Abstract
ABSTRACT Using dense soil moisture (SM) measurements in the upper Huai River basin of China, this study evaluated the spatial patterns of L-band satellite-based SM products, including Soil Moisture Active Passive (SMAP) L3, Soil Moisture and Ocean Salinity (SMOS) L3 and the European Space Agency’s Climate Change Initiative (ESA CCI) SM products. The mean difference (MD), root mean squared error (RMSE), unbiased root mean square error (ubRMSE) and Pearson correlation coefficient (R), were used in the evaluation. The evaluation results presented that SMAP and ESA CCI products can well capture the temporal variation of SM at single points quite well, with average R values of 0.51 and 0.46, respectively. And SMAP had the highest overall accuracy among the three satellite-based products in study area. We also analyzed the correlations between the four accuracy indexes and six environmental factors including the proportions of five land use/land cover types (i.e. water bodies, paddy fields, construction land, dryland and forest) and the average NDVI (Normalized Difference Vegetation Index) in 2016 in each grid. Analysis showed that the proportions of paddy fields and water bodies in each grid had significant positive correlations with MD, RMSE and ubRMSE, while NDVI, and the proportions of dryland and construction land had significant negative correlations with these three indexes. The significant correlations between the accuracy of SMAP, SMOS and ESA CCI SM products and environmental factors indicate that there exist systematic biases in these products, which can provide valuable insights into algorithm improvements.
Highlights
Soil moisture (SM) has essential impacts on the partition of energy and water over land surface, and it exerts a critical control on land–atmosphere interaction, hydrological and biogeochemical cycles (Brocca et al, 2016; Paloscia, Pettinato, & Santi, 2012)
The ranges of satellite-based SM products were larger than the interpolated in situ SM; they were 0.13–0.45 m3/m3, 0.09–0.47 m3/m3, 0.25–0.64 m3/m3 and 0.15–0.38 m3/m3 for Soil Moisture Active Passive (SMAP), Soil Moisture and Ocean Salinity (SMOS), European Space Agency’s Climate Change Initiative (ESA CCI) and the interpolated in situ values, respectively
Compared with SMAP and ESA CCI, the spatial distribution of SMOS SM was more fragmented in space, which may be because that SMOS suffered more significantly from radio frequency interference (Colliander et al, 2017)
Summary
Soil moisture (SM) has essential impacts on the partition of energy and water over land surface, and it exerts a critical control on land–atmosphere interaction, hydrological and biogeochemical cycles (Brocca et al, 2016; Paloscia, Pettinato, & Santi, 2012). The knowledge of surface SM is crucial for many studies, such as climate simulation, flood forecasting as well as land surface modeling (Brocca, Ciabatta, Massari, Camici, & Tarpanelli, 2017; Vittucci et al, 2013). During the past few decades, passive microwaves of different frequency (e.g. X, C and L bands) have been widely used to estimate SM (Kolassa, Gentine, Prigent, & Aires, 2016; Schmugge, Gloersen, Wilheit, & Geiger, 1974; Vittucci et al, 2013). Soil Moisture Active Passive (SMAP, 1.41 GHz) and Soil Moisture and Ocean Salinity (SMOS, 1.4 GHz) are currently available satellites carrying L-band microwave radiometers. Global surface SM products retrieved from the observations of SMAP and SMOS have been released to the public, which have great value for related researches and applications (Anam, Chishtie, Ghuffar, Qazi, & Shahid, 2017; Shellito et al, 2016)
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