Retrieval of Soil Moisture Content Based on a Modified Hapke Photometric Model: A Novel Method Applied to Laboratory Hyperspectral and Sentinel-2 MSI Data.

Soil moisture is the crucial carrier of the global hydrologic cycle and the dynamic energy balance regulation process. Therefore, it is of great significance to monitor surface soil moisture content (SMC) accurately for the study of the natural ecological environment. The Hapke model is the most widely used photometric model in soil remote sensing research, but the development of this model is limited by the lack of valid multi-angular data. The main innovations of this paper have two aspects: (1) A novel soil moisture retrieval approach based on the Hapke (SMR-Hapke) model is derived by exploring the relationship between single scattering albedo (SSA) and SMC on the optical bands from 400 to 2400 nm. The performance of the proposed model was verified on a dataset consisting of four different soil samples, and the experimental results indicated that the inverted soil moisture from SMR-Hapke model coincided with the measurement values, with theR(2)being generally more than 0.9 in the solar domain. (2) The SMR-Hapke model has been reduced to a linear form on the SWIR field and a physically-based normalized difference soil moisture indexNDSMIHapke has been proposed. Based on the laboratory-based hyperspectral data, we compared the performance ofNDSMIHapke with other traditional soil moisture indices using linear regression analysis, and the results demonstrate that the proposedNDSMIHapke had a great potential for estimating SMC withR(2)values of 0.88. Finally, high-resolution SMC map was produced by combining the Sentinel-2 MSI data withNDSMIHapke. This study provides a novel extended Hapke model for the estimation of surface soil moisture content.