Remote sensing of root zone soil moisture: A review of methods and products

Abstract

The demand for accurate and reliable estimates of root zone soil moisture (RZSM) has been increasing because of the growing need to address global concerns, including climate change and food security. In recent decades, significant advances have been made for inferring RZSM from remotely sensed observations, providing many methods and products that are available for Earth science fields. However, there are conflicting views in the recent literature regarding the accuracy of these methods due to regional differences in climate, soil, and vegetation conditions, as well as variations in parameterization and calibration approaches. These techniques have not been adequately discussed and documented in the literature. This article comprehensively reviews the satellite-based RZSM estimation methods (however some methods are not exclusively remote sensing-based, but in theory satellite data can be applicable to them), discusses their basic principles, and highlights their strengths, weaknesses, and potential research directions. We categorize these methods into two groups: those that estimate RZSM with knowledge of surface soil moisture (SSM) and those that estimate RZSM without SSM knowledge. Although these methods show varying levels of accuracy, periodic review required to address neglected assumptions, modify algorithms, and develop new ones in line with technological advancements of the 21st Century. In addition, we adequately describe and compare the major satellite-based global RZSM products at present to understand their global and local patterns and uncover existing problems. This review will serve as a reference for scientists in various fields to make informed decisions on the preferred methods for estimating RZSM from remote sensing measurements.