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MAIN HYDROLOGICAL MAIN HYDROLOGICAL CONCEPTSCONCEPTS

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To assist the students to develop and enhance the ability and knowledge in

main hydrological concepts such as

1. hydrological cycle2. water balance equation

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At the end of the course, students should be able to:

• Define hydrology.• Apply fundamental knowledge of

hydrology particularly use in civil and environmental engineering.

• Apply water balance equation as the base of a modeling of hydrology which covers processes of precipitation, evaporation, infiltration, runoff and groundwater.

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HydrologyHydrology

• is a multidisciplinary subject that deals with the occurrence, circulation, storage, and distribution of surface and ground water on the earth.

• The domain of hydrology includes the physical, chemical, and biological reactions of water in natural and man-made environments.

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Hydrology is…. • a science that studies the

availability and movement of water in the earth.

• also defined as a science related to the occurrence and distribution of natural water on the earth.

• hydrology covers many type of water, including transformation among liquid, solid and gas in atmosphere, surface and subsurface land.

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To civil and environmental engineers, hydrogeologists, and other earth scientists because of the environmental significance of

water supply, major floods droughts drainage and urban stormwater

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Hydrologic cycle Hydrologic cycle is a continuous process in which water is evaporated from water surfaces and oceans, moves inland as moist air masses, and produce precipitation if the correct vertical lifting conditions exist. The precipitation that falls from clouds onto the land surface of the earth is dispersed to the hydrologic cycle via several pathways (Fig.1-1).

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impermeable layer

Evaporationfrom Lake/river

Evaporationfrom Land

Evaporationfrom Ocean

Transpiration

Run-offInfiltration

Groundwater Flow

Precipitation

Wind

Figure 1-1: Hydrology Cycle

Cloud

Cloud Cloud

Precipitation Precipitation

Run-off

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P = PrecipitationT = Transpiration I = InfiltrationR = RunoffG = Groundwater flow E = Evaporation from lake, land surface and oceanET = EvapotranspirationLS = Land surfaceWT = Water table

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Main River

River

River

River

River

Catchment boundary

Lake

Spring

Spring

Spring

HIGHLAND

AREA

COASTAL

AREA

Sketch of Movement of Water on the Land Surface

A watershed is a contiguous (sharing a boundary) area that drains to an outlet, such that precipitation that falls within the watershed runs off through that single outlet.

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Outlet

Outleta. Elongated shape b. Concentrated shape

Figure 1-2Typical watershed areas

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Water Balance Equation

In quantitative terms, hydrology cycle can be represented by a closed equation which represents the principle of conservation of mass. And many forms of this expression, called the water balance.

Water balance equation is the base of a modelling of hydrology.

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Location Area of Water

km2

Volumeof Water

km3

Percentage

Total of Water

(%)

Surface Water- Fresh Water in Lake- Salt Water in Lake- Fresh Water in River &

Stream

Subsurface Water- Groundwater, upper

800m depth- Groundwater, below

800m depth

Soil Moisture

Ice and Glacier

Atmosphere

Ocean

854,330.73698,997.87

129,444,050.00

129,444,050.00

129,444,050.00

17,863,278.90

510,009,557.00

361,148,899.50

124,965.285104,137.7381,249.652

4,165,509.5294,165,509.529

66,648.152

29,158,566.703

12,913.080

1,320,466,520.000

0.0090.0080.001

0.310.31

0.005

2.15

0.001

97.20

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Volume of water in the world 1,358 million km3.

Volume of fresh water 2.8 %, most of fresh water are in the form of ice and glacier

Fresh water of groundwater, lake, cloud and rain 8.54 million km3 or only 0.63%.

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dtdS

Q I

where: I = inflow [L3/t]

Q = outflow [L3/t]dS/dt = change in storage per time [L3/t]

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The same concept can be applied to small basins or large watersheds

P – R – G – E – T = ΔS where:

P = precipitation, R = surface runoff, G = groundwater flow,

E = evaporation, T = transpiration,ΔS = change in storage in a specified time

period.

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For a given month, a 121 ha lake has 0.43 m3/s of inflow, 0.37 m3/s of outflow, and total storage increase of 1.97 ha-m. A USGS gage next to the lake recorded a total of 3.3 cm precipitation for the lake for the month. Assuming that infiltration loss is insignificant for the lake, determine the evaporation loss, in cm, over the lake for the month.

Solution: Solving the water balance for inflow I and outflow Q in a lake gives,

for evaporation, E = I – O + P – ΔS,

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1ha10,000m

121ha

1hour3,600sec

1day24hr

1month30day

1monthsecm

0.43

2

3

I

1ha10,000m

121ha

1hour3,600sec

1day24hr

1month30day

1monthsecm

0.37

2

3

O

= 0.92 m = 92 cm

= 0.79 m = 79 cm

P = 3.3 cm

121ha

m1.97haΔS

= 0.0163 m = 1.63 cm

E = 92 – 79 + 3.3 – 1.63 = 14.67 cm

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A swimming pool (6m 6m 1.5m) has a small leak at the bottom. Measurements of rainfall, evaporation, and water level are taken daily for 10 days to determine what should be done for repair. Estimate the average daily leakage out of the swimming pool in cm3/day. Assume the pool is exactly 1.5 m deep at the end of day 1.

Day Evaporation

(mm)

Rainfall(mm)

Measured Level(mm)

12345678910

12.70

12.70

12.712.7

012.712.712.7

-25.4

-50.8

--

101.6---

1,524

1,321

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Solution

The water balance equation becomes: O = P – E – ΔSTotal change in storage, S = 1,321 – 1,524 = -203 mmTotal precipitation, P = 25.4 + 50.8 + 101.6 = 177.8 mmEvaporation, E = (7) (12.7) = 88.9 mm

Thus, outflow,O = 177.8 – 88.9 – (-203) = 291.9 mm

Outflow should be in cm3/day. The height change is distributed over the pool area. Q =

10days1m

100cm6m

1m100cm

6m10mm1cm

291.9mm

Q = 1,050,840 cm3/day

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Fundamental to analyses, forecasting, and modeling.

Hydrologic data consists of; Wind Temperature Humidity Evaporation Precipitation Solar radiation

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Hydrology is the science of water.

It embraces the occurrence, distribution, movement and properties of the waters of the earth.

A mathematical accounting system may be constructed for the inputs, outputs and water storages of a region so that a history of water movement over time can be estimated.

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