land suitability for irrigation and land irrigability classification
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LAND SUITABILITY FOR IRRIGATION AND LAND
IRRIGABILITY CLASSIFICATION
Selection of Soils
The selection of lands for irrigation encompasses factors of climate, topography, drainage
physical and chemical characteristics of soil. The suitability of soils for different crops in relation
to the proposed irrigation system planning depends upon following consideration:
Climate
Climate exerts important influences on the selection of lands for irrigation. Soils tend to
show a strong geographical correlation with climate, especially at the global scale. Energy and
precipitation strongly influence physical and chemical reactions on parent material. Climate also
determines vegetation cover which in turn influences soil development. Precipitation also affects
horizon development factors like the translocation of dissolved ions through the soil. As time
passes, climate tends to be a prime influence on soil properties while the influence of parent
material is less.
Climate affects both vegetative production and the activity of organisms. Hot, dry desert
regions have sparse vegetation and hence limited organic material available for the soil. The lack of
precipitation inhibits chemical weathering leading to coarse textured soil in arid regions. Bacterial
activity is limited by the cold temperatures in the tundra causing organic matter to build up. In the
warm and wet tropics, bacterial activity proceeds at a rapid rate, thoroughly decomposing leaf
litter. Under the lush tropical forest vegetation, available nutrients are rapidly taken back up by the
trees. The high annual precipitation also flushes some organic material from the soil. These factors
combine to create soils lacking much organic matter in their upper horizons.
The character of the soil, drainage conditions, distribution of native vegetation, and crop
adaptation are strongly related to climate. To a lesser extent climate also influences the relief of the
land surface. These factors affect needs of the area, type of plan formulated, design of facilities,
and economic impacts from irrigation. As a result, the lands selected to be irrigated will have
different characteristics and qualities in different climate settings.
The climatic influences on the farm input-output relationship have considerable effect on
the quality of lands included in irrigation projects. In areas producing high income crops such as
citrus, vegetables, and cotton (Gossypium herbaceum), expenditures as high as $ 550/- / acre can be
made to develop the farm for irrigation. In areas producing general field crops similar expenditures
would probably not exceed $ 250, while areas growing hay, pasture, and small grains would have
limits of about $ 100/acre. (These dollar values are for Federal Reclamation Project areas and
would be different for private development having different institutional constraints).
In general, as climate favours higher farm income, greater expenditure can be made for
land forming, farm distribution systems, leaching salt and exchangeable sodium, profile
modification practices, and farm surface and subsurface drainage. When considered in terms of
land class determining factors such as uneven microrelief; soil texture, structure, and depth; ESP
and soluble salt levels; permeability of substrata; and depth to groundwater barrier, then more
severe deficiencies involving such factors can be tolerated in climates favoring high farm incomes
than in those favoring lower incomes.
Within the broad regional climatic patterns important localized influences occur. The
average climatic conditions on individual farms or small tracts may be different from those of the
region mainly because of slope and exposure. In selecting irrigable lands, this factor can be of
considerable importance. In areas to be devoted to fruit production, land should be differentiated
with respect to frost hazard. This is typical practice in selecting irrigable lands on projects in the
Pacific Northwest where the cropping pattern is dominated by fruit. Here frost hazard lands are
intricately associated with lands, rarely experiencing frost. In regions where experience is lacking
to guide such differentiation, detailed studies of the microclimate may be necessary. Mason (1958)
describes such a study in which lands are identified as frost-free, frost-rare, intermediate, frost-
liable, and frost-subject, all occurring within an area of 10 square miles.
Topography
The principal topographic attributes determining suitability of land for irrigation are degree
of slope, relief, and position. These factors influence land development needs and costs, design of
on-farm water conveyance systems, erosion hazards, crop adaptability, drainage requirements,
water use practices, and selection of management systems. The topographic attributes are
correlated with soil and drainage conditions in selecting the irrigable lands. The quality of land for
irrigation is then derived by appraising the correlated effect of topography on productivity, cost of
land development, and production costs. Size and shape of the areas are usually considered as part
of the topographic factor because of their common relationship to land development and production
costs.
1. Slope : The effect of slope on the suitability of land for irrigation is primarily influenced by the
crops to be grown, the duration and intensity of rainfall, erodibilty of the soil, farm income
potential, and the method of irrigation to be used. In selecting irrigable lands, an initial decision
needs to be made as to whether or not the slope conditions should be modified by landforming.
That is, whether the slope factor is to be treated as permanent or changeable. If the dominant slope
is to remain, then the land is appraised for irrigation suitability primarily on the basis of
productivity and labor inputs.
In this case, an increase in per cent or complexity of slope generally results in a decrease of
the quality of the land for irrigation. The decrease will vary with the irrigation method, being
particularly pronounced with gravity methods and somewhat subdued under the sprinkler method.
As slope increases, shifts in cropping patterns from row to close-growing crops are frequently
made. in general, the row crops produce higher net incomes than the close-growing crops. Hence,
there is a decline in net productivity. Also, the labor costs involved in water management on
sloping lands are usually higher than that of nearly level lands. Surface drainage problems
frequently arise on lands lacking gradient and these can usually be corrected by land forming and
installing surface drainage systems. In areas producing fruit or vegetables, relief and position need
to be considered with respect to frost hazard.
Harris and Hansen (1958) find that the relationship between agricultural land value and
slope is governed by the relative importance of surface drainage, erosion, and water intake. The
relationship is shown in Fig. Values for a particular area will, according to Harriss and Hansen, lie
somewhere within the shaded area depending upon the relative importance of the factor cited.
As the complexity of slope increases, there is a successive reduction in the amount of
gradient that should be permitted within an irrigable land class. Complexity of slope affects the size
and shape of fields, type and number of water control structures needed, size and length of water
conveyance systems, and requirements for relift pumps to reach high areas. In some situations
involving complex slopes the irrigation problems involved can be most efficiently handled by
adopting sprinkler irrigation.
2. Land development : In selecting lands for irrigation the more important land development
factors considered are : clearing vegetation such as trees or brush; removing stones, cobble, or
outcrops of bedrock; leveling land; terracing land; floating and planning; deep plowing or
chiseling; subsoiling and hardpan shattering and perforation; constructing laterals, ditches, borders,
dikes, basins, drops, checks, weirs, flumes, and small storage ponds; installing wells, pumps, main
farm laterals, pipelines, fixed and portable sprinkler systems, nozzles and rotating devices, farm
surface drains, ditch and lateral lining, grassed waterways, inlet and outlet structures, open and
closed subsurface drain laterals, and inverted wells; and by leaching. The requirements for land
development vary from tract to tract. The method of irrigation, value of crops, climatic conditions,
volume of water available for irrigation, slope of land, surface irregularities, soil properties, and
drainability of the substrata are the principal variables considered in determining costs of land
development. The cost will also vary in accordance with preferences of individual farmers. In the
selection process, assumptions are therefore made regarding the amount of land development to be
performed on a project.
Drainage
Prediction of the drainage requirement is the crucial element in selecting land for
irrigation. The adequacy of this prediction and its engineering implementation are among the prime
physical determinants of the success of irrigation enterprises.
Under the concept of conservational use, irrigable land must be drainable land. Systems
devised for selecting irrigable lands should therefore encompass drainability evaluations. This
includes investigating the substrata as well as the soil. Some lands are endowed with adequate
natural drainage to sustain irrigation. Unfortunately, naturally drainable areas occupy but a minor
portion of the landscape. Requirements for engineering works to remove excess water and salts will
therefore arise in most irrigated areas.
In selecting lands for irrigation both surface and subsurface drainage are considered.
Surface drainage is the removal of excess water from the surface of the land. The excess may arise
from precipitation, irrigation, losses from conveyance and storage facilities, or seepage from
groundwater at a higher elevation. Subsurface drainage is the removal of excess water from within
the soil by downward or lateral flow through the soil and substrata ... it involves control of
groundwater levels and the corresponding salt levels in the soil. The source of the water may arise
from deep percolation from precipitation or irrigation; leakage from canals, drains, or surface water
bodies at higher elevations; or upward leakage from artesian aquifers. Surface and subsurface
drainage problems are frequently co-incident and inseparable.
Recognition of Drainage Problems : Existing drainage problems may be identified by careful
field observations. The following generally indicate adverse drainage conditions : (i) water
standing in topographic depressions for prolonged periods; (ii) occurrence of salt-affected soils
with barren surfaces; (iii) soils containing high concentrations of soluble salts in surface layers or
having a distinct surface crust; (iv) shallow water table; (v) mottling or presence of gleyed
horizons; (vi) crop symptoms of unfavorable drainage such as stunted growth or late maturity,
disease, and shallow root development; and (vii) presence of phreatophytic vegetation.
Recognizing a potential drainage problem requires field observations and measurements
coupled with careful engineering analyses. Drainage investigations are designed to identify (i)
possible sources of excess water, (ii) adequacy of outlets for conducting excess water out of the
project area, (iii) capacity of the soil and substrata to conduct water, (iv) volume of excess water to
be removed, and (v) design required to achieve most efficient drainage. The investigations are
initiated by collecting, reviewing, and analyzing relevant data regarding geology and landforms,
soils, topography, well logs, water levels and fluctuations, precipitation and surface flow, and
similar factors.
In general a potential drainage problem may exist when (i) soil and substrata have
hydraulic conductivities of < 1 inch/hr; (ii) soil or unconsolidated substrata consist of textures of
fine sandy loam or finer and contain exchangeable sodium usually > 15%; (iii) soils contain excess
soluble salts for the climate under which they developed; (iv) impervious strata of shale or
sandstone occur within depths of 10 ft or less and have an unevenly weathered or eroded surface
obstructing both lateral and vertical water movement; (v)vertical or nearly vertical barriers occur
such as faults, dikes, and intrusions accompanied by topographic and substrata conditions that
intercept groundwater movement, constrict flow, and favour high water table conditions; (vi)
obstructions to surface drainage occur such as road and railroad embankments; (vii) lands lie
adjacent to a large proposed unlined canal, particularly the lower lying lands enclosed by a U-
shaped canal alignment; (viii) lands border natural drainage channels such as stream bottoms and
low terraces; (ix) lands lie adjacent to lakes or reservoirs whose water elevation may rise
sufficiently to unfavorably influence the groundwater levels; (x) tracts are subjected to sidehill
runoff or seepage from higher lands; and (xi) irrigation water, groundwater, or both contain an
ionic composition that may induce imperviousness in the soil or substrata through chemical
reactions. Recognizing such factors assists in determining the type and detail of drainage
investigations needed in a particular area to select irrigable lands and design drainage facilities.
Soil physical and chemical characteristics
1. Effective Soil Depth : This is a soil mentle resting over parent material which could be either
undifferentiated alluvium, hard pan (murrum layer) weathered (unconsolidated) or unweathered
(consolidated) rock below where roots are unable to pass and even a permanently moistured
saturated zone in which plant roots cannot survive.
The various soil depth classes recognised for this purpose in connection with soil survey
are following :
a) Very Shallow - less than 7.5 cm.
b) Shallow - 7.5 to 22.5 cm.
c) Medium deep - 22.5 to 45.0 cm.
d) Deep - 45 to 90 cm.
e) Very deep - more than 90 cm.
The area, therefore, can be classified into different soil depth classes and average area
under each soil depth class be recorded.
2. Soil Texture : It represents the distribution of particle size in different soils of the command.
Three important particles sizes are being considered for assessing soil texture :
i) Sand (2.0 to 0.02 mm dia as per International method)
ii) Sild (0.02 to 0.002 mm dia as per International method)
iii) Clay (0.002 mm and below).
The number of texture classes of surface and below by auger hole method upto 1 metre
should be grouped as under after profile examinations in the area :
1. Coarse textured soils e.g., sandy, and loamy sand.
2. Medium textured soils e.g. loam, silt loam and sandy loam.
3. Fine textured soils e.g. sandy clay-loam, clay-loam, clay-loam, clayey, fatty clay, silty-clay
etc.
3. Soil Structure : The natural arrangement and orientation of soil particles forming a soil
aggregates of different shapes and sizes and termed as soil structure. The various soil structures
recognised are :
a) Structureless : (a) Single grain (e.g. sand) and (b) Massive (single grain particles clining
together with small amount of clay and organic matter but are without definite lines of
cleavage).
b) With structure : (a) Granular or crumb (rounded granules of soil aggregated in either
uniform or irregular size and shape) and (b) Platty Soil aggregates having horizontal
dimensions greater than vertical as in mica (Biotite, muscovite).
c) Blocky : Soil aggregates conglomerating in (non-angular or angular) a shape having same
horizontal and vertical dimensions.
d) Prismatic and Collumner : Soil aggregates having vertically elongated shape as in prism.
If elongated portions are rounded it is columner (common in sodic soils).
4. Structure Destroyed (puddle soil mess) : The hydro-physical properties of soils are
considerably affected by this parameter.
5. Available Water Capacity : Available water capacity is the amount of water that a soil can
store that is available for use by plants. For designing efficient water delivery or distribution
system for fixing frequency or interval of irrigation and depth etc. the available water capacity of
the different established soil classes has to be worked out. This term when represented on per cent
basis is called available water per cent or A.W.P. It is a difference of moisture at field capacity
(F.C.) minus moisture at permanent wilting point (PWP) for different soil classes. The AWP,
Irrigation frequency and depth of water in a soil is expressed as cm/m soil depth which can be
calculated by formula :
i) AWP = FC-PWP
P x D x AWPii) Irrigation Interval, i = -------------------
ETC
P x D x AWPiii) Application depth, d = -------------------
Water or irrigation Eadepth
Where i = irrigation in days
P = means allowable moisture depletion for particular soil
D = rootzone depth
Ea = application efficiency
ETC = crop evapotranspiration.
6. Infiltration Rate : Infitration is the process of entry of water into the soil. Infiltration rate is the
maximum rate (mm/h) at which water penetrates into the soil at a given moment, the surface being
in contact with water at atmospheric pressure. The basic (Ib) and cumulative infiltration must be
determined at various locations in all the classes and average value for each class should be
recorded. Cumulative infiltration is total quantity of water enters the soil in a given time
Y = a tb + D
Y - cumulative infiltration in cm in time
t - elapsed time (minutes)
a and b - constants for various soils
7. Stream Size : For clayey soils, the stream size should be kept at 10 lits/sec for medium type and
medium textured 15 lits/sec. for medium to medium coarse land, it may be kept 30 lit per second
and for coarse texture upto 60 lits/sec (2 cusecs).
8. Permeability : This term is now designated as hydraulic conductivity and can be determined in
laboratory and in situation fields by auger hole test method in presence of water table or by pit
method in absence of water table. The test be conducted for all soil classes and averaged for each
class.
Permeability may be defined as the characteristic of a porous medium of its readiness to
transmit a liquid. The equation expressing the flow considers the fluidity of liquid and the
permeability factor called intrinsic permeability. Darcy's law according to the definition of
permeability may be written as,
A = K'
Where,
Q = volume of flow, cm3/s
K' = intrinsic permeability, cm/s2
= density of liquid
g = acceleration due to gravity, 9.81 cm/s2
= viscosity of liquid
H = loss of hydraulic head, cm
L = Length of tube, cm
A = cross -sectional area, cm2
From the expression, we find that the hydraulic conductivity K is,
K = = K'f
Where,
f = = fluidity of liquid
Again K' =
The intrinsic permeability has the physical dimension of L2T-1. Only the size and shape of
soil particles and pores influence it. Intrinsic permeability is the same as the hydraulic conductivity
except that it is independent of the fluid properties such as specific weight and viscosity, while the
hydraulic conductivity is dependent on fluid properties and changes with quality of water. For
most studies of water flow in irrigation and drainage, the influence of specific weight and viscosity
is relatively small.
Permeability is dependent on the pore size distribution in the soil. the larger the proportion
of macro-pores, the greater is the permeability. Permeability usually decreases with depth as
subsoil layers are more compact and have a smaller number of macro-pores compared to the
surface soil layer. The organic matter content, soil aggregates, texture, structure, colloidal matters,
plough pan, sodium concentrations of water, tillage and crop management practices influence
greatly the permeability of soil. Permeability decreases as the soil becomes drier following
saturation.
Smith and Browning (1946) described six classes of permeability based on hydraulic
conductivity of soil.
Table Permeability classes based on hydraulic conductivity of soil
Permeability classes Hydraulic conductivity (cm hr-1)
Extremely slow <0.0025
Very slow 0.0025-0.025
Slow 0.025-0.25
Moderate 0.25-2.5
Rapid 2.5-25.0
Very rapid >25.0
Source : Smith and Browning (1946)
9. Soil Salinity and Sodicity : The amount of salts, pH and exchangeable sodium per cent would
determine the type of problem in the area and accordingly different soil classes affected may be
regrouped for design of drainage system and leaching of salts for their suitability to irrigation and
irrigation planning. The reclamation measures be done by gypsum/lime as per requirement and
green manuring with dhaincha and salt resistant crops/paddy with special agronomical practices.
10. Water Quality : The quality of under-ground and irrigation water must be tested prior to
planning or clearance of wetting of any irrigation project. Also a periodic test of water quality be
made to accommodate any change in future planning and maintained soil fertility and sustain land
use in any type of soil in a command. For water quality rating sec Table 1.10.
Water quality rating as proposed by workers in India (Pai and Mukkeri, 1979)
Nature of soil Crops to be grown Upper permissible safe limits to Ec of
irrigation water micromphos/cm
Deep black soils and alluvial
soils clay >30%, fairly to well
drained
Semi-tolerant crops
Tolerant crops
1500
2000
Soils with clay 20-30% well
internal drainage and good
surface drainage
Semi-tolerant crops
Tolerant crops
2000
4000
Soils with 10-20% clay well
internal drainage and good
surface drainage
Semi-tolerant crops
Tolerant crops
4000
6000
Soils with clay <10% excellent
internal and surface drainage
Tolerant crops 8000
11. Classification of Different Soil Classes into Irritability Classes
Five to six irrigable classes are made for command. These are :
Class I = None to slight soil limitation (slope 1%, water table < 3 m).
Class II = Moderate soil limitations (soil, drainage, salt, topography and erosion) slope 1-
3%, water table 3-5 m
Class III = Severe above listed limitations for sustained (slope 3-5%), water table (1.5-3.0
m) use under irrigation.
Class IV = Very severe soil limitations for sustained use under irrigation (slope 5 to 10%),
water table 1.5 m and greater).
Class V (slope 7 to 15%) = Unsuitable for irrigation under prevailing conditions but may
be brought under irrigation by reclamation and management at high cost.
Class VI = Totally unsuitable for irrigation due to poor soil cover, and rock out crops –
Hence not considered for irrigability rating.
Land Development Needs
The irrigation is economical when applied to only I and II irrigability classes where slopes
are not steeper. For gravity or flow irrigation system the ideal slopes is 0.3% to 0.4% in flow
direction. However, by manipulating stream size and application method the soils up to 1.0% may
be irrigated. The land leveling programme should be taken only on deep to very deep soils and
should be avoided where soil depth is less than 0.50 meter. However, since 1'' soil layer of top
takes 300 to 1000 years to develop under natural conditions and 25 to 30 years under highly and
intensive soil management practices the cuttings should be planned to bare minimum and emphasis
should be on terrace making and contour cultivation, contour stripping cropping and other soil
conservation measures for other class of soils as per details of soil survey.
References
Darra B L and Raghuvanshi C S (1999) Irrigation management. Atlantic Publishers, New Delhi. p 8-13
Majumdar D K (2000) Irrigation water management (Principle and practices). New Delhi. p 83-84
Hangon R M, Haise H R, Edminster T W (1967) Irrigation of Agricultural Lands. American Society of Agronomy, Publisher Madison, Wisconsin, USA. p 125-89
Reddi G H S and Reddi T Y (2005) Efficient Use of Irrigation Water. Kalyani Publishers. p 324-25
Mason B (1958) An example of climatic control of land utilization. Climatology and microclimatology. Canberra Symp. Proc, UNESCO 11: 188-94
Harris K and Hansen V W (1958) Relative productive value of land. Utah Stat Univ. Agr. Appl. Sci. Eng. Exp. Sta. p 29
Assignment
On
LAND SUITABILITY FOR IRRIGATION AND
LAND IRRIGABILITY CLASSIFICATION
Course No. : Agron-602Evapo-transpiration
Submitted to :
Dr. S.S. Mahal
Submitted by :
Jagjot Singh Gill L-2009-A-3-D
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