WATER-USE EFFICIENCY OF FLOODED RICE FIELDS .2. PERCOLATION AND SEEPAGE LOSSES

The concept of a constant seepage and percolation (SP) rate in monitor ing the water balance of flooded rice fields, as often used in e.g. ir rigation system design and management, was investigated. First, magnit ude and variability of percolation rate were studied for different com binations of soil-hydraulic properties and hydrologic conditions using the validated water balance model SAWAH. Percolation losses from fiel ds with relatively low subsoil permeability (k(s,sub) < 10(-1) cm d-1) are either limited by a poorly permeable plow sole (k(s,top) < 10(-2) cm d-1) or by the low hydraulic conductivity of the subsoil itself. T ypical percolation losses of 0-0.5 and 1-1.5 cm.d-1 respectively are h ardly affected by ponded water depth, subsurface water content and dep th of ground water table. Percolation losses from fields with relative ly high subsoil permeability (k(s,sub) > 10(1) cm.d-1) may vary from 0 -0.5 cm d-1 with a poorly permeable plow sole, to 1-5 cm.d-1 or more f or a relatively permeable plow sole (k(s,top) > 10(-2) cm d-1). Only i n 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 of bunds to estimate th e magnitude and variation of SP rates, and to decide whether book-keep ing with a fixed SP rate is an appropriate tool in monitoring the wate r balance of paddy fields.