The synergistic use of microwave coarse-scale measurements and two adopted high-resolution indices driven from long-term T-V scatter plot for fine-scale soil moisture estimation

Abstract

ABSTRACT In an attempt to retrieve soil moisture content (SMC) from remote sensing techniques, this article suggests and evaluates a developed approach that overcomes three of the most fundamental limitations of the temperature–vegetation (T-V) scatter plot method that are (1) low accuracy of the T-V scatter plot method in cases that the corresponding scatter plot and its wet and dry edges are not formed appropriately, (2) incompatibility of the T-V index maps in different days, and (3) inability to obtain the absolute SMC values from the T-V indices. The research consists of three main steps. In the first step, the measurements of eight global in situ SMC networks were applied to select the most appropriate and accurate microwave remote sensing mission among from Advanced Microwave Scanning Radiometer 2, Soil Moisture and Ocean Salinity, and Soil Moisture Active Passive. The results outlined the superiority of SMAP’s products in terms of RMSE and correlation coefficient (root mean square difference (RMSE) = 0.3–0.12 m3/m3 and R = 0.53–0.93). At the second step, the 1-year T-V scatter plot was formed and then, two adopted soil moisture indices, namely the annual soil moisture index (ASMIHR) and daily soil moisture index (DSMIHR), were extracted. The ASMIHR was driven from the annual wet and dry edges and a novel concept called “co-moisture line” was introduced to obtain the DSMIHR. In the third step of the proposed method, Disaggregation based on Physical and Theoretical scale Change was applied as the downscaling algorithm with a novelty to identify the parameter of partial derivative of microwave data relative to SMC indices using the relationship between ASMIHR and coarse-scale SMAP pixels. Across four SMAP coarse-scale pixels located in the study area, the corresponding parameter was obtained with the average correlation coefficient 0.7. This step was followed by integrating DSMIHR and SMAP products to provide absolute SMC values at intermediate spatial resolution. The proposed method was evaluated on the agricultural site of Soil Moisture Active Passive Validation Experiment 2016-Manitoba. The ultimate results of the proposed method in terms of absolute SMC values proved promising consistency with field measurements (R = 0.66 and no-bias RMSE = 0.06 m3/m3).