Abstract
Soil moisture content (SMC) plays an important role in the hydrological functioning of wetlands. Remote sensing shows potential for the quantification and monitoring of the SMC of palustrine wetlands; however, this technique remains to be assessed across a wetland–terrestrial gradient in South Africa. The ability of the Sentinel Synthetic Aperture Radar (SAR) and optical sensors, which are freely available from the European Space Agency, were evaluated to predict SMC for a palustrine wetland and surrounding terrestrial areas in the grassland biome of South Africa. The percentage of volumetric water content (%VWC) was measured across the wetland and terrestrial areas of the Colbyn Wetland Nature Reserve, located in the City of Tshwane Metropolitan Municipality of the Gauteng Province, using a handheld SMT-100 soil moisture meter at a depth of 5 cm during the peak and end of the hydroperiod in 2018. The %VWC was regressed against the Sentinel imagery, using random forest, simple linear and support vector machine regression models. Random forest yielded the highest prediction accuracies in comparison to the other models. The results indicate that the Sentinel images have the potential to be used to predict SMC with a high coefficient of determination (Sentinel-1 SAR = R²>0.9; Sentinel-2 optical = R²>0.9) and a relatively low root mean square error (Sentinel-1 RMSE =<17%; Sentinel-2 optical = RMSE <21%). Predicted maps show higher ranges of SMC for wetlands (> 50%VWC; p<0.05) compared to terrestrial areas, and therefore SMC monitoring may benefit the inventorying of wetlands, as well as monitoring of their extent and ecological condition.
 Significance:
 
 The freely available and space-borne Sentinel sensors show potential for the quantification of surface soil moisture across a wetland–terrestrial gradient.
 Significant differences between the surface soil moisture of palustrine wetlands and terrestrial areas, imply that inventorying and monitoring of the extent and hydroperiod of palustrine wetlands can potentially be done.
Highlights
It is estimated that more than 85% of wetlands have been transformed, primarily owing to the loss of natural habitat resulting from land conversion, and as a consequence of other pressures including changes to the hydrological regime, water pollution and invasive species.[1,2] In South Africa, the extent of natural and transformed wetlands is unknown[3,4], sub-national studies have shown that 58% of wetlands in the Umfolozi secondary catchment, had already been transformed irreversibly already by the 1990s5
Of the various modelling scenarios, the Sentinel images were capable of predicting the %soil moisture content (SMC) with the majority of coefficients of determination (R2)>0.7 and RMSEs
This study proves that the freely available Sentinel-1 (SAR) and Sentinel-2 sensors have potential in the estimation of the extent and degree of soil saturation in palustrine wetlands in the grassland biome of South Africa
Summary
It is estimated that more than 85% of wetlands have been transformed, primarily owing to the loss of natural habitat resulting from land conversion, and as a consequence of other pressures including changes to the hydrological regime, water pollution and invasive species.[1,2] In South Africa, the extent of natural and transformed wetlands is unknown[3,4], sub-national studies have shown that 58% of wetlands in the Umfolozi secondary catchment (in the KwaZulu-Natal Province), had already been transformed irreversibly already by the 1990s5. Remote sensing has played an important role in the detection and monitoring of the inundated sections of wetlands, both internationally and in South Africa. The Global Inundation Extent from Multi-Satellites (GIEMS) and Global Surface Water Explorer products have been generated from coarse-scale satellite imagery which reflect the extent of inundation of larger artificial and natural wetlands.[6,7] In South Africa, the national land-cover products include open water classes[8], and, more recently, monitoring of the monthly extent of inundation[9] has improved our ability to characterise the hydroperiod of wetlands. The development of indices which would characterise the extent and nature of palustrine wetlands is, a gap and top priority for South Africa
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