Evapotranspiration in Semi-Arid Climate: Remote Sensing vs. Soil Water Simulation

Estimating crop evapotranspiration (ET_a) is an important requirement for a rational assessment and management of water resources. The various remote sensing products allow the determination of crops' biophysical variables integrated in the evaluation of ET_a by using surface energy balance (SEB) models. This study compares ET_a estimated by the simplified surface energy balance index (S-SEBI) using Landsat 8 optical and thermal infra-red spectral bands and transit model HYDRUS-1D. In semi-arid Tunisia, real time measurements of soil water content (θ) and pore electrical conductivity (EC_p) were made in the crop root zone using capacitive sensors (5TE) for rainfed and drip irrigated crops (barley and potato). Results show that HYDRUS model is a fast and cost-effective assessment tool for water flow and salt movement in the crop root layer. ET_a estimated by S-SEBI varies according to the available energy resulting from the difference between the net radiation and soil flux G₀, and more specifically according to the assessed G₀ from remote sensing. Compared to HYDRUS, the ET_a from S-SEBI was estimated to have an R² of 0.86 and 0.70 for barley and potato, respectively. The S-SEBI performed better for rainfed barley (RMSE between 0.35 and 0.46 mm·d^-1) than for drip irrigated potato (RMSE between 1.5 and 1.9 mm·d^-1).