Hydrology: Objectives of Hydrology, Hydrograph and Site Selection for Hydropower Plants


• First requirement – Q (discharge)

• Hydrology deals with occurrence and distribution of water over and under earth’s surface.

– Surface Water Hydrology

– Ground Water Hydrology

• Watershed, catchment area or drainage area: length of the river, size and shape of the area it affects, tributaries, lakes, reservoirs etc.

• Investigation of run-off for past few years is required for power potential studies of a HPP.

Objectives of Hydrology

• To obtain data regarding the stream flow of water that would be available,

• To predict the yearly possible flow

• To calculate the mean annual rainfall in the area under consideration from a record of the annual rainfall for a number of years, say 25 to 30

• To note the frequency of dry years

• To find maximum rainfall and flood frequency Various terms related to Hydrology

• Rainfall is also known as precipitation and can be measured by rain gauges.

• Some part of precipitation is lost due to evaporation, interception and transpiration.

• Transpiration: Plants absorbing moisture and giving it off to the atmosphere

• Stream flow = precipitation – losses

• Stream flow = surface flow + percolation to ground

• Surface flow is also known as run-off.


– shows the variation of stream flow in m3/s with time for a particular river site. The time may be hour, week, month or a year.

– The area under hydrograph gives the total volume of flow

• Flow duration curve

– shows the percentage of time during the period when the flow was equal to greater than the given flow.

– The area under FDC gives the total quantity of run-off during a period

• Mass curve

– indicates the total volume of run-off in cubic meters up to a certain time.

– the slope of the curve at any point shows the rate of flow at that time

– Used for estimating the capacity of storage reservoir

• Storage

– to ensure water availability during deficient flow and thus increasing the firm capacity

– Storage also results in more energy production

• Pondage

– Storing water in small ponds near the power plant as the storage reservoir is away from plant

– To meet the power demand fluctuations over a short period of time e.g. 24 hours

• Primary Power: power that will be available 90 % of the time

• Secondary Power: power that will be available 75 % of the time

• Dump Power: power that will be available 50 % of the time.

• Maximum flow estimation: gives estimation of floods and helps in design of dam and spillway.

Site Selection for Hydropower Plants

• Availability of Water: Run-off data for many years available

• Water Storage: for water availability throughout the year

• Head of Water: most economic head, possibility of constructing a dam to get required head

• Geological Investigations: strong foundation, earthquake frequency is less

• Water Pollution: excessive corrosion and damage to metallic structures

• Sedimentation: capacity reduces due to gradual deposition of silt

• Social and Environmental Effects: submergence of areas, effect on biodiversity (e.g. western ghat), cultural and historic aspects

• Access to Site: for transportation of construction material and heavy machinery new railway lines or roads may be needed

• Multipurpose: power generation, irrigation, flood control, navigation, recreation; because initial cost of power plant is high because of civil engineering construction work

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