Calibration and Validation of the SEBS Model for Actual Evapotranspiration Assessment of Medicinal Herbs in Arid and Semi‐Arid Iran: Implications for Irrigation Management

ABSTRACT This study assesses the surface energy balance system (SEBS) model for estimating farm‐scale actual evapotranspiration (ETa) of medicinal herbs in three diverse climatic regions of southern Iran: Pasargad (semi‐dry, semi‐cold), Arjan (semi‐wet, semi‐cold) and Darab (dry, warm). The research aimed to establish ground truth ETa using the soil water balance (SWB) approach and to calibrate and validate the SEBS model utilizing Landsat 8 imagery. SEBS was calibrated against FAO Penman–Monteith (FAO P‐M) potential evapotranspiration (ETo) data from nearby water bodies, improving the root mean square error (RMSE) to 0.081–0.088 mm/day and R 2 values to 0.90–0.96 across sites. Validation against field‐observed ETa, derived from the SWB (considering irrigation, effective rainfall and return flow, with negligible soil moisture change), revealed an R 2 of 0.81 and an RMSE of 246.0 m 3 /ha (approximately 5% of average SEBS ETa) after one outlier was removed. Although SEBS generally underestimates the observed ETa by approximately 10%, it effectively demonstrates that plant ETa is governed by water needs rather than excessive application. The findings confirm SEBS's capability for large‐scale water management, highlighting current overirrigation and the model's utility for optimizing water use and acknowledging a challenge in applying the model to small farm plots owing to image resolution.