Evaluation of radiant canopy temperature and soil water measurement for quantifying the contribution of shallow watertables to crop evaporation.

Abstract

Accurate estimation of the contribution of shallow watertables to crop water consumption is of major importance for improved irrigation practice and watertable management. A water balance model based on radiant canopy temperature estimations of crop evaporation and measurements of the change in the soil water content with a neutron probe was used to estimate the net upward flux from the watertable. The model was tested against measurements made by using two weighing lysimeters containing loam and clay loam soils, and the estimation of errors involved was analysed. The watertable in the lysimeters was maintained 1 m below the ground surface. Evaporation from a soybean crop was estimated by using an energy balance model with measured values of net radiant energy and soil heat flux. Sensible heat flux was calculated from the difference between the radiant canopy temperature and air temperature, together with an estimated aerodynamic resistance of the crop. Two sources of error were associated with the model, one due to the estimation of crop evaporation and the other due to the measurement of the change in soil water content. Errors of 0.7 and 5.1 mm per day were estimated for the first and second sources respectively. The accuracy of the method was determined by the length of the period considered and by the contribution of the watertable itself. An error of 20% was estimated when periods of 20 and 80 days were considered in the loam and clay loam soils respectively. Shorter time periods will result in larger uncertainty. The implications of the these results for watertable management are discussed.