Abstract
In this study, we address the variations of bare soil surface microwave brightness temperatures and evaluate the performance of a dielectric mixing model over the desert of Kuwait. We use data collected in a field survey and data obtained from NASA Soil Moisture Active Passive (SMAP), European Space Agency Soil Moisture and Ocean Salinity (SMOS), Advanced Microwave Scanning Radiometer 2 (AMSR2), and Special Sensor Microwave/Imager (SSM/I). In situ measurements are collected during two intensive field campaigns over bare, flat, and homogeneous soil terrains in the desert of Kuwait. Despite the prevailing dry desert environment, a large range of soil moisture values was monitored, due to precedent rain events and subsequent dry down. The mean relative difference (MRD) is within the range of ±0.005 m3·m−3 during the two sampling days. This reflects consistency of soil moisture in space and time. As predicted by the model, the higher frequency channels (18 to 19 GHz) demonstrate reduced sensitivity to surface soil moisture even in the absence of vegetation, topography and heterogeneity. In the 6.9 to 10.7 GHz range, only the horizontal polarization is sensitive to surface soil moisture. Instead, at the frequency of 1.4 GHz, both polarizations are sensitive to soil moisture and span a large dynamic range as predicted by the model. The error statistics of the difference between observed satellite brightness temperature (Tb) (excluding SMOS data due to radio frequency interference, RFI) and simulated brightness temperatures (Tbs) show values of Root Mean Square Deviation (RMSD) of 5.05 K at vertical polarization and 4.88 K at horizontal polarization. Such error could be due to the performance of the dielectric mixing model, soil moisture sampling depth and the impact of parametrization of effective temperature and roughness.
Highlights
Soil moisture is an important parameter for hydrological, climate, and weather model predictions
The objective of this paper is to assess microwave brightness temperatures simulated at specific channels coincident with those onboard Soil Moisture Active Passive (SMAP), Advanced Microwave Scanning Radiometer 2 (AMSR2), Soil Moisture and Ocean Salinity (SMOS), and Sensor Microwave/Imager (SSM/I) in microwave frequencies ranging from 1.4 GHz to 19 GHz in the Kuwait desert environment
The bare soil test site of 36 km × 36 km in the open arid area of Kuwait exhibited dynamics of soil moisture that ranged between 0.01 m3·m−3 and 0.1 m3·m−3
Summary
Soil moisture is an important parameter for hydrological, climate, and weather model predictions. Low frequency microwave instruments are used to remotely sense and map soil moisture. The spatial and temporal variations of soil moisture from in situ measurements are important for the validation of satellite measurements. Temimi et al [3] analyzed the diurnal variability of L-band brightness temperature and its impact on soil moisture retrievals in upstate New York, USA. According to Schneider et al [7] and Ryu et al [8], temporal stability is important to validate hydrological, remote sensing models, and to upscale soil moisture information to larger scales. Several studies investigate the retrieval and the validation of soil moisture at low microwave frequency L-band (1.4 GHz) from the SMAP satellite [9,10,11]. Attempts to retrieve soil moisture from higher frequencies utilized by such as the SSM/I and AMSR2 satellites were conducted and investigated in several studies [14,15,16,17,18,19]
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