Abstract
Abstract. Satellite-derived soil moisture provides more spatially and temporally extensive data than in situ observations. However, satellites can only measure water in the top few centimeters of the soil. Root zone soil moisture is more important, particularly in vegetated regions. Therefore estimates of root zone soil moisture must be inferred from near-surface soil moisture retrievals. The accuracy of this inference is contingent on the relationship between soil moisture in the near-surface and the soil moisture at greater depths. This study uses cross correlation analysis to quantify the association between near-surface and root zone soil moisture using in situ data from the United States Great Plains. Our analysis demonstrates that there is generally a strong relationship between near-surface (5–10 cm) and root zone (25–60 cm) soil moisture. An exponential decay filter is used to estimate root zone soil moisture using near-surface soil moisture derived from the Soil Moisture and Ocean Salinity (SMOS) satellite. Root zone soil moisture derived from SMOS surface retrievals is compared to in situ soil moisture observations in the United States Great Plains. The SMOS-based root zone soil moisture had a mean R2 of 0.57 and a mean Nash–Sutcliffe score of 0.61 based on 33 stations in Oklahoma. In Nebraska, the SMOS-based root zone soil moisture had a mean R2 of 0.24 and a mean Nash–Sutcliffe score of 0.22 based on 22 stations. Although the performance of the exponential filter method varies over space and time, we conclude that it is a useful approach for estimating root zone soil moisture from SMOS surface retrievals.
Highlights
Root zone soil moisture in vegetated regions has a significant influence on evapotranspiration rates (McPherson, 2007; Alfieri et al, 2008)
It is important to evaluate the degree of association between near-surface and root zone soil moisture when attempting to estimate root zone moisture using satellite retrievals
Albergel et al (2008) found that The primary coefficient of the exponential filter (Topt) varied strongly between stations in their study, using the overall average Topt based on all stations did not result in a significant decrease in model accuracy. We evaluated this finding in our study region by initializing the exponential filter model using three different Topt parameters: (1) the overall average Topt, which was 8 days for Oklahoma sites and 9 days for Nebraska sites, (2) site-specific Topt parameters, and (3) Topt based on the near-surface soil wetness index (SWI), ms(tn), conditions
Summary
Root zone soil moisture in vegetated regions has a significant influence on evapotranspiration rates (McPherson, 2007; Alfieri et al, 2008). Frye and Mote (2010) found that soil moisture and soil moisture gradients in the southern Great Plains significantly influence convective initiation under synoptic conditions not otherwise conducive to convection. Taylor et al (2012) found that afternoon convective precipitation in the Sahel region of Africa preferentially falls over dry soil, most likely due to enhanced sensible heat flux by anomalously low soil moisture. Despite the important role that soil moisture plays in the climate system (Legates et al, 2011), there are relatively few stations that measure soil moisture as compared to stations that measure temperature and precipitation. This impedes observation-based analyses of soil moisture–climate interactions
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