Abstract
Understanding the effects of Earth’s surface topography on Global Navigation Satellite Systems Reflectometry (GNSS-R) space-borne data is important to calibrate experimental measurements, so as to provide accurate soil moisture content (SMC) retrievals. In this study, several scientific observables obtained from delay-Doppler maps (DDMs) ⟨ | Y r , t o p o ( τ , f ) | 2 ⟩ generated on board the Cyclone Global Navigation Satellite System (CyGNSS) mission were evaluated as a function of several topographic parameters derived from a digital elevation model (DEM). This assessment was performed as a function of Soil Moisture Active Passive (SMAP)-derived SMC at grazing angles θ e ~ [20,30] ° and in a nadir-looking configuration θ e ~ [80,90] °. Global scale results showed that the width of the trailing edge (TE) was small T E ~ [100, 250] m and the reflectivity was high Γ ~ [–10, –3] dB over flat areas with low topographic heterogeneity, because of an increasing coherence of Earth-reflected Global Positioning System (GPS) signals. However, the strong impact of several topographic features over areas with rough topography provided motivation to perform a parametric analysis. A specific target area with little vegetation, low small-scale surface roughness, and a wide variety of terrains in South Asia was selected. A significant influence of several topographic parameters i.e., surface slopes and curvatures was observed. This triggered our study of the sensitivity of T E and Γ to SMC and topographic wetness index ( T W I ). Regional scale results showed that T E and Γ are strongly correlated with the T W I , while the sensitivity to SMC was almost negligible. The Pearson correlation coefficients of T E and Γ with T W I are r Γ ~ 0.59 and r T E ~−0.63 at θ e ~ [20, 30] ° and r Γ ~ 0.48 and r T E ~ −0.50 at θ e ~ [80, 90] °, respectively.
Highlights
Soil moisture content (SMC) is an important component in the global water and energy balance because it determines the flux of water that infiltrates to groundwater or drains via surface water [1]
It was found that topographic roughness was a much more complex phenomenon, which depended on several topographic features in a differentiated manner
TPI, TRI, VRM, β, TC, and PC were derived from the ~250 m GMTED2010 digital elevation model (DEM)
Summary
Soil moisture content (SMC) is an important component in the global water and energy balance because it determines the flux of water that infiltrates to groundwater or drains via surface water [1]. The dependence of SMC on topographic convergence suggests that topography is an important element in the spatial distribution of SMC [2]. The dependence of SMC on topographic convergence [2] suggests that topography is an important factor that partially determines the spatial distribution of SMC. Wetlands mapping and spatial patterns of wetness can be generated in catchments under the assumption that groundwater tables follow topography holds. The coordinates of the selected target area were as follows: latitude = [5, 36] ◦ and longitude = [64, 94] ◦ The characteristics of this region enabled this study to be performed over a rich variety of terrains, including the Tibetan Plateau, Great Himalaya, Indo-Gangetic Plains, and Hindustan. Significant SMC levels could be found over mountains and over rivers basins
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