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The concept of a constant seepage and percolation ( SP) rate in monitoring the water balance of flooded rice fields, as often used in e.g. irrigation system design and management, was investigated. First, magnitude and variability of percolation rate were studied for different combinations of soilhydraulic properties and hydrologic conditions using the validated water balance model SAW AH.Percolation losses from fields with relatively low subsoil permeability ( ks,sub < 10- Only in the latter case, percolation rates are largely affected by the depth of ponded water. Next, the constancy of combined SP rates was studied in a field experiment on a permeable subsoil. Simple book-keeping of the water balance using a fixed SP rate proved accurate to predict the depth of ponded water in time in case of a poorly permeable plow sole and a small seepage component. A decision tree was suggested based on soil-hydraulic properties and characteristics ofbunds to estimate the magnitude and variation of SP rates, and to decide whether book-keeping with a fixed SP rate is an appropriate tool in monitoring the water balance of paddy fields.

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

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