Assessment of Root Zone Soil Moisture Products from the Passive Microwave sensors over Iran

Accurate and continuous root zone soil moisture (RZSM) data are crucial for important applications such as water resources management, flood and drought monitoring, irrigation planning and timing, agricultural productivity, climate change adaptation, and climate modeling. Direct measurement of RZSM using in-situ sensors can be time-consuming and costly, particularly over large areas. The Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) satellites provide surface soil moisture (SSM) data at L-Band and on a global scale. Due to the relationship between SSM and RZSM, satellite SSM data can be assimilated into land surface models to estimate RZSM data. The SMAP Level-4 Surface and Root Zone Soil Moisture (SMAP RZSM) product includes RZSM (0–100 cm), 3-hourly, 9 km on EASE-Grid 2; and the SMOS Level 4 RZSM CATDS (SMOS RZSM) product provides daily RZSM (0–100 cm), 25 km spatial sampling on EASE-Grid 2 on a global scale. This study aimed to evaluate the accuracy and efficiency of RZSM data derived from SMAP RZSM and SMOS RZSM products compared to in-situ measurements collected from IRIMO (Islamic Republic of Iran Meteorological Organization) sites from 2017 to 2020 in different regions of Iran. The evaluation results indicate that the SMOS RZSM data has higher accuracy compared to the SMAP RZSM data. The SMOS RZSM data has a root mean square error (RMSE) ranging from 0.03 to 0.09 m³m⁻³ and an unbiased root mean square error (ubRMSE) ranging from 0.03 to 0.05 m³m⁻³. The SMAP RZSM data has an RMSE range of 0.03 to 0.21 m³m⁻³ and an ubRMSE range of 0.02 to 0.08 m³m⁻³.