Critical soil moisture thresholds for evapotranspiration in rainfed semi-arid maize in Kenya

Understanding how maize ( Zea mays L. ) evapotranspiration (ET) and its drivers vary under rainfed semi-arid conditions remains limited, despite general estimates across growth stages. This study aimed to quantify growth stage–specific ET in rainfed maize, characterize its soil and atmospheric controls, and evaluate seasonal and interannual variability using the Surface Energy Balance System (SEBS) model. ET was measured using the eddy covariance method over two long rainy seasons. Crop growth stages were identified using the normalized difference vegetation index (NDVI) from Sentinel-2, and ERA5 reanalysis data were used as inputs for the SEBS model. The measured growing‐season ET ranged from 221 to 269 mm season⁻¹ for the two wet years. However, long‐term SEBS estimates (2014–2022) revealed rainfall control on soil moisture and ET, with seasonal ET spanning 94–408 mm season⁻¹. Results show that the critical soil moisture threshold ( θ crit ) separating non-stressed and water-stressed ET conditions is not a fixed soil property but varies with crop growth stage and soil depth. Growth-stage-specific θ crit at 0.1 m depth ranged from 0.05 to 0.07, while growing-season θ crit derived from ET SEBS varied from 0.04 to 0.09 across years. The ET–soil matric potential relationship exhibited stage-specific hydraulic thresholds (–1.5 MPa during early vegetative and –1.3 MPa during reproductive stages), reflecting shifts in soil–plant hydraulic functioning across the season. Together, these stage‐specific moisture and stress thresholds clarify ET responses to soil and rainfall dynamics, enhancing crop‐model parameterization and informing water‐management strategies in rainfed semi‐arid maize systems.