Linking NDVI to METRIC‐Based Stress‐Adjusted Crop Coefficients ( K c_adj ) for Operational Evapotranspiration Estimation Under Semiarid Conditions

ABSTRACT Accurate estimation of evapotranspiration (ET) is essential for sustainable water management, particularly in arid and semiarid regions. This study aimed to establish relationships between the normalised difference vegetation index (NDVI) derived from satellite imagery and stress‐adjusted crop coefficients ( K c_adj ) obtained using the METRIC (mapping ET at high resolution with internalised calibration) model and to develop efficient models for rapid ET estimation. The study was conducted during the 2023 and 2024 growing seasons in Ergani (Diyarbakır, Türkiye). The water stress coefficient ( K s ) was derived from METRIC‐based ET and used to obtain K c_adj values. Regression analysis was performed to quantify the relationships between NDVI and K c_adj , and NDVI‐based K c ( K c_NDVI ) values were developed. The results indicated strong relationships between NDVI and K c_adj for maize and tobacco ( R 2 = 0.90–0.93 and 0.89–0.95, respectively), whereas the resulting NDVI‐based ET estimates showed strong internal consistency with the METRIC‐informed stress‐adjusted estimates. Conversely, weaker relationships were observed for potato and onion ( R 2 = 0.52–0.58 and 0.0003–0.56, respectively), likely driven by planting–harvest timing heterogeneity, incomplete satellite coverage and the influence of stress factors. Overall, the approach is most suitable for crops showing strong and stable NDVI– K c_adj relationships under adequate temporal data coverage conditions.