Relationship between Actual Evapotranspiration and Potential Evapotranspiration Calculated by the Equation Simplified from the FAO Penman-Monteith Equation in a Paddy Field

Abstract

To estimate reliable actual evapotranspiration from a paddy field with flooding water using a simple measurement system, first, the relationship between actual and potential evapotranspiration (LF_w,ef and LF_PM) was examined. LF_w,ef was estimated with the energy balance flux ratio method, and LF_PM was calculated by the FAO Penman-Monteith equation. LF_w,ef was sufficiently proportional to LF_PM in each month of June, July, Aug. and Sept.. Next, actual evapotranspiration (LF_w,eb or LF_w,ef) was estimated by the energy balance Bowen ratio method or the energy balance flux ratio method, after unreliable values obtained by those methods were corrected from the relationship between reliable values by those methods and LF_PM. Potential evapotranspiration (LF_pe) was calculated by the equation simplified from the FAO Penman-Monteith equation. The sum of LF_w,eb or LF_w,ef for a day was sufficiently proportional to that of LF_pe, although the value of proportional coefficient in the early growth stage was different from that in the late growth stage. The former value was 0.885, the latter was 0.996. In conclusion, reliable daily actual evapotranspiration could be estimated from daily potential evapotranspiration calculated by the FAO Penman-Monteith simple equation.