OVERESTIMATION OF SOYBEAN CROP TRANSPIRATION BY SAP FLOW MEASUREMENTSUNDER FIELD CONDITIONS IN CENTRAL PORTUGAL

Direct measurements of the xylem sap flow by the stem heat balance tec hnique can be a valuable aid for determining the irrigation demand of field crops. In the present study, soybean (Glycine max (L.) Merr.) sa p flow was evaluated under well-watered and water-stressed conditions using Dynamax SGA10 sap flow gauges. Solar radiation was measured cont inuously throughout the growing season. Soil water content was measure d before and after each irrigation. There was a close relationship bet ween solar radiation and xylem sap flow. The water flux in the soybean stems responded realistically to changes in the soil water content. H owever, the absolute values of sap flow were highly questionable. Calc ulating crop transpiration from sap flow measurements, the results wer e up to 4 times as high as calculated transpiration from soil moisture data and simulated transpiration using the locally calibrated soybean crop growth model SOYGRO. A sensitivity analysis of the stem heat bal ance technique gave no indications of technique or input errors. The g auge design was possibly not appropriate for the outdoor installation on soybeans.