Scale effects on water use and water productivity in a rice-based irrigation system (UPRIIS) in the Philippines

Between 25% and 85% of water inputs to rice fields are lost by seepage and percolation. These losses can be reused downstream and do not necessarily lead to true water depletion at the irrigation system level. Because of this potential for reuse, the general efficiency of water use can increase with increasing spatial scale. To test this hypothesis, a multi-scale water accounting study was undertaken in District I of the rice-based Upper Pampanga River Integrated Irrigation System (UPRIIS) in the Philippines. Daily measurements of all surface water inflows and outflows, rainfall, evapotranspiration, and amounts of water internally reused through check dams and shallow pumping were summed into seasonal totals for 10 spatial scale units ranging from 1500 ha to 18,000 ha.