SAMPLING STRATEGIES FOR MEASUREMENT OF SOIL HYDRAULIC-PROPERTIES TO PREDICT RICE YIELD USING SIMULATION-MODELS

Measurements of soil hydraulic conductivity and water retention functi ons that are needed to run soil water balance and/or crop growth simul ation models are expensive and time consuming. The number of such meas urements can be reduced considerably if ''classical'' soil horizons th at are to be sampled can be merged into broader, hydraulic functional horizons. A hydraulic-functional horizon comprises one or more soil ho rizons that are similar in terms of soil hydraulic functions. A detail ed soil survey was carried out in a 50 ha dryland rice area to test th e concept of hydraulic-functional horizons and to obtain soil hydrauli c input data for a crop growth simulation model. The feasibility of id entifying hydraulic-functional horizons from visual appraisal of textu re and structure during the field survey was demonstrated. Soil water retention and conductivity data determined for the functional horizons and weather data from a nearby weather station were used to simulate potential and rainfed rice yield for 25 years. A variability analysis showed that the effect of variation in soil hydraulic properties on si mulated rice yield was relatively small compared to the effect of vari ation in year to year weather conditions. It was concluded that sampli ng strategies for soil hydraulic functions should be based on a first rough comparison of the impact of weather variability and soil heterog eneity on yield variability using simulation and sensitivity analyses.