Abstract
The Soil Moisture Active Passive (SMAP) satellite makes coincident global measurements of soil moisture using an L-band radar instrument and an L-band radiometer. It is crucial to evaluate the errors in the newest L-band SMAP satellite-derived soil moisture products, before they are routinely used in scientific research and applications. This study represents the first evaluation of the SMAP radiometer soil moisture product over China. In this paper, a preliminary evaluation was performed using sparse in situ measurements from 655 China Meteorological Administration (CMA) monitoring stations between 1 April 2015 and 31 August 2016. The SMAP radiometer-derived soil moisture product was evaluated against two schemes of original soil moisture and the soil moisture anomaly in different geographical zones and land cover types. Four performance metrics, i.e., bias, root mean square error (RMSE), unbiased root mean square error (ubRMSE), and the correlation coefficient (R), were used in the accuracy evaluation. The results indicated that the SMAP radiometer-derived soil moisture product agreed relatively well with the in situ measurements, with ubRMSE values of 0.058 cm3·cm−3 and 0.039 cm3·cm−3 based on original data and anomaly data, respectively. The values of the SMAP radiometer-based soil moisture product were overestimated in wet areas, especially in the Southwest China, South China, Southeast China, East China, and Central China zones. The accuracies over croplands and in Northeast China were the worst. Soil moisture, surface roughness, and vegetation are crucial factors contributing to the error in the soil moisture product. Moreover, radio frequency interference contributes to the overestimation over the northern portion of the East China zone. This study provides guidelines for the application of the SMAP-derived soil moisture product in China and acts as a reference for improving the retrieval algorithm.
Highlights
Soil moisture, a primary state variable of hydrology, plays an important role in the linkage between terrestrial water, energy, and carbon cycles, which control a variety of the hydro-meteorological, hydro-climatic, and biogeochemical processes at various spatial and temporal scales [1,2,3,4]
Soil Moisture Active Passive (SMAP) soil moisture retrievals from 1 April 2015 to 31 August 2016
This paper made a preliminary evaluation of the SMAP radiometer soil moisture product using sparse in situ measurements to offer a realistic understanding of the errors associated with the L-band
Summary
A primary state variable of hydrology, plays an important role in the linkage between terrestrial water, energy, and carbon cycles, which control a variety of the hydro-meteorological, hydro-climatic, and biogeochemical processes at various spatial and temporal scales [1,2,3,4]. The conventional approach for detecting the soil moisture content involves relatively accurate ground measurements at a point scale using site networks [5,6,7,8], such as the United States Department of Agriculture (USDA) Soil Climate. 2017, 9, 292 regions, the availability of in situ measurement data is insufficient to accurately capture the spatial distribution of surface soil moisture at a continental or global scale. Due to the advantage of remote sensing observations, many efforts have been dedicated to determining the surface soil moisture for regional or even global applications [9,10,11,12,13,14]. Several soil moisture products have been derived from various operational satellite sensors with the required accuracy at different spatial resolutions (i.e., a few of kilometers), such as the Topical Rainfall Measuring (TRMM) Microwave Imager (TMI) [15], the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) aboard
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