Improving ETa Estimation for Cucurbita moschata Using Remote Sensing-Based FAO-56 Crop Coefficients in the Lis Valley, Portugal.

Efficient water management is essential for optimizing agricultural productivity in water-scarce regions such as the Lis Valley, Portugal. In situ measurements of soil moisture content (SMC) and electrical conductivity (EC), together with Sentinel-2-derived vegetation indices, were used to assess the crop water status and evapotranspiration dynamics of pumpkin (Cucurbita moschata 'Butternut') during the 2020 growing season. SMC and EC were measured at depths of 10, 20, 30, 50, and 70 cm using a TDR sensor, with strong correlations observed in the upper layers, indicating that EC can complement direct SMC measurements in characterizing near-surface moisture conditions. Sentinel-2 imagery was acquired to compute NDVI, SAVI, EVI, and GCI. In addition, NDVI values obtained from both a GreenSeeker® sensor and Sentinel-2 imagery were compared, showing a similar temporal pattern during the season. By replacing the standard FAO-56 Kc values with those derived from each vegetation index, ETa was recalculated to incorporate actual crop condition variability detected via satellite. ETa estimates from RS-assisted vegetation indices agreed with those obtained using the FAO-56 method; independent ETa measurements were not available for validation. Although such agreement is partly expected due to calibration, its confirmation for Cucurbita moschata under Mediterranean conditions-where published references are scarce-reinforces the method's practical applicability for water management in data-limited settings. Water Productivity (WP) was estimated as 8.32 kg m-3, and Water Use Efficiency (WUE FAO-56) was calculated as 0.64 kg m-3, indicating high water use efficiency under Mediterranean smallholder irrigation conditions. These findings demonstrate that integrating high-resolution RS with continuous soil moisture monitoring can enhance precision irrigation strategies, increase crop yields, and conserve water resources in the Lis Valley.