Abstract
This study evaluates the accuracy of several recent remote sensing Surface Soil Moisture (SSM) products at sites in southwestern France. The products used are Soil Moisture Active Passive “SMAP” (level 3: 36 km × 36 km, level 3 enhanced: 9 km × 9 km, and Level 2 SMAP/Sentinel-1: 1 km × 1km), Advanced Scatterometer “ASCAT” (level 2 with three spatial resolution 25 km × 25 km, 12.5 km × 12.5 km, and 1 km × 1 km), Soil Moisture and Ocean Salinity “SMOS” (SMOS INRA-CESBIO “SMOS-IC”, SMOS Near-Real-Time “SMOS-NRT”, SMOS Centre Aval de Traitement des Données SMOS level 3 “SMOS-CATDS”, 25 km × 25 km) and Sentinel-1(S1) (25 km × 25 km, 9 km × 9 km, and 1 km × 1 km). The accuracy of SSM products was computed using in situ measurements of SSM observed at a depth of 5 cm. In situ measurements were obtained from the SMOSMANIA ThetaProbe (Time Domaine reflectometry) network (7 stations between 1 January 2016 and 30 June 2017) and additional field campaigns (near Montpellier city in France, between 1 January 2017 and 31 May 2017) in southwestern France. For our study sites, results showed that (i) the accuracy of the Level 2 SMAP/Sentinel-1 was lower than that of SMAP-36 km and SMAP-9 km; (ii) the SMAP-36 km and SMAP-9 km products provide more precise SSM estimates than SMOS products (SMOS-IC, SMOS-NRT, and SMOS-CATDS), mainly due to higher sensitivity of SMOS to RFI (Radio Frequency Interference) noise; and (iii) the accuracy of SMAP-36 km and SMAP-9 km products was similar to that of ASCAT (ASCAT-25 km, ASCAT-12.5 km and ASCAT-1 km) and S1 (S1-25 km, S1-9 km, and S1-1 km) products. The accuracy of SMAP, Sentinel-1 and ASCAT SSM products calculated using the average of statistics obtained on each site is defined by a bias of about −3.2 vol. %, RMSD (Root Mean Square Difference) about 7.6 vol. %, ubRMSD (unbiased Root Mean Square Difference)about 5.6 vol. %, and R coefficient about 0.57. For SMOS products, the station average bias, RMSD, ubRMSD, and R coefficient were about −10.6 vol. %, 12.7 vol. %, 5.9 vol. %, and 0.49, respectively.
Highlights
Understanding and simulating water cycle behavior allows forecasting of important natural phenomena such as drought, flood, climate change, and landslides [1]
The objective of this study is to evaluate the accuracy of SMAP, SMOS, ASCAT and S1 Surface Soil Moisture (SSM) products over the SMOSMANIA sites in southwestern France
Station average bias obtained with SMAP-36 km and SMAP-9 km products is about −4.5 vol % comparatively to about −10.5 vol % obtained with SMOS products (SMOS-IC, SMOS-NRT, and SMOS-Centre Aval de Traitement des Données SMOS (CATDS))
Summary
Understanding and simulating water cycle behavior allows forecasting of important natural phenomena such as drought, flood, climate change, and landslides [1]. Several satellite missions and instruments including SMOS (Soil Moisture and Ocean Salinity) [7,8], SMAP (Soil moisture Active and Passive) [9], and ASCAT (Advanced Scatterometer) [10] provide SSM estimates at coarse spatial resolution and very high revisit time (up to 1 day). SSM maps at very high spatial resolution (up to plot scale) and high revisit time (6 days) derived from Sentinel-1 (S1) satellite were provided for the Occitanie region in the southern part of France [11]. It is the first satellite dedicated to SSM retrievals over continental surfaces and Seas Surface Salinity (SSS) over the oceans using an L-band interferometric radiometer at 1.4 GHz. SMOS has a sun-synchronous orbit at 757 km altitude with a 06:00 LST (Local Solar Time) ascending equator crossing time and an 18:00 LST descending equator crossing time. Radio Frequency Interference (RFI) pollutes part of the SMOS data several regions of the globe are impacted [12]
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