Water Balance Variability Across Sri Lanka for Assessing Agricultural and Environmental Water Use W.G.M
Agricultural Water Management 58 (2003) 171±192 Water balance variability across Sri Lanka for assessing agricultural and environmental water use W.G.M. Bastiaanssena,*, L. Chandrapalab aInternational Water Management Institute (IWMI), P.O. Box 2075, Colombo, Sri Lanka bDepartment of Meteorology, 383 Bauddaloka Mawatha, Colombo 7, Sri Lanka Abstract This paper describes a new procedure for hydrological data collection and assessment of agricultural and environmental water use using public domain satellite data. The variability of the annual water balance for Sri Lanka is estimated using observed rainfall and remotely sensed actual evaporation rates at a 1 km grid resolution. The Surface Energy Balance Algorithm for Land (SEBAL) has been used to assess the actual evaporation and storage changes in the root zone on a 10- day basis. The water balance was closed with a runoff component and a remainder term. Evaporation and runoff estimates were veri®ed against ground measurements using scintillometry and gauge readings respectively. The annual water balance for each of the 103 river basins of Sri Lanka is presented. The remainder term appeared to be less than 10% of the rainfall, which implies that the water balance is suf®ciently understood for policy and decision making. Access to water balance data is necessary as input into water accounting procedures, which simply describe the water status in hydrological systems (e.g. nation wide, river basin, irrigation scheme). The results show that the irrigation sector uses not more than 7% of the net water in¯ow. The total agricultural water use and the environmental systems usage is 15 and 51%, respectively of the net water in¯ow.
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