Measurements and predictions of evapotranspiration rates from irrigated wheat in the Cerrados Region of central Brazil

The purpose of this study was to characterize evapotranspiration for irrigated wheat growing in the Cerrados Region of Central Brazil and to evaluate and test various methods for predicting evapotranspiration. Bowen ratio measurements were made on an hourly basis over irrigated wheat crops grown in the dry seasons of 1982 and 1983. This data was analyzed in order to evaluate factors affecting crop evapotranspiration and also was compared to rates of evapotranspiration predicted using the Priestly-Taylor equation and Monteith combination model. By comparing measured evapotranspiration to net radiation it was shown that sensible heat advection, of local and regional origin, can significantly affect crop water requirements in this region.;Bulk canopy resistance required in the Monteith combination model was calculated using the energy balance method and the residual method. Both methods produced comparable results and indicated that this resistance varied within well defined limits dictated by irrigation timing. When the combination model was used to predict crop evapotranspiration, results were in good agreement with those obtained from energy balance measurements. In calculating the aerodynamic resistance it was found that the effects of stability could be ignored. Also crop surface temperature predicted from measurements of wind speed and air temperature at one height above the canopy were in good agreement with infrared measured canopy temperature.;In order to evaluate the supply function for models predicting water movement in the soil-plant-atmosphere continuum the Libardi and Campbell methods for estimating soil hydraulic conductivity as a function of water content were compared. They produced similar results. The parameters required in Campbell's method could be obtained from routine soil characterization data, but field saturated hydraulic conductivity and field saturated water content must be known.;Measured and predicted daily evapotranspiration rates and soil water potential at the upper 10 cm soil-layer were in good agreement when the Monteith combination model was used as a demand function in a single-layer canopy and multi-layer soil model (GAPS). Predictionis of hourly energy fluxes using a multi-layer canopy and multi-layer soil model (Cupid) were in good agreement with measured energy fluxes.;The proportionality constant in the Priestly-Taylor equation had an average value of 1.2 for potential conditions under non-advective conditions and decrease gradually between irrigations. For irrigation management purposes the use of a value of 1.5 was deemed appropriate.