Modeling water balance components in rice field irrigation

Abstract

In this study, a water balance equation approach was applied to the irrigation practiced at the Besut Rice Irrigation Scheme. Each component of the equilibrium equation, where not measured, was modeled using equations sourced from the literature. Results obtained from the modeling exercise were compared with measured data whenever available. The reference evapotranspiration (ETo) modeled with the Penman-Monteith equation for the monthly time series; was evaluated to be 3.6 mm/day and 3.5 mm/day for off-season (May – October) and main season (November – April) crops respectively. Effective rainfall was estimated using the water use-rainfall ratio method, with monthly rainfalls data from 1951 to 1998, for three rainfall stations. It has been observed that 68% and 54% of rainfall accounts as effective rainfall during the pre-saturation and crop growth periods respectively. The water subsidence technique was used to estimate the average daily seepage cum percolation rate, and results showed this to be 2.4 mm/day for the site. Comparisons between model component outputs and observed and recorded field data are good. The water balance components can be adequately used to monitor field conditions for rice irrigation in Malaysia.