By G.INDUMATHY LITERATURE ON ESTIMATION OF EVAPOTRANSPIRATION BY WATER BALANCE METHOD

EVAPORATION The term evapotranspiration denotes the water transpired by crop plants and the water evaporated from the soil or water surface in the crop field and the intercepted precipitation by crop aerial parts in any specified period. It is usually expressed as surface depth of water in millimeters or centimeters. Evapotranspiration (ET) = evaporation from the crop field (E) + transpiration (T) + intercepted precipitated by crop plants lost as evaporation (IP)

Methods of estimating evapotranspiration: Direct methods Pan evaporimeter method Empirical methods

Direct methods: Direct methods are the water balance or hydrologic methods and include Lysimeter Field experimentation Soil water depletion Water balance method

Water balance method: The other name of this method is inflow- outflow method. This method is applied for estimating the yearly CU overlarge area. It is also called water balance method. It may be formulated as follows: CU (or U) = P+I+ GW-R

Where, CU (or U) = yearly consumptive use over a large area,hectare metres P = yearly precipitation hectare metres (areas in hectares*precipitation in metres ) I = surface water that flows into the area in a year, hectare metres GW = change in ground water storage, hectare metres R = yearly outflow (run-off) from the area, hectare metres The change in the profile is not included as it is considered negligible. It is assumed that the subsurface inflow into the areas is same as the subsurface out flow.

SOIL WATER BALANCE METHOD IN GROUND WATER RECHARGE Water balance models were developed in the 1940s by Thornthwaite (1948) and were later revised. The method is essentially a bookkeeping procedure, which estimates the balance between the inflow and outflow of water. Here, the volume of water required to saturate the soil is expressed as an equivalent depth of water and is called soil water deficit.

The soil water balance can be represented by, Ri = P – Ea + W – Ro where, Ri = Recharge P = precipitation Ea = Actual evapotranspiration W = Change in water storage in soil Rn = runoff

GROUND WATER BALANCE METHOD In general I – O = W / t where I = inflow in during time t O = Out flow during time t W = Change in water volume

REFERENCE: Irrigation water management principles and practice – Dilip Kumar Majumdar. Kumar C.P., "Estimation of Natural Ground Water Recharge". ISH Journal of Hydraulic Engineering, Vol.3, No.1, pp Sathish Chandra, "Estimation and measurement of recharge to ground water for rainfall, irrigation and influent seepage" - International seminar on development and management of ground water resources (November 5-20, 1979)

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