Improved estimation of vegetation water content and its impact on L-band soil moisture retrieval over cropland

•Relative to climatology VI, dynamic VI improve VWC and SM retrieval in SMAP algorithm.•Red-edge VI estimate more accurate VWC than non-red-edge VI do in peak growth stage.•Surface roughness along with VI climatology should be updated to improve SM retrieval. Satellite-based soil moisture (SM) retrieval within agricultural regions is challenging due to pronounced vegetation and land cover variations during the growing season. Vegetation water content (VWC)–used to describe vegetation opacity in both the Soil Moisture Active Passive (SMAP) Dual Channel Algorithm (DCA) and V-polarization Single Channel Algorithm (SCA-V)–is approximated using a climatological Normalized Difference Vegetation Index (NDVI) time series. To understand the impact of VWC algorithms on SMAP SM retrievals, we investigated optimal vegetation indices for VWC estimation, and further evaluated the impact of VWC uncertainty on DCA and SCA-V SM retrievals. Besides NDVI, we applied four dynamic vegetation indices (VIs) acquired from Sentinel-2–including two red-edge VIs (Modified Chlorophyll Absorption Ratio Index (MCARI) and Inverted Red-edge Chlorophyll Index (IRECI)) and two non-red-edge VIs (Normalized Difference Water Index (NDWI1 and NDWI2)) to estimate growing-season corn VWC and the impact of different empirical relationships on VWC estimates. We find that climatological NDVI from Moderate Resolution Imaging Spectroradiometer (MODIS) significantly underestimates VWC, and therefore underestimates SM over croplands in Northwest China during the growing season, whereas dynamic VIs can improve the accuracy of VWC and SM retrievals. With respect to the choice of VI for VWC estimation, MCARI is more accurate during the peak growing season compared to its counterpart non-red-edge VIs; however, it also overestimates VWC during the early growing stage. In addition, the influence of VWC uncertainty on SM accuracy is shown to be higher in SCA-V than in DCA. The results of this study provide insight into preferred approaches for the accurate estimation of VWC and associated improvement in passive microwave algorithms for SM retrieval.