51 5.3 32 3.0 18

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1 ha watershed 5 ha watershed 25 ha watershedYear Rainfall

mm Runoff

mm

Runoff,

% of

rainfall

Runoff

mm

Runoff,

% of

rainfall

Runoff

mm

Runoff,

% of

rainfall

21.4 10.8 51 13.1 61 7.1 33

1998 12.6 7.7 61 7.5 60 1.5 1281.0 56.2 69 28.7 35 27.1 34

30.6 23.2 76 18.4 60 8.7 28

38.7 24.9 64 7.6 20 4.0 10

17.8 2.4 14 1.4 08 0.4 02

32.1 25.9 81 21.0 65 10.8 34

13.5 2.6 19 2.7 20 1.9 14

12.1 4.1 34 3.5 29 0.7 06

44.5 27.1 61 19.1 43 4.4 10

1999

59.9 38.9 65 34.5 58 14.0 23

Mean 31.18 20.94 61 14.10 42 9.09 24.5

Standarddeviation 20.95 15.33 20 10.23 0.16 8.83 0.135

Coefficient of

variability, %

67.19 73.21 32.79 72.55 38.10 97.14 55.10

Source: Anonymous, 2000, 2001.

Assessment of the surface water resources through hydrograph analysis like the one

explained in Fig. 10 is cumbersome and time consuming. Besides, the runoff variability (see

Table 4) makes it extremely uncertain as to which of the hydrographs may be considered for

water resource assessment. One may get what is called a design hydrograph, which is the

hydrograph generated by the design rainfall of a chosen recurrence interval. The recurrence

interval for the design rainfall is 5 or 10 years in case of small size works executed over

agricultural lands and may be 1000 years for large multipurpose dams and reservoirs, whichcater to and also affect much larger area and the corresponding population by producing

hydroelectric power, providing irrigation and drinking water, inundating large area, releasing

large volumes of water for dam safety when the inflow is large than anticipated, etc.

Hydrographs such as the one shown in Fig. 10 are, therefore, useful in deciding the

dimensions of a spillway, the capacity of a small farm pond storing runoff water for life

saving irrigation, design of drains, etc.

Fig. 10. A simple hydrograph for the 97.5 km2

Nagwan

watershed in Upper Damodar Valley, Jharkhand.

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For large works also, hydrographs are used for water resource assessment but there it shows

the relation of mean monthly flows with the months. In the relation, the mean monthly flow is

often substituted by the monthly flow of various per cent dependence such as 90 per cent

dependable flow or 70 per cent dependable flow and the like. The area under the hydrograph

so obtained is the total volume of flow available in a year. A 90 per cent dependable flow

implies a flow that is expected 90 per cent of the time or 9 out of 10 years or 18 out of 20years and so on. Dependable flows on a monthly basis are, nevertheless, obtained by

summing up the daily flows over a month and subjecting the resulting data to probability

analysis. Thus, stream gauging is an important prerequisite for generating the data base,

which is processed to obtain the surface flow volume at the outlet of a watershed at desirable

recurrence interval. Examples of the type of hydrograph used for planning large works is

given in Fig. 11 A and B.

A: Total hydrograph

2000

1600

1200

800

400

0

Dec

Nov

Oct

Sep

Augu

l

May

un

Apr

Ma

Feba

n

Meanmonthlyflow,mm/month

Month

Hydrograph area represents

4495.5 mm of runoff from 84 km2

watershed (equivalent to 377.4

million cubic meter of water

during May through November.

B:1800

1600

1200

800

400

0

Direct runoff

aphhydrogr

Dec

Nov

Oct

Sep

Aug

Jul

May

Jun

Meanmonthlyflow,m

m/month

MonthFig. 11. Surface water resource assessment through hydrograph

analysis of an 84 km2

watershed.

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A shows the total hydrograph drawn between mean monthly flows expressed in millimeter

over a watershed of 84 km2 area and the time expressed in month. To do this, the daily flows,

usually recorded in cubic meter per second, were converted to cubic meter per day, were

added up for all the days in a month and then averaged for the month. The monthly mean

flow was then converted to millimeter per month from cubic meter per month by dividing the

latter by the watershed area, expressed in consistent units. The base flow (also called the dryweather flow) was deducted from the total hydrograph and the resulting hydrograph, the

direct runoff hydrograph, was plotted in Fig. 11 B. The area under the direct runoff

hydrograph represents the total flow of runoff water at the outlet of the watershed (more

correctly, at the location where stream gauging was done). In the present case, the direct

runoff volume was 4495.5 mm, which is equivalent to 377.4 million cubic meter. This much

water was obtained during May December. If this water were to be stored in a reservoir and

if the reservoir capacity was not possible to keep above 200 million cubic meters due to site

constraint, much of the balance 177.4 milliom cubic meter of water would have to be safely

passed through spillways. The remaining would be taken care by seepage and evaporation.

There are many empirical relationships expressing runoff as a function of certain parametersone of which is invariably the rainfall over the watershed (Raghunath, 1985 and Varshney,

1986). Some of these formulae are:

i. For Nilgiris: (1))AT/()P511.1(R 0613.034.1m44.1=

ii. Inglis formula for Bombay Deccan catchment

Ghat areas: 5.30P85.0R += (2)

Plain areas: 254/}P)8.17P{(R = (3)

iii. Lacys formula for Indo-Gangetic plains: ]P/)}S/F(8.340{1/[P[R += (4)

iv. Khoslas formula for north India: )75.3/T(PR m= (5)

v. Chambal river basin (Rajasthan): 4945P120R = (6)

vi. Ganga basin: (7)64.0P4.2R=

vii. Yamuna basin: (8)1.1P14.0R=

viii. Rihand basin (U.P.): (9)86.0P17.1PR =

ix. Tawa basin (M.P.): (10)4800P5.90R =

x. Tapi basin (Gujarat): (11)7200P435R =

In the above formulae (where applicable), R is the annual runoff, cm; P is the average annual

rainfall, cm; A is the watershed area, km2; Tm is the mean annual temperature,0C; F is a

monsoon duration factor varying between 0.5 and 1.5 and S is the catchment factor varying

from 0.25 for flat areas to 3.45 for hilly areas.

A dam constructed at the outlet of a watershed impounds water in the reservoir behind it. The

reservoir water is taken through conveyance network by gravity flow to the downstream areas

for irrigating crops. With respect to an irrigation reservoir, the area upstream to it is called the

watershed or more commonly, the catchment area and the area downstream to it is called thecommand area (see Fig. 4). When certain portions of the command area are at a relatively

higher elevation, water from the reservoir may not flow under gravity. In such cases, a

23


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smaller modulating reservoir is constructed in the vicinity of the elevated portions of the

command area but at a level where water from the main reservoir can flow under gravity.

Water is pumped from the smaller reservoir to take it to higher elevation of the command

area. If this is not feasible, then certain portion of the earlier defined command area remains

out of command. Many a time, water is lifted by pumping directly from the river when it

carries sufficient flow to irrigate lands at a higher elevation. This is done during the monsoonseason when the river flow is high. For irrigation by surface water gravity flow from

reservoir to the land to be irrigated is preferred, as pumping involves consumption of energy,

which is expensive and its supply is uncertain in many states.

The Central Water Commission under the Ministry of Water Resources, Government of India

regularly monitors the flow in all the major rivers at certain specified locations through

stream gauging. This data base is utilized in working out the river basin-wise surface water

availability. However, the best location of constructing a dam may not be the locations where

stream flow is monitored. At different locations along the stream, the flow varies, gradually

increasing from upstream to downstream. For example, if the dam shown in Fig. 4 were to be

constructed at A instead of the location shown, the reservoir storage would be less and if itwere to be constructed at B, the reservoir storage would be more. The reservoir design in

such cases may be based on the data of nearest stream gauging station or the information

from similar watershed and rainfall features may be assumed to apply, and where recorded

data are not available, by estimating the runoff through empirical relationships or methods, or

by applying hydrologic models to predict runoff based on recorded data of rainfall and

watershed features. Interlinking of rivers is an important concept of water resources

development in India. The Ministry of Water Resources has plans to link some of the rivers

of the water surplus basins with those in the water deficit basins.

Availability of groundwater depends upon aquifer characteristics and the annual

replenishment, which again depends upon rainfall, land surface and subsurface features. An

aquifer is an underground water bearing stratum that can be pumped to extract water.

Crop Water Requirement

Plants need water to live, grow and produce yield. Water dissolves the nutrients in the soil,

either naturally present or applied externally and the plant roots take up the nutrient solution

from the soil and supply to other parts of the plant. While nutrients are used up by the plant

while it is growing, most of the water taken up by the plant roots transpires to the atmosphere

through the leaves. Transpiration is an essential physiological process of plant growth.

Nutrients and water supports vegetative growth of the plant through thickening and

lengthening of the stem, development of the root system and increase in the number and size

of the green leaves. These green leaves in the presence of sunlight and atmospheric carbon

dioxide prepare the plant food through a process called photosynthesis. As a result of this

process, the plants produce yield such as rice, wheat, maize, sorghum, pearl millet,

groundnut, tapioca, fruits, vegetables, etc. In the entire process of plant growth and

production of useful yields, water is an essential component. Right quantity of water is

needed at the right time during the development of plants. To enable this, crop water

requirement is to be known

Crop water requirement is the sum of evapotranspiration, some unavoidable losses and water

required for special needs such as while puddling rice field, for leaching of salts, etc. The

unavoidable losses are relevant in most surface irrigation methods drawing water from the

canals, which are scheduled to release a fixed quantity of water at a certain time interval

though at the initial stages of plant growth, the water actually needed by the plants is less.Similarly, in clay soils, some of the applied water is lost through the cracks that form on

24


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drying of the soil between successive irrigations. The losses that occur due to inefficiency of

irrigation method are not counted in crop water requirement. The major sources of meeting

crop water requirement are irrigation, rainfall and contribution from soil moisture storage.

Different crops have different water requirements. The same crop grown in climatically

different regions also requires different amounts of water. Crop water requirement is

governed by the physiological features of the plant, the soil characteristics and atmospheric

parameters such as temperature, relative humidity, wind velocity, duration of sunshine hours

and the net radiation received from the sun on the earth surface. Since out of the total crop

water requirement, some portion may be supplied by effective rainfall and contribution from

soil moisture storage, Irrigation water demand, therefore, equals the crop water demand

minus the sum of effective rainfall and the moisture contribution from the soil profile.

Effective rainfall is that part of total rainfall, which is stored in the soil profile and which can

be taken up by the plant roots. Of the total crop water requirement, the major component is

the evapotranspiration, which can be measured and also estimated.

Measurement of evapotranspiration

Either non-weighing type or weighing type lysimeters are employed to measure

evapotranspiration. Lysineters are large containers as much deep as the effective root zone

depth of the plants. Crops are grown in the lysimeter keeping it within the cropped area to

ensure a similar environment to the lysimeter crops as the crops elsewhere in the field. They

are irrigated along with the surrounding field and moisture increase due to irrigation and the

moisture depleted between two irrigations are monitored by soil moisture sampling. Non-

destructive methods of soil moisture sampling using tensiometer, neutron moisture probe,

TDR meter, etc. are preferred in the lysimeter in lieu of gravimetric method in which every

time moisture sampling is done, some soil is lost from the limited volume of soil and that

might affect the uniformity of the soil in the lysimeter and the surrounding. Lysimeters also

have a constructional feature to monitor the deep percolation loss of water beyond the rootzone. Analysis of the soil moisture data monitored over the cropping season enables one to

calculate total water lost through evapotranspiration from the water applied in irrigation.

Wieghing type lysimeters are more sophisticated in their construction and the soil moisture

changes in it are monitored through the change in mass of the lysimeter as a result of

irrigation application and subsequent drying of the soil. For this, the lysimeter container is

mounted on a platform balance. Besides using lysimeters, evapotranspiration can also be

measured through water balance studies conducted in the field. In water balance, a complete

accounting is done of all the water that is input to the soil-crop system and all the water that

is out put from the system. In the field, it is difficult to measure all the components

accurately. Hence, field experimental results are not as accurate as the lysimetric

experimental results.

Estimation of evapotranspiration (ET)

Since, lysimeter experiment is time consuming, cumbersome and expensive, and field

experiments based on water balance concept is inaccurate, evapotranspiration for irrigation

planning is more often estimated than measured. The various terms representing

evapotranspiration are: Consumptive use (CU), Potential evapotranspiration (PET), Pan

evaporation (Epan), Reference crop evapotranspiration (ET0), actual evapotranspiration

(AET), etc. CU is the sum of evaporation from soil surface, transpiration through the plant

and the water needed for the metabolic processes in the plant. The water needed for the plant

metabolic activities is very small as compared to PET or ET0 and hence, many times, CU is

not considered or calculated separately, as PET or ET0 is considered to take care of the smallamount needed for plant metabolic activity. PET and ET0 are synonymous and is the

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maximum rate of evapotranspiration by reference crop when the water supply is unlimited.

Reference crop is not a specific crop but is used as a standard. Epan is the evaporation from

Class A pan of US Weather Bureau. AET is the actual evapotranspiration by a crop under a

given set of climatic and water supply conditions.

A simple approach is to estimate ET is by using an evaporation pan of 120 cm diameter and

25 cm depth , made of 20 gauge galvanized iron sheet, painted white and mounted on a

wooden platform such that air may circulate below the pan also. The pan is filled with water

to a depth of 20 cm and water surface reading is taken daily. The evaporation, as recorded

from the daily decline of the water surface in the pan is multiplied by 0.7, which is assumed

as the pan coefficient, to give an estimate of daily evaporation from open water body. In

using the pan evaporimeter, the depth of water in the pan should be maintained at or close to

the initial value of 20 cm by adding water to the pan. The estimated open water body

evaporation is multiplied by a crop coefficient (Kc) to estimate crop ET. The crop coefficient

(also called crop factor) depends on foliage features, stage of crop growth, climate and

location and is to be determined experimentally for a given crop and for a given region.

There are many well established formulae based on climatic parameters for estimatingevapotranspiration. Many of them require referring to various Tables and graphs for their

solution. For them, only the names and the formula will be mentioned in this text. Some of

the important ET determination formulae are:

i. Blaney-Criddle formula: === 100/P.t.KuF.KU c (12)where, U = Seasonal ET of water for a given period, inches, u = Monthly ET, inches,

K = Empirical crop coefficient for the growing season of the crop,

P = Sum of monthly ET factor (f) for the growing season,

Kc = Empirical crop coefficient for the month (= u/f), f = t.P/100,

where, t = mean monthly temperature, 0F and p = monthly day light hours expressed

as percentage of day light hours of the year.

Blaney-Criddle formula is expressed in F.P.S. system of units and the result is obtained in

inches. For the result to be compatible for use with rainfall, irrigation, soil moisture, etc.,

which are expressed in cm or mm, the Blaney-Criddle result is to be multiplied by 2.54 to get

the result in cm.

Dastane (1972) has given the monthly crop coefficient values for some of the important crops

grown in India. These values vary from a minimum of 0.5 for maize in the month of October

to a maximum of 1.3 for rice in the month of July. Also, U.S.D.A. (1962) has prepared Tables

of monthly percentage of day light hours of the annual day light hours for regions of differentlatitudes and for different months. Using these information in the Blaney-Criddle method, the

procedure of calculating ET may be explained as follows:

Month: February; Crop Wheat; Location: Delhi (Lat: 28.40N); Mean

temperature in February t = 15.80C = 60.4

0F; Monthly crop coefficient Kc =

0.7; Per cent day light hours p = 7.02; u = ktp/100 = (0.7) (60.4) (7.02)/100

= 2.97 inches = 7.5 cm. Using this procedure and considering the active

growing season of wheat in Delhi is spread over from November to March (5

months), the ET for these five months can be calculated.

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ii. Thornthwaite formula: (13)a)I/t10(6.1e =

where, e = uncorrected ET, cm/month (assuming a month of 30 days with 12 hours of day

light each day), t = mean air temperature, 0C, I = annual or seasonal heat index,

a = empirical constant

The uncorrected e is corrected by multiplying with a correction factor determined by

considering actual day light hours. Tables of correction factor have been prepared

corresponding to month of the year and latitude of the place.

iii. Christiansen formula: (14))C)(C)(C)(C)(C)(C)(R)(473.0(E mehwtv s=

where, Ev = computed pan evaporation, R = extra terrestrial radiation expressed in the same

unit as that of Ev, the C terms are, respectively, linear or non-linear functions of

temperature, wind speed, relative humidity, per cent of possible sunshine, elevation of

the place above mean sea level and a monthly coefficient varying between 0.9 and 1.1

according to the latitude of the place.

iv. Penman formula: )]/()EQ[(CET an0 ++= (15)

where, ET0 = Reference crop evapotranspiration, mm/day, C = an adjustment factor, which is

a function of relative humidity, short wave radiation, day time wind speed and the

ratio of day time wind speed and night time wind spee (numerically, C varies in the

range of 0.27 to 1.33), = slope of the curve drawn between saturation vapourpressure (millibar) and temperature (0C) evaluated at mean air temperature, Qn = net

radiation converted to equivalent mm of water, = psychrometric constant and Ea =an aerodynamic term and is a function of saturation vapour pressure of air (millibar),

actual vapour pressure of air (millibar), wind speed at 2 m height from the ground

surface (km/day).v. Hargreavess formula: (16)5.0DCA )T()8.17T()R()0023.0(PET +=

where, PET = potential (or reference crop) evapotranspiration, (mm/day), RA =

extraterrestrial radiation equivalent evaporation (mm/day), TC = average of daily

maximum and minimum temperature (0C) and TD = difference between daily

maximum and minimum temperature (0C).

vi. Ramdas formula: (17))E()6.0(PET pan=

where, PET = reference crop evapotranspiration (mm/day) and Epan is the evaporation from

Class A pan of US Weather Bureau (mm/day).

The evapotranspiration formulae estimate the open pan evaporation (Epan) or the reference

crop evapotranspiration (PET or ET0). These are multiplied by pan coefficient and/or crop

coefficient to get the actual crop evapotranspiration (AET). Having determined or estimated

the AET, the crop water requirement is obtained as the sum of the ET, the unavoidable losses,

and the water needed for special purposes, as mentioned earlier. The reference crop

evapotranspiration, as mentioned earlier also, does not refer to a specific crop but is used as a

standard parameter. Currently, use of the term potential evapotranspiration is discouraged.

In lieu, it is recommended to use the term Reference crop evapotranspiration (ET0).

Irrigation Water Demand

Irrigation water demand being the crop water requirement minus the sum of effective rainfalland moisture contribution from soil profile, there should be a mechanism to evaluate the last

two components to know irrigation requirement. Effective rainfall is the directly or indirectly

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(from the soil) utilizable rainwater locally where the rain falls and is a function of the

intensity, duration and amount of rainfall, the land slope and cover and the soil texture.

Considering monthly rainfall as the criterion, the monthly effective rainfall is taken as 90, 85.

75, 50, 30, 10 and 0 per cent of the monthly rainfalls of 25, 50, 75, 100, 125, 150 and greater

than 150 mm, respectively. Thus if the rainfall in a month is 50 mm, the effective rainfall will

be (50)(85/100) = 42.5 mm. If the crop ET for the month were 100 mm, there was no specialneed of water and the unavoidable losses were 10 mm, the crop water requirement would be

(100 + 10) = 110 mm. Irrigation water demand IR would be IR = (110 42.5) = 67.5 mm,

assuming that the water table at the place is deep and does not contribute water from capillary

rise to meet a part of the crop water need. This amount is the net irrigation requirement or

demand. The gross irrigation demand, which must be applied to the crop land to satisfy the

net demand is the net demand divided by the irrigation application efficiency, expressed in

fraction. Thus, if the irrigation application efficiency were 70 per cent, the gross water

demand would be (67.5/0.7) = 96.4 mm in one month (or, 100 mm in a month.). This amount

of water may be applied in two irrigations of 50 mm each.

Another way to determine irrigation water demand is based on soil moisture depletion. Thisconcept was discussed in the section: Soil Moisture Storage. There, we found that for a

loam soil of certain specific soil-water properties, 9 cm of water would be needed to bring

back the soil to field capacity moisture content. If there is no rainfall or c0ontribution from

soil moisture storage, all of this net requirement of 9 cm must come from irrigation. The

gross amount of water required, for an irrigation application efficiency of 70 per cent, as

mentioned in the previous paragraph, will be 9/0.7 = 12.7 or about 13 cm. To adopt this

method of calculating net and gross water requirement, one should either have the soil

moisture depletion data or a good estimate of crop evapotranspiration to ascertain when 50

per cent of available water is depleted and apply irrigation then of an amount by which the

ET has taken place. This amount is to be divided by the irrigation application efficiency to

know the gross depth of water application.Irrigation Water Supply

Irrigation from surface sources

In large reservoir based or diversion based irrigation system, water from the source is

supplied through a network of canals to the crop land. At the top of the network hierarchy,

there is the main canal, usually two of them, one taking off from the right side of the reservoir

and the other from the left side. Accordingly, they are called right main canal and left main

canal, respectively. In a large command area there are crop lands far away from these main

canals. Therefore, to convey water to those areas, the main canal branches into distributaries,

minors and water courses. The entire command area cannot be irrigated simultaneously.

Hence, water is released in the various branches of the network following a certain schedule.

For this, control structures are built on the canal at the required places to regulate and divert

the flow. The schedule and the rate of flow are decided before the irrigation season and

attempts are made to follow the schedule so that the cultivators know before hand when they

will receive water. Some differences do take place between the prepared schedule and the

schedule that is actually followed due to less than anticipated storage in the reservoir if the

rainfall was below normal in the catchment area of the reservoir, due to maintenance

requirements, due to unauthorized withdrawal of water by some users, etc.

In the canal network, substantial quantity of water is lost due to seepage. Seepage has three

main repercussions. First, it reduces the irrigation water supply, secondly, it causes

waterlogging in the vicinity of the canal and thirdly, it recharges the groundwater reservoir.Though groundwater recharge is desirable, but that is not the primary aim of expensive

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irrigation development. Seepage losses are reduced by lining the canal. However, due to high

cost, only a small length of the canal network is lined leaving a very large total length

unlined. Waterlogging and reduced irrigable area due to seepage loss is a common feature of

large canal irrigated regions in India.

Irrigation from groundwater sources

The size of groundwater based irrigation works is much smaller and hence, the canal net

work is small and does not have the various hierarchical order of canals, as in the reservoir

based large irrigation systems. Efficiency of groundwater use is in general, higher than the

surface water, as it is available on demand at the point of use requiring little conveyance.

Most of the groundwater development and use for irrigation is restricted to the shallow

aquifer zone within 50-meter depth and by individual irrigators. Development of deeper (50

to 300 meters below ground level) aquifers is under public sector or done in community

irrigation projects. Shallow groundwater structures are dug wells, dug cum bore wells,

shallow tube wells (both cavity type and strainer type), bore wells, etc. The deeper structures

are deep tube wells and bore wells. A dug well is a large diameter well lined with brick or

stone masonry, or with concrete or baked clay collars placed one above the other. A dug-cum-bore well consists of a dug well and a tube well at its base to enable drawing water from

deeper depth to which the dug well cannot be constructed. Tube wells are commissioned by

driving down hollow steel or strong rigid PVC pipes usually of 10 cm to 20 cm diameter to

the desired depth where water is available for pumping. Percussion method or drilling

machines are used for this purpose. In a strainer type tube well, slotted pipe is used for the

length that matches with the thickness of the aquifer (water bearing formation below the

ground). In cavity type tube well, the pipe is taken down to the top of the aquifer puncturing

through a stiff and thick layer of heavy and compact soil. In both this types of wells, after

installation, they are initially pumped at a high rate for 48 to 72 hours to wash out finer

parcels in the aquifer making water available freely when pumped later at a normal rate. In

strainer type tube well, the initial pumping creates a coarse zone with good watertransmission property around the slotted pipe. In cavity type tube well, the initial high rate of

discharge removes some aquifer material, forming a cavity below the pipe end to store and

supply water when the tube well is pumped normally later. Wells constructed in the hard

rock regions of the country are mainly of two types namely, large size, usually rectangular in

cross section dug wells, which tap the rainwater stored in the top weathered zone of the soil

and bore wells, which are drilled through hard rock and intercepts water in the cracks and

fissures in the rock. Bore wells do not require a hollow pipe as the tube well because the rock

is strong and stable and the bore stands on its own. Water availability from bore wells in the

hard rock regions is low and uncertain.

Formation characteristics

The supply of water from groundwater structures depends much upon the characteristics of

the geological formation below the earth surface through which the well has been

constructed. There are broadly three types of geological formation namely unconsolidated,

semi-consolidated and consolidated. The unconsolidated formation comprise about one-third

of the total land area but account for about 50 to 60 per cent of the total usable groundwater

resources. The lithology includes zone of sands, gravel, pebbles etc., which store large

quantities of groundwater and have good water transmission characteristics favouring

groundwater extraction. The semi-consolidated sandstone formations are next in importance

but hardly cover about 5% of the total land area. The lithology is generally favourable for

groundwater storage and extraction. The major part of the peninsular region consists ofconsolidated formations. The rocks in these formations have no primary pore spaces and hold

limited quantities of groundwater contained in the weathered and the fractured zones.

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However, siezable quantities are available at some locations in Vescicular lava flows,

intertrappean beds and caverneous lime stone.

Different types of water bearing formationsA water bearing formation is called aquifer. An aquifer may be unconfin

irrigation+water+resources+ +formatted

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