Abstract
In this study, over 36,000 ground‐based soil moisture measurements collected during the SGP97, SGP99, SMEX02, and SMEX03 field campaigns were analyzed to characterize the behavior of soil moisture variability across scales. The field campaigns were conducted in Oklahoma and Iowa in the central USA. The Oklahoma study region is sub‐humid with moderately rolling topography, while the Iowa study region is humid with low‐relief topography. The relationship of soil moisture standard deviation, skewness and the coefficient of variation versus mean moisture content was explored at six distinct extent scales, ranging from 2.5 m to 50 km. Results showed that variability generally increases with extent scale. The standard deviation increased from 0.036 cm3/cm3 at the 2.5‐m scale to 0.071 cm3/cm3 at the 50‐km scale. The log standard deviation of soil moisture increased linearly with the log extent scale, from 16 m to 1.6 km, indicative of fractal scaling. The soil moisture standard deviation versus mean moisture content exhibited a convex upward relationship at the 800‐m and 50‐km scales, with maximum values at mean moisture contents of roughly 0.17 cm3/cm3 and 0.19 cm3/cm3, respectively. An empirical model derived from the observed behavior of soil moisture variability was used to estimate uncertainty in the mean moisture content for a fixed number of samples at the 800‐m and 50‐km scales, as well as the number of ground‐truth samples needed to achieve 0.05 cm3/cm3 and 0.03 cm3/cm3 accuracies. The empirical relationships can also be used to parameterize surface soil moisture variations in land surface and hydrological models across a range of scales. To our knowledge, this is the first study to document the behavior of soil moisture variability over this range of extent scales using ground‐based measurements. Our results will contribute not only to efficient and reliable satellite validation, but also to better utilization of remotely sensed soil moisture products for enhanced modeling and prediction.
Highlights
[2] Numerous studies have suggested that the realistic representation of spatial variability of surface soil moisture content can improve the predictive skill of hydrologic, weather prediction, and general circulation models, including processes such as evapotranspiration and runoff [Famiglietti and Wood, 1994], precipitation [Koster et al, 2000], and atmospheric variability [Delworth and Manabe, 1993]
[9] we review previous studies of soil moisture spatial variability, and in particular, those dealing with variability in relation to mean moisture content and spatial scale
[14] The ground-based measurements of surface moisture content used in this study were obtained during the Southern Great Plains Hydrology Experiment (SGP97), Southern Great Plains Hydrology Experiment (SGP99), Soil Moisture Experiment in 2002 (SMEX02) and Soil Moisture Experiments in 2003 (SMEX03) campaigns
Summary
[2] Numerous studies have suggested that the realistic representation of spatial variability of surface soil moisture content can improve the predictive skill of hydrologic, weather prediction, and general circulation models, including processes such as evapotranspiration and runoff [Famiglietti and Wood, 1994], precipitation [Koster et al, 2000], and atmospheric variability [Delworth and Manabe, 1993]. The footprint scale ranges from a few hundred meters for an airborne radiometer, to about 60-km for a spaceborne radiometer For this reason, a large number of distributed ground-based samples is needed to accurately estimate the mean soil moisture content within a remotely sensed foot-. [7] During the Southern Great Plains (SGP) Hydrology Experiments of 1997 and 1999, and the Soil Moisture Experiments (SMEX) of 2002 and 2003, intensive sampling was conducted at both field- and regional-scales for the purpose of validating airborne and spaceborne remotely sensed soil moisture products. Implications of this study for satellite validation and subgrid-scale parameterization of surface soil moisture are discussed in the final section To our knowledge, this is the first study to document the behavior of soil moisture variability over this range of extent scales using ground-based measurements
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