lec.01.introduction to hydrology

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The study of water, including rain, snow and water on the earth’s surface, covering its properties, distribution, utilisation, etc.

(Chambers Science and Technology Dictionary)

The study of water in all its forms, and from its origins to all its destinations on the earth.

(Bras, 1990))

The science dealing with the waters of the earth, their occurrence, distribution and circulation, their chemical and physical properties, and their interaction with the environment.

(Ward & Robinson, 1999)

HYDROLOGHYDROLOGYY

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Main Branches

HYDROLOGY

Ground Water

Hydrology

Surface Water

Hydrology

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• Water is one the most valuable natural resources essential for human and animal life, industry and agriculture.

• It is also used for power generation, navigation and fisheries.

• Tremendous importance is given to the hydrology all over the world in the development and management of water resources for irrigation, water supply, flood control, water-logging and salinity control, Hydro power and navigation.

Scope of Hydrology

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Engineering Hydrology

• It uses hydrologic principles in the solution of engineering problems arising from human exploitation of water resources of the earth.

• The engineering hydrologist, or water resources engineer, is involved in the planning, analysis, design, construction and operation of projects for the control, utilization and management of water resources.

• Hydrologic calculations are estimates because mostly the empirical and approximate nature of methods are used to describe various hydrological processes.

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Engineering Hydrology seeks to answer questions of the following types:

• What is the maximum probable flood at a proposed dam site?

• How does a catchment’s water yield vary from season to season and from year to year?

• What is the relationship between a catchment’s surface water and groundwater resources?

• What flood flows can be expected over a spillway, at a highway culvert, or in an urban storm drainage system?

• What reservoir capacity is required to assure adequate water for irrigation or municipal water supply in droughts condition?

• What hydrologic hardware (e.g. rain gauges, stream gauges etc) and software (computer models) are needed for real-time flood forecasting?

Uses of Engineering Hydrology

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Major Aspects of HydrologyThe main jobs of a hydrologist are collection and analysis of data, and making prediction out of this data.

1. Collection of Data: The hydrologic data comprises:

Rainfall data, snowfall and snowmelt data, runoff data, topographic maps, groundwater data.

2. Analysis of Data

Analysis of hydrologic data includes checking it for consistency and homogeneity as well as finding its various statistical parameters.

3. Prediction

Means to find design values and maximum possible events (rainfall, floods, droughts). Various approaches used are:

Statistical, Physical, Deterministic

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HYDROLOGIC CYCLE

• The hydrologic cycle describes the continues re-circulating

transport of the waters of the earth, linking atmosphere, land

and oceans.

• To explain it briefly, water evaporates from the ocean surface, driven by energy from the Sun, and joins the atmosphere, moving inland as clouds. Once inland, atmospheric conditions act to condense and precipitate water onto the land surface, where, driven by gravitational forces, it returns to the ocean through river and streams.

• The process is quite complex, containing many sub-cycles.

• Engineering Hydrology takes a quantitative view of the hydrologic cycle.

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Hydrological ProcessesHydrological Processes• Precipitation• Interception• Evaporation• Transpiration• Infiltration• Overland flow• Surface Runoff• Groundwater outflow

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• The quantification of the hydrologic cycle which is an open system, can be represented by a mass balance equation, where inputs minus outputs are equal to the change in storage.

I - O = ∆S

• The water holding elements of the hydrological cycle are:1. Atmosphere 2. Vegetation3. Snow packs 4. Land surface5. Soil 6. Streams, lakes and rivers7. Aquifers 8 Oceans

Hydrologic EquationHydrologic Equation

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Inflow:1. Precipitation2. Import defined as water channeled into a given area.3. Groundwater inflow from adjoining areas.

Outflow:1. Surface runoff outflow2. Export defined as water channeled out of the same area.3. Evaporation4. Transpiration5. Interception

Change in Storage: This occurs as change in:1. Groundwater2. Soil moisture3. Surface reservoir water and depression storage

Water Balance Components

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Global Hydrologic Cycle

• The global hydrologic cycle can be represented as a system containing three subsystems:

the atmospheric water system,

the surface water system, and

the subsurface water system.

• Block-diagram (flow chart) representation of GHC is shown in Figure#1.

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Precipitation

Infiltration

Transpiration

Interception

Groundwater recharge

Overland flow

Subsurface flow

Runoff to streams and ocean

Surface runoff

Groundwater flow

Evaporation

∑

∑

Atm

osph

eric

Wat

erSu

bsur

face

Wat

erSu

rfac

e W

ater

Block-diagram representation of the global hydrologic system (Chow et al. 1988).

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Evapotranspiration from land

Evaporation from ocean

Moisture over land

Precipitation on land

100

61

39

424

Precipitation on ocean

385

Groundwater outflow

Surface outflow 38

1

Surface flow

Groundwater flow

Infiltration

Global Water Balance of The hydrological cycle

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In the atmosphere:Precipitation (P) = Evapotranspiration (ET)100+385 = 61+424

On land:P = Evapotranspiration (ET) + Surface runoff (R) +

Groundwater outflow100 = 61 + 38 + 1

Over oceans and seas:Ocean precipitation + Surface runoff + Groundwater

outflow = Evaporation (E)385 + 38 + 1 = 424

Global Water Balance

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Table 1. Estimated Distribution of World's Water.

Component Volume 1000 km3 % of Total Water

Atmospheric water 13 0.001

Surface Water Salt Water in Oceans Salt water in lakes & inland seas Fresh water in lakes Fresh water in stream channels Fresh water in glaciers and icecaps Water in the biomass

13200001041251.25

2900050

97.20.0080.0090.00012.15

0.004

Subsurface water Vadose water G/W within depth of 0.8 km G/W between 0.8 and 4 km depth

6742004200

0.0050.310.31

Total (rounded) 1360000 100

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Catchment and Basin

A catchment is a portion of the earth’s surface that collects runoff and concentrates it at its furthest downstream point, referred to as the catchment outlet.

The runoff concentrated by a catchment flows either into a larger catchment or into the ocean.

The place where a stream enters a larger stream or body of water is referred to as the mouth.

The terms watershed and basin are commonly used to refer to catchments. Generally, watershed is used to describe a small catchment (stream watershed), whereas basin is reserved for large catchments (river basins).

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Regional Water Balance (Water Budget)Precipitation (P) Evapotranspiration (ET)

Surfacerunoff (R)

Infiltration (F)

A mass balance over time from t = 0 to T, i.e.

Inputs - Outputs = Change in Storage P - (R+ET+F) = ΔS

All terms in the hydrologic equation should be in the same units.

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Infiltration (F)

Storage (S)

Time t = 0

Time t = TChange in storage (∆S)

Precipitation (P) Evapotranspiration (ET)

Surface runoff (R)

Schematic representation of the mass balance equation

∆S = P - (R + F + ET)

∆S = +ve if P > (R + F + ET)∆S = -ve if P < (R + F + ET)∆S = 0 if P = (R + F + ET)

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In a given year, a catchment with an area of 2500 km2 received 1.3 m of precipitation. The average rate of flow measured in a river draining the catchment was 30 m3s-1.

(i). How much total river runoff occurred in the year (in m3)?

(ii). What is the runoff coefficient?

(iii).How much water is lost due to the combined effects of evaporation, transpiration, and infiltration. (Express in m).

Problem #1

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Solution

(i). Total runoff volume = number of seconds in a year × average flow rate= 31 536 000 × 30= 9.4608×108 m3

(ii). Runoff coefficient = runoff volume/ precipitation volume= (9.4608×108) / (1.3 × 2500 × 106)= 0.29 (29 %)

Problem #1

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(iii). The water balance equation can be arranged to produce:

ET+F= P - R - ΔSwhere:P = (1.3 × 2500×106)

= 3.25×109 m3

R = 9.4608×108 m3 (from [i])

ΔS = 0 (i.e. no change in storage)

So,ET + F = 3.25×109 - 9.4608×108

= 2.30392×109 m3

= (2.30392×109) / (2500×106)= 0.92 m

Problem #1

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Water at a constant rate of 370 cumec was observed to be entering into Tarbela Reservoir in a certain season. If outflow from the reservoir including infiltration and evaporation losses is 280 cumec, find out the change in storage of reservoir for 10 days. Also convert your answer into Hectare-meter.

Problem #2

I = 370 cumec O = 280 cumec

∆t = 10 days = 10 x 24 x 3600 = 864,000 sec

∆S = ?

According to water balance equation

∆S/∆t = I – O = 370 – 280 = 90 cumec

Total change in storage = ∆S = 90 x 864,000 = 7776000 m3

= 7776000/10000 = 777.6 hectare-m

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In a given year, a catchment with an area of 1750 km2 received 1250 mm of precipitation. The average rate of flow measured in a river draining the catchment was 25 m3s-1.

(i). Calculate how much total river runoff occurred in the year (in m3).

(ii).Calculate the runoff coefficient. What is the percentage runoff ?

Problem #3

Area of the catchment = 1750 km2 = 1750 x 10^6 m2

Flow rate in the river = 25 m3/s

Precipitation received = 1250 mm = 1.25 m

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Solution:

Total annual precipitation = (1.25) x (1750 x 10^6)= 2187.5 x 10^6 m3

Flow rate during the year = 2187.5 x 10^6 / (365 x 24 x 60 x 60) = 69.36 m3/s

Runoff Coefficient = Actual flow in river / Total precipitation occurred

= 25 / 69.36= 0.36

Percentage of flow = 0.36 x 100 = 36%

Problem #3