A Comparison of Two Soil Moisture Products S2MP and Copernicus-SSM Over Southern France

This paper presents a comparison between the Sentinel-1/Sentinel-2-derived soil moisture product at plot scale ((SMP)-M-2) and the new Copernicus surface soil moisture (C-SSM) product at 1-km scale over a wide region in southern France. In this study, both products were first evaluated using in situ measurements obtained by the calibrated time delay reflectometer in field campaigns. The accuracy against the in situ measurements was defined by the correlation coefficient R, the root mean square difference (RMSD), and the bias and the unbiased root mean square difference (ubRMSD). Then, the soil moisture estimations from both SSM products were intercompared over one year (October 2016-October 2017). Both products show generally good agreement with in situ measurements. The results show that using in situ measurements collected over agricultural areas and grasslands, the accuracy of the C-SSM is good (RMSD = 6.0 vol%, ubRMSD = 6.0 vol%, and R = 0.48) but less accurate than the (SMP)-M-2 (RMSD = 4.0 vol%, ubRMSD = 3.9 vol%, and R = 0.77). The intercomparison between the two SSM products over one year shows that both products are highly correlated over agricultural areas that are mainly used for cereals (R value between 0.5 and 0.9 and RMSE between 4 and 6 vol%). Over areas containing forests and vineyards, the C-SSM values tend to overestimate the (SMP)-M-2 values (bias > 5 vol%). In the case of well-developed vegetation cover, the (SMP)-M-2 does not provide SSM estimations while C-SSM sometimes provides underestimated SSM values.