WATER-USE EFFICIENCY OF FLOODED RICE FIELDS .1. VALIDATION OF THE SOIL-WATER BALANCE MODEL SAWAH

The water use efficiency of a flooded puddled rice field was studied t hrough analysis of the components of the water balance in the field an d through simulation modelling. Seepage and percolation (SP) losses we re the main determinants of water use efficiency in a field experiment conducted in the Philippines. Seepage through and underneath bunds ca n greatly increase total water loss. Seepage and percolation rates in well-puddled soil varied from 0.4 cm.d-1 without seepage to 3.62 cm.d- 1 with seepage, and cumulative SP losses varied between 90 and 350 cm per crop cycle, respectively. The vertical profile of an irrigated pud dled rice soil can schematically be described by a layer of ponded wat er, a muddy layer with little resistance to water flow, a plow sole wi th large resistance to water flow, and the non-puddled subsoil. Using this concept, the one-dimensional flow model SAWAH (Simulation Algorit hm for Water flow in Aquic Habitats) accurately simulated ponded water depth and pressure head gradients within the soil profile for the tes t field without seepage, using measured soil-hydraulic input data.