Abstract
Abstract. Past studies on soil moisture spatial variability have been mainly conducted at catchment scales where soil moisture is often sampled over a short time period; as a result, the observed soil moisture often exhibited smaller dynamic ranges, which prevented the complete revelation of soil moisture spatial variability as a function of mean soil moisture. In this study, spatial statistics (mean, spatial variability and skewness) of in situ soil moisture, modeled and satellite-retrieved soil moisture obtained in a warm season (198 days) were examined over three large climate regions in the US. The study found that spatial moments of in situ measurements strongly depend on climates, with distinct mean, spatial variability and skewness observed in each climate zone. In addition, an upward convex shape, which was revealed in several smaller scale studies, was observed for the relationship between spatial variability of in situ soil moisture and its spatial mean when statistics from dry, intermediate, and wet climates were combined. This upward convex shape was vaguely or partially observable in modeled and satellite-retrieved soil moisture estimates due to their smaller dynamic ranges. Despite different environmental controls on large-scale soil moisture spatial variability, the correlation between spatial variability and mean soil moisture remained similar to that observed at small scales, which is attributed to the boundedness of soil moisture. From the smaller support (effective area or volume represented by a measurement or estimate) to larger ones, soil moisture spatial variability decreased in each climate region. The scale dependency of spatial variability all followed the power law, but data with large supports showed stronger scale dependency than those with smaller supports. The scale dependency of soil moisture variability also varied with climates, which may be linked to the scale dependency of precipitation spatial variability. Influences of environmental controls on soil moisture spatial variability at large scales are discussed. The results of this study should be useful for diagnosing large scale soil moisture estimates and for improving the estimation of land surface processes.
Highlights
Spatial variability of soil moisture plays an important role in the estimation of land surface fluxes (evapo-transpiration (ET) and runoff), due to the non-linear relationship between soil moisture and the associated physical processes
Soil moisture of SCAN, Noah and Advanced Microwave Scanning RadiometerEOS (AMSR-E) was first calculated at each SCAN location and used to compute spatial statistics – mean, variability and skewness – in each region for each data set
Because AMSR-E retrievals are surface observations only, the statistical analysis was limited to the SCAN measurements at the 5cm depth and the top layer of Noah estimates
Summary
Spatial variability of soil moisture plays an important role in the estimation of land surface fluxes (evapo-transpiration (ET) and runoff), due to the non-linear relationship between soil moisture and the associated physical processes. Characterizing soil moisture spatial variability is important for validating satellite-derived soil moisture, as the number of soil samples needed to accurately represent the spatial mean within the footprint of satellites depends on soil moisture spatial variability (Brocca et al, 2012; Famiglietti et al, 2008; Wang et al, 2008). Since the footprint of satellite-derived soil moisture is often larger than the resolution of many hydrological applications, downscaling retrieved soil moisture is often necessary, which can be helped with knowledge of soil moisture spatial variability and its scale dependency. Various conclusions have been reached about this relationship
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