| Summary: | Estimating crop evapotranspiration (ET<sub>a</sub>) 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<sub>a</sub> by using surface energy balance (SEB) models. This study compares ET<sub>a</sub> 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<sub>p</sub>) 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<sub>a</sub> estimated by S-SEBI varies according to the available energy resulting from the difference between the net radiation and soil flux G<sub>0</sub>, and more specifically according to the assessed G<sub>0</sub> from remote sensing. Compared to HYDRUS, the ET<sub>a</sub> from S-SEBI was estimated to have an R<sup>2</sup> 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<sup>−1</sup>) than for drip irrigated potato (RMSE between 1.5 and 1.9 mm·d<sup>−1</sup>).
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