Application of FAO-56 for evaluating evapotranspiration in simulation of pollutant runoff from paddy rice field in Japan

Applicability of FAO-56 method in estimation of evapotranspiration for the simulation of pollutant runoff from rice paddy field in Japan was investigated. Crop evapotranspiration and crop coefficient recommended by FAO-56 method relative to those values obtained in the field monitoring for three Japanese rice varieties, namely, Nihonbare, Mangetsumochi and Koganemochi during first three growing stages were compared. Also, the pesticide fate model, PCPF-1, which incorporates the FAO-56 method was evaluated for the applicability of the FAO-56 method towards the accurate prediction of herbicide concentration in paddy water. The estimation of cumulative ETc in paddy rice by FAO-56 method using the recommended Kc value resulted in estimation error of up to 17% from the observed values. The recommended values of Kc-ini in FAO-56 method are appropriate if reliable atmospheric data are available. However, the Kc-mid was found to be a sensitive parameter affecting ETc estimation and the careful calibration according to the regional conditions and varieties seemed to be required for the accurate prediction. Considering the effect of random errors, FAO-56 method is more reliable when calculating cumulative ETc longer than 7 days of period. Despite the relatively large error in cumulative ETc resulted from the FAO-56 method with the recommended Kc value, the maximum error expected to have on the prediction of the herbicide concentration in paddy field is 5.6%, and that of herbicide runoff loss can be 3.2%. Therefore, it can be concluded that application of FAO-56 method with the recommended Kc value is acceptable in the simulation of pesticide fate and transport.