WATER-BALANCE OF FIELD PLOTS PLANTED WITH SOYBEAN AND PUMPKIN

A field water balance was studied within the context of the Japanese p lan for upland field agriculture. As part of this study, soil physical conditions were monitored in the field, soil hydraulic characteristic s were measured in the laboratory, and the water balance was modeled. The soil hydraulic characteristics were described by the analytical fu nctions of van Genuchten and Mualem, and were used as inputs in the SW AP 1993 water balance model. The model calculation of actual evapotran spiration, ETa agreed well in some instances with the field ETa calcul ated using the Energy balance - Bowen ratio method but did not agree a t other times. Good results were obtained for the two situations consi dered as calculated soil water contents agreed fairly well with indire ctly measured values (i.e., from tensiometer data) in the field. Howev er a sensitivity analysis will be necessary if quantitative conclusion s are to be made.