Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 6, 2004
Publication Date: September 1, 2004
Citation: Cosh, M.H., Jackson, T.J., Bindlish, R., Prueger, J.H. 2004. Watershed scale temporal persistence of soil moisture and its role in validation satellite estimates. Remote Sensing of Environment. 92(4):427-435.
Interpretive Summary: Soil moisture at the surface is critical to the proper understanding and modeling of many hydrologic fluxes. Satellites have recently been launched to help monitor and estimate this moisture, howeveer, the validation of the products is necessary before the data can be used effectively. Large scale monitoring of soil moisture is not practical, so alternative methods of estimation are needed. One such method is to use the temporal stability of a region to identify representative locations. From these locations, smaller scale monitoring systems can be installed and used as an accurate estimate of the large scale parameter. This study looks at one such system installed south of Ames, Iowa and operated during the SMEX02 experiment. This watershed scale soil moisture network continued operation throughout the summer, thus extending the duration of the experiment at a fraction of the research expense.
Watershed scale soil moisture estimates are necessary to validate current remote sensing products, such as those from the Advanced Microwave Scanning Radiometer (AMSR). Unfortunately, remote sensing technology does not currently resolve the land surface at a scale that is easily observed with ground measurements. One approach to validation is to use existing soil moisture measurement networks and scale these point observations up to the resolution of remote sensing footprints. As part of the Soil Moisture Experiment 2002 (SMEX02), one such soil moisture gaging system in the Walnut Creek Watershed, Iowa, provided robust estimates of the soil moisture average for a watershed throughout the summer of 2002. Twelve in situ soil moisture probes were installed across the watershed. These probes recorded soil moisture at a depth of 5 cm from June 29th, 2002 to August 19th, 2002. The sampling sites were analyzed for temporal and spatial stability by several measures including mean relative difference, Spearman rank, and correlation coefficient analysis. Representative point measurements were used to estimate the watershed scale (~25 km) soil moisture average and shown to be accurate indicators with low variance and bias of the watershed scale soil moisture distribution. This work establishes the validity of this approach to provide watershed scale soil moisture estimates in this study region for the purposes of satellite validation with estimation errors as small at 3%. Also, the potential sources of error in this type of analysis are explored. This study is a first step in the implementation of large-scale soil moisture validation using existing networks such as the Soil Climate Analysis Network (SCAN) and several Agricultural Research Service watersheds as a basis for calibrating satellite soil moisture products.