WATER BALANCE STUDY OF A STORAGE TANK IN DROUGHT PRONE AREA

Abstract

Water scarcity problems in a drought prone area can be mitigated by conservation and management of water available in the area. Water balance study is a pre-requisite for planning of an efficient water management practice. In water scarcity areas, run off originating from rainfall is conserved in storage tanks. These storage tanks are shallow reservoirs formed by constructing earthen embankments across seasonal streams. In some places water from the seasonal streams is diverted to natural depressions. These storage tanks support supplementary irrigation, function as important water source for people, livestock, pisciculture and are places of recreation and worship. In India, tank irrigation is about 20% of the total area irrigated. Storage tanks also facilitate ground water recharge. A water balance study in a storage tank reveals whether the prevailing water source meets the water demand in its command area. The component processes involved in water balance in a storage tank are: (i) evaporation from the water body, (ii) run off from contributing catchment of the tank, (iii) direct rainfall (iv) infiltration from the tank bed and (v) outflow from the storage tank. These process level models have been integrated in this thesis to quantify the depth of water available in the storage tank during a water year. Penman's method and heat balance method are used for computation of evaporation from a water body. In this thesis it is found that the Penman's method computes an average evaporation rate as compared to heat balance method. The heat balance method is more accurate as the method computes zero evaporation on the day, the water temperature in the tank coincides with the dew point temperature of the atmosphere. In shallow storage tank (less than 3m), the variation in depth of water due to evaporation has nominal effect on evaporation rate from the water body. SCS (Soil Conservation Service) method is widely used for computation of runoff from a catchment. Since, the storage parameter changes during a water year owing to several rainfall events, it is pertinent that the curve number should be updated. In the present study the curve number is updated by taking account of change in soil moisture due to evaporation and drainage from the soil profile. The depth of water in a storage tank is primarily governed by infiltration. The infiltration rate has been computed using Green and Ampt infiltration theory. The assumption of Green and Ampt infiltration theory, that the soil is saturated behind a moving front, is valid for a two layered soil system, if the upper layer has higher hydraulic conductivity than that of lower layer. Incase, the upper-layer in two layered soil system has lower hydraulic conductivity than that of the underlying layer, the soil moisture behind the moving front in the first layer is equal to the saturation moisture content of the upper layer. When the moving front surpasses the upper layer, the moisture content behind the moving front in the lower layer is less than the saturated moisture content of the underlying layer. Green and Ampt infiltration theory is inapplicable for computing infiltration from a storage tank, underlain by a layered soil system if the upper soil layer is less permeable than that of the lower layer, as the simulated infiltration rate does not follow the decreasing trend of infiltration given by Horton's equation. With a postulation that water front moves in unsaturated state in the lower layer at a moisture content corresponding to which the unsaturated hydraulic conductivity of the lower layer is same as the saturated hydraulic conductivity of the upper layer, the simulated infiltration rate follows the decreasing trend as given by Horton's infiltration equation. The temporal variation in depth of water in a storage tank has been presented for four soil groups. The soil moisture characteristics for these four groups of soil have been taken from published data. The variation of the drying time of a storage tank in a ii homogeneous soil layer has been computed for different initial depth. It is found that the variation of drying time with initial depth of water is quasi-linear in nature. The drying time is very closely inversely proportional to the hydraulic conductivity of the sub soil. But, in case of a two layered soil system, where the bottom soil layer has lower hydraulic conductivity, the variation of drying time with initial depth of filling is very much non-linear in nature. Water availability in a storage tank has been predicted as a case study. The tank is situated in a drought prone area. The present height of the spillway crest needs to be raised from 1.525m to 3.155m to maintain a minimum depth of 1.0m in the Asha Sagar storage tank at the end of the water year. Otherwise, the tank would remain dry during the later half of the water year. If the crest level will be raised to a height of 6.8m; all the potential runoff from the contributing catchment will get stored and water can be supplied for irrigation during post monsoon period to a command area of 121 ha, besides supporting for pisciculture in the storage tank.