A Modified Surface Energy Balance to Estimate Crop Transpiration and Soil Evaporation in Micro-Irrigated Orchards

A surface energy balance model was conceived to estimate crop transpiration and soil evaporation in orchards and vineyards where the floor is partially wetted by micro-irrigation systems. The proposed surface energy balance model for partial wetting (SEB-PW) builds upon previous multiple-layer modelling approaches to estimate the latent, sensible, and soil heat fluxes, while partitioning the total evapotranspiration (ET) into dry and wet soil evaporation (lambda E-soil) and crop transpiration (T). The model estimates the energy balance and flux resistances for the evaporation from dry and wet soil areas below the canopy, evaporation from dry and wet soil areas between plant rows, crop transpiration, and total crop ET. This article describes the model development, sensitivity analysis and a preliminary model evaluation. The evaluation shows that simulated hourly ET values have a good correlation with field measurements conducted with the surface renewal method and micro-lysimeter measurements in a micro-irrigated winegrape vineyard of Northern California for a range of fractional crop canopy cover conditions. Evaluation showed that hourly LE estimates had root mean square error (RMSE) of 58.6 W m(-2), mean absolute error (MAE) of 35.6 W m(-2), Nash-Sutcliffe coefficient (C-NS) of 0.85, and index of agreement (d(u)) of 0.94. Daily soil evaporation (E-s) estimations had RMSE of 0.30 mm d(-1), MAE of 0.24 mm d(-1), C-NS of 0.87, and d(a) of 0.94. E-s estimation had a coefficient of determination (r(2)) of 0.95, when compared with the micro-lysimeter measurements, which showed that E-s can reach values from 28% to 46% of the total ET after an irrigation event. The proposed SEB-PW model can be used to estimate the effect and significance of soil evaporation from wet and dry soil areas on the total ET, and to inform water balance studies for optimizing irrigation management. Further evaluation is needed to test the model in other partially wetted orchards and to test the model performance during all growing seasons and for different environmental conditions.