Evaluation of WaPOR Evapotranspiration Product for Irrigated Orchards in an Arid Region

With the scarcity of water resources and climatic variability aggravated by climate change, the Mediterranean region's agricultural sector faces serious water allocation challenges. In the central region of Tunisia, characterized by an arid climate, the arboriculture sector is moving forward with the intensification of groundwater resources’ exploitation. Hence, accurate water management becomes a crucial need. Currently, remote sensing products, such as the FAO’s portal to monitor Water Productivity (WAPOR), offer the possibility to assess water consumption in near real-time and for a large area. In this work, the accuracy of WAPOR's evapotranspiration product is evaluated on irrigated arboriculture areas based on comparison with the water balance and FAO-56 methods deployed using in situ measurements. During 2021, six orchards of almond, pistachio, and olive trees in full production were surveyed. Two weather stations in the region measure the rainfall and the meteorological variables needed to calculate the reference evapotranspiration (ET0) with the daily time step FAO-56 Penman–Monteith. The crop cover’s phenological stages are surveyed weekly, and each species’ crop coefficient is adjusted to local conditions. On the other hand, from the WAPOR database, we used the 10-day ETa available at 100 m (ETaWaPOR). Based on the FAO-56 method, the actual evapotranspiration (ETaFAO56) is calculated using the adjusted single crop coefficients and then used to validate ETaWaPOR. Despite that WaPOR is assumed to overestimate ETa of irrigated fields, the comparison with in situ values shows an underestimation for the three studied crops and mainly for pistachio and almond, for which ETaFAO56 is seven times higher than ETaWaPOR. Due to the high divergence between the two estimated ETaFAO56 and ETaWaPOR, the estimated datasets need to be evaluated based on another ETa measurement.