Abstract
The scale difference between point in situ soil moisture measurements and low resolution satellite products limits the quality of any validation efforts in heterogeneous regions. Cosmic Ray Neutron Probes (CRNP) could be an option to fill the scale gap between both systems, as they provide area-average soil moisture within a 150–250 m radius footprint. In this study, we evaluate differences and similarities between CRNP observations, and surface soil moisture products from the Advanced Microwave Scanning Radiometer 2 (AMSR2), the METOP-A/B Advanced Scatterometer (ASCAT), the Soil Moisture Active and Passive (SMAP), the Soil Moisture and Ocean Salinity (SMOS), as well as simulations from the Global Land Data Assimilation System Version 2 (GLDAS2). Six CRNPs located on five continents have been selected as test sites: the Rur catchment in Germany, the COSMOS sites in Arizona and California (USA), and Kenya, one CosmOz site in New South Wales (Australia), and a site in Karnataka (India). Standard validation scores as well as the Triple Collocation (TC) method identified SMAP to provide a high accuracy soil moisture product with low noise or uncertainties as compared to CRNPs. The potential of CRNPs for satellite soil moisture validation has been proven; however, biomass correction methods should be implemented to improve its application in regions with large vegetation dynamics.
Highlights
Soil moisture partitions the incoming rainfall into runoff and infiltration, and controls water and energy fluxes, as well as exchange of trace gases at the Earths’ surface [1,2,3,4]
Satellite surface soil moisture products from Advanced Microwave Scanning Radiometer 2 (AMSR2), Advanced Scatterometer (ASCAT), Soil Moisture Active and Passive (SMAP), and Soil Moisture and Ocean Salinity (SMOS), as well as the land data assimilation product Global Land Data Assimilation System Version 2 (GLDAS2) are analyzed in comparison to Cosmic Ray Neutron Probes (CRNP) over the six test regions
For the L-band systems SMAP and SMOS βY is consistently lower than 1 except for Karnataka, indicating that in those sites the dynamic scaling needs to reduce the dynamic range to match the range of the CRNP observations
Summary
Soil moisture partitions the incoming rainfall into runoff and infiltration, and controls water and energy fluxes, as well as exchange of trace gases at the Earths’ surface [1,2,3,4]. But moderate temporal resolution global surface soil moisture can be obtained by satellite remote sensing, mostly by microwave sensors [8,9]. The footprint of the CRNP typically has a radius between 150 m and 250 m, weakly dependent on soil moisture content [54] and depths between 0.1 m and 0.7 m, strongly dependent on soil moisture content [55] Given this large footprint area, the CRNP method is better suited for the validation of satellite based soil moisture products [56] because it integrates out small-scale spatial variations in soil moisture such as those in Figure 1 of Zreda et al [52]. In this study a more in-depth analysis and comparison of different currently available soil moisture products from SMOS, SMAP, AMSR-2, ASCAT and the Global Land Data Assimilation System Version 2 (GLDAS2) simulation with CRNP time series is performed to find out how useful cosmic-ray soil moisture data can be. In addition to standard validation scores the Triple Collocation method is performed to give more in-depths views on retrieval accuracies of the different sensors and their causes
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