Hydrological analysis of farm reservoirs in rainfed rice areas

Abstract

Farm reservoirs are water storage structures used to collect rainfall and runoff on the farm. In central Luzon, Philippines, farm reservoirs are about 800 to 5000 m 2 in area and have an average depth of about 1 m. The average farm size is about 3 ha. The stored water is used to supplement rainfall for wet season rice production and to irrigate a dry season rice crop on approximately one third of the farm area. Farmers without reservoirs cannot grow a dry season crop. This study quantified the major hydrological parameters of four selected farm reservoirs using a water balance approach. Direct rainfall and runoff from the catchment area contributed, respectively, about 36% and 64% to reservoir inflow. Typically, farmer's use of stored water for land preparation and irrigation of rice accounted for about 30% of the total water outflow from a reservoir. Seepage and percolation losses of water accounted for about 45% of the outflow volume, and evaporation loss of about 25%. The minimum catchment area required to support a reservoir of given capacity was calculated to be nearly 5 times higher for a grassed catchment than for a catchment under paddy rice. The hydraulic conductivity of reservoir embankments reduced with reservoir age due to continuous settlement of poorly compacted materials, affecting the rate of water loss through seepage.