Water requirement during crop growth

Water is lost from a rice field mainly through evapotranspiration, seepage, percolation, surface runoff & bund leakages which could vary depending on crop, environment and the field management factors. Evapotranspiration from a rice crop canopy is a function of the size of the crop (leaf area), water availability and the environmental conditions. Evapotranspiration increases with increase leaf area. Evapotranspiration is low at early stages of crop growth and achieve maximum towards heading. Hence the frequency of irrigation should increase accordingly towards flowering to meet the increasing demand for water. Experiment conducted at Agriculture Research Station, Mahailluppallama showed that the total ET in the dry zone during in Yala season in higher than during Maha season (Table 1).

Table 1.Total Evapotranspiration (mm) from a 4 1/2 and 3 month rice crop during Yala and Maha seasons at Mahailluppallama

Calculated using modified Penman method using long term average climatic values