Surface Soil Moisture Retrieval Using the L-Band Synthetic Aperture Radar Onboard the Soil Moisture Active–Passive Satellite and Evaluation at Core Validation Sites

This paper evaluates the retrieval of soil moisture in the top 5-cm layer at 3-km spatial resolution using L-band dual-copolarized Soil Moisture Active-Passive (SMAP) synthetic aperture radar (SAR) data that mapped the globe every three days from mid-April to early July, 2015. Surface soil moisture retrievals using radar observations have been challenging in the past due to complicating factors of surface roughness and vegetation scattering. Here, physically based forward models of radar scattering for individual vegetation types are inverted using a time-series approach to retrieve soil moisture while correcting for the effects of static roughness and dynamic vegetation. Compared with the past studies in homogeneous field scales, this paper performs a stringent test with the satellite data in the presence of terrain slope, subpixel heterogeneity, and vegetation growth. The retrieval process also addresses any deficiencies in the forward model by removing any time-averaged bias between model and observations and by adjusting the strength of vegetation contributions. The retrievals are assessed at 14 core validation sites representing a wide range of global soil and vegetation conditions over grass, pasture, shrub, woody savanna, corn, wheat, and soybean fields. The predictions of the forward models used agree with SMAP measurements to within 0.5 dB unbiased-root-mean-square error (ubRMSE) and −0.05 dB (bias) for both copolarizations. Soil moisture retrievals have an accuracy of 0.052 m³/m³ ubRMSE, −0.015 m³/m³ bias, and a correlation of 0.50, compared to in situ measurements, thus meeting the accuracy target of 0.06 m³/m³ ubRMSE. The successful retrieval demonstrates the feasibility of a physically based time series retrieval with L-band SAR data for characterizing soil moisture over diverse conditions of soil moisture, surface roughness, and vegetation.

I. Introduction

Surface soil moisture retrievals using radar observations have been challenging in the past due to the complicating factors of surface roughness and vegetation. Vegetation changes may alter measured normalized backscattering coefficients ( by 5–10 dB (soybean and corn, see [1, Fig. 5]), a variability larger than the dynamic range of associated with soil moisture changes even at the L-band. In general, surface roughness (in terms of rms height) has a greater influence on as well than that of soil moisture (see [2, Figs. 5 and 6], [3, Fig. 4]). The correlation length of surface roughness also should be accounted for, although its contribution to ° is less significant than the rms height and vegetation effects. Despite these challenges, the radar-based retrievals are important mainly because of the high spatial resolution offered by synthetic aperture radars (SARs).