Evaluation of SMAP and SMOS Microwave Satellite Soil Moisture Products for Agricultural Drought Monitoring: Karkheh River Basin Case Study, Iran

Agricultural drought occurs when soil moisture available to crops is inadequate to meet their water requirements, resulting in reduced crop yields and agricultural production. Therefore, it is crucial to accurately monitor agricultural drought for effective irrigation planning and scheduling, water resources management and allocation, and food security protection. Soil moisture is a fundamental variable for estimating agricultural drought indicators; thus, reliable, accurate, continuous, and long-term datasets are essential for effective drought monitoring. L-band radiometry is the most effective passive microwave remote sensing technique for estimating global soil moisture. The ESA Soil Moisture and Ocean Salinity (SMOS) and NASA Soil Moisture Active Passive (SMAP) missions, launched in 2009 and 2015, respectively, were developed specifically to retrieve Surface Soil Moisture (SSM) at L-band (1.4 GHz) for the top 5 cm of soil, with a target accuracy of 0.04 m³ m⁻³. These missions also generate Root Zone Soil Moisture (RZSM) at 0–100 cm depth through assimilation of SSM into land surface models. Agricultural drought can be detected using the Soil Water Deficit Index (SWDI) and Soil Moisture Drought Index (SMDI) derived from SMAP and SMOS satellite-based soil moisture products. The Karkheh River Basin is one of the most important watersheds and agricultural regions in southwestern Iran, with a semi-arid to arid climate, which has experienced frequent droughts in recent years. This study evaluates the efficiency of SMOS and SMAP products for agricultural drought monitoring using the SMDI and SWDI indices over the Karkheh River Basin.