Water balance of field plots planted with soybean and pumpkin

A field water balance was studied within the context of the Japanese plan for upland field agriculture. As part of this study, soil physical conditions were monitored in the field, soil hydraulic characteristics were measured in the laboratory, and the water balance was modeled. The soil hydraulic characteristics were described by the analytical functions of van Genuchten and Mualem, and were used as inputs in the SWAP 1993 water balance model. The model calculation of actual evapotranspiration, ET(a) agreed well in some instances with the field ET(a) calculated using the Energy balance - Bowen ratio method but did not agree at other times. Good results were obtained for the two situations considered as calculated soil water contents agreed fairly well with indirectly measured values (i.e., from tensiometer data) in the field. However, a sensitivity analysis will be necessary if quantitative conclusions are to be made.