Water balance and water productivity of rice paddy in unpuddled sandy loam soil

In this study, water balance components of rice paddy field under unpuddled sandy loam soil conditions were determined using the water balance and irrigation scheduling model WinISAREG. The WinISAREG water balance and irrigation scheduling model has been calibrated and validated for field soil and crop parameters using the measured soil moisture contents in the experimental field. Fairly similar values of seasonal evapotran-spiration and soil moisture storage across the field plots were computed. Deep percolation was identified as the most important water balance component causing reduced field water productivity. It has been observed that nearly 82-86% of input water was accounted for deep percolation as measured in lysimeters during the continuous irrigation crop season (crop season 1), while about 76-78% of the input water was lost as deep percolation during the intermittent irrigation crop season (crop season 2). Input water during crop season 2 has been reduced 2.2 to 2.6 (or irrigation size 2.9 to 3.9) times that of crop season 1 which resulted in input water saving of 55-62% (or irrigation water saving of 65-74%) with comparatively small yield decrease. A comparative yield decrease ranging from 17 to 36% was observed during crop season 2 with the exception to one of the field plots where yield has been, contrarily, increased. However, yield decrease was accompanied with increase in comparative field water productivity ranging between 42 and 174%. This study shows that there exists large possibility for increasing field water productivity in rice paddy fields even under unpuddled course textured soil conditions.