Effect of multi-temporal satellite images on soil moisture prediction using a digital soil mapping approach

Soil moisture (SM), a critical component of the global hydrological cycle, is affected by individual or combinations of multiple factors including soil properties, climate, and topography. Despite its importance to many disciplines, predicting SM continuously, accurately, and inexpensively over a large area is a great challenge due to its dynamic nature controlled mostly by the spatial and temporal variability of these factors. Static environmental covariates, such as those derived from a digital elevation model, are commonly used in digital soil mapping (DSM); these are typically less suitable for predicting dynamic properties. Easily available multitemporal satellite images show strong promise to capture this variability. The objective of this study was to predict SM from multi-temporal satellite images using a DSM approach. Specifically, we examined the feasibility of using dynamic, static, and combinations of environmental covariates (ECs) to predict SM in the Balikhli_Chay watershed in Iran on four separate dates in June, July, August, and September 2018 coincident with satellite overpass. Cubist and random forest (RF) machine learning algorithms (MLAs) were trained for making SM predictions for individual dates, and the data was then compiled without considering the date to create generalized models. The baseline for comparisons were the models developed using only static ECs. For June, July, August, and September, Cubist R-2 improvements were 96%, 78%, 185% and 120%, respectively. Using the generalized models, R-2 improved by as much as 223% and RMSE decreased by as much as 47% when comparing the best SM prediction model in each month to models developed using only static ECs for that same month using the Cubist model. Similar model improvements were seen for the RF model. The generalized Cubist and RF MLAs performed equally well with concordance of 0.91 and 0.90 for Cubist and RF respectively, and low RMSE of 3.04 and 2.98. The best Cubist and RF MLAs by month were always those developed with dynamic, or satellite-derived, ECs. Based on the variable importance statistics, land surface temperature (LST) was the most important EC. This study showed the strong predictions, and the practical feasibility of using multi-temporal satellite data as a dynamic EC that could help to capture the spatial and temporal variations of soil moisture. This approach could likely be extended to other dynamic soil property (e.g., soil temperature).