Estimating Maize Crop Water Requirements in Tamale, Northern Ghana Using Sentinel-2 and a Hybrid FAO-56/ASCE Reference Evapotranspiration Approach

Understanding crop water requirements is essential for advancing climate-resilient agriculture, particularly in rainfed systems where variability in rainfall and evapotranspiration undermines productivity. This study presents a spatially explicit approach to estimate maize water demand during the 2022 growing season in Tamale, Northern Ghana, a region characterized by unimodal rainfall, limited irrigation infrastructure, and data scarcity. The research integrates global datasets, including Sentinel-2 NDVI to monitor canopy development, CHIRPS for rainfall estimation, and TerraClimate for reference evapotranspiration (ETo). While NDVI was used to track phenological stages, crop coefficients (Kc) were sourced from FAO-56 literature and temporally aligned with observed crop development. The resulting approach constitutes a hybrid methodology, combining FAO-56 Kc values with ASCE Penman-Monteith–derived ETo from TerraClimate. Using the single crop coefficient method, actual crop evapotranspiration (ETc) was calculated monthly. Results indicate a peak mid-season ETc of 368.7 mm, with a critical water deficit of –60.87 mm during the development phase. Spatial analysis revealed significant intra-seasonal variability in water demand and supply across croplands, underscoring the vulnerability of smallholder maize production to rainfall mismatches. By leveraging freely accessible datasets and bypassing the need for ground-based calibration, this study offers a replicable, low-cost methodology for crop water assessment in data-limited regions. The findings provide valuable insights for irrigation planning, agricultural water management, and policy formulation in Ghana’s Guinea Savannah and comparable agro-ecological zones.