Comparisons of In Situ Ground Observations and Satellite Measurements of Soil Moisture Standardized Using a Consistent Method

Abstract:

The United States Climate Reference Network (USCRN) soil moisture measurements have taken place at 114 locations across the conterminous U.S. since installation during 2009-2011. Each station site is relatively open, but may have a variety of confounding factors for satellite determination of soil moisture in the surrounding area, including deciduous and evergreen forests, wetlands, lakes, and oceans. In this study, Copernicus gridded soil moisture data (1978-present) are compared to USCRN in situ soil moisture measurements in raw volumetric form. The figure in this abstract shows the mean difference in daily values of volumetric soil moisture for the 2009-2019 period, when both data sets are present. Many of the patterns are understood in terms of forest interference in the West and Southeast, but some differences are less well understood, such as in the relatively open landscapes of the central U.S. region. This may result from the use of modeled data to scale the satellite-based inputs. The scaling also creates lower and upper bounds for the satellite soil moisture data that vary by location.

The objective of this study is to apply a novel method to standardize both the USCRN in situ measurements and satellite measurements independently, and then ascertain the degree of agreement between the standardized measurements. The methodology subtracts the local median and divides by the interquartile range, both of which are from a climatology derived for a moving window of 31 days centered on the target day. This approach generates a stable climatology for a brief 9-11 year window of observations. Preliminary results are promising and full results will be presented.