Water balance variability across Sri Lanka for assessing agricultural and environmental water use

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 verified 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 sufficiently 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 inflow. The total agricultural water use and the environmental systems usage is 15 and 51%, respectively of the net water inflow. The consumptive use of rain-fed and irrigated agriculture are approximately equal. The evaporation rates in agriculture and mixed vegetation are similar, so that low productivity rangelands can be transformed into rain-fed agriculture without detrimental effects on water availability to downstream users. The unused water flow to the Indian Ocean is 34% of the net inflow, hence there is scope for further water developments in Sri Lanka.