ARS | Publication request: Surface and Profile Soil Moisture Spatial Analysis During an Excessive Rainfall Period in the Southern Great Plains

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 18, 2009
Publication Date: N/A

Interpretive Summary: Soil moisture is a key state variable in understanding and describing the dynamics of land surface hydrological processes involving a broad variety of applications in agriculture, meteorology, water resource management, and hydrologic modeling. Soil moisture is one of the most significant factors influencing infiltration and runoff dynamics, in addition to its role in the partitioning of sensible and latent heat transfer at the land-atmosphere interface. In order to better describe the processes involving soil moisture dynamics it is essential that we are able to characterize the spatial and temporal behavior of soil moisture at various scales and at different depths in the soil profile. The aim of this work was to analyze the temporal stability and spatial characteristics of soil moisture at watershed and field scales and to identify locations within the watershed, as well as within each field, that could be considered representative of the aerial mean soil moisture content. We also determined the relationship between sites found to be temporally stable for surface soil moisture versus those determined stable for average profile soil moisture content. Surface and profile soil water content were measured in four 64 ha fields during the remote sensing Cloud and Land Surface Interaction Campaign 2007 (CLASIC07) experiment in the Little Washita River Experimental Watershed (LWREW), in south-central Oklahoma. Also, soil moisture data from 20 LWREW measurement stations obtained during the experimental period were used for temporal stability analysis across the 61,000 ha watershed. For the unusually wet experimental period, results at the watershed scale show that LWREW two stations, 133 and 134, have very high non-zero temporal stability at all depths indicating that they could be used as representative watershed sites provided a constant offset value is used to acquire a watershed mean soil water content value. At the field scale, measured average profile soil moisture was higher in the winter wheat fields than the rangeland fields with the majority of the winter wheat depth intervals having high non-zero temporal stability. Field scale temporal stability analysis revealed that 4 of the 16 sampling sites in the rangeland fields and 3 of the 16 sampling sites in the winter wheat fields did not maintain stability at the 0-5 and 0-60 cm depth intervals. This finding is significant in terms of soil moisture ground-truth sampling for calibrating and validating airborne remotely sensed soil moisture products under extremely wet conditions. In addition, identification of temporal stable sites at the watershed and field scales in the LWREW provide insight in determining future measurement station locations and field scale ground sampling protocol, as well as providing data sets for hydrologic modeling.

Technical Abstract: In this work we analyze the temporal stability of soil moisture content across the 61,000 ha Little Washita River Experimental Watershed (LWREW) and at a field scale of 64 ha as part of the remote sensing Cloud and Land Surface Interaction Campaign (CLASIC07) during June 2007 in south-central Oklahoma. Temporal stability of surface and profile soil moisture data were investigated for 20 LWREW soil moisture measurement stations. In addition, daily surface and profile soil moisture measurements were obtained in four 800 m by 800 m fields (remote sensing footprint), including two rangeland sites and two winter wheat fields. The work aimed to analyze the temporal stability of soil moisture at the watershed and field scale and to identify stations within the watershed, as well as locations within each field, that were representative of the mean aerial soil moisture content. We also determined the relationship between sites found to be temporally stable for surface soil moisture versus those determined stable for average profile soil moisture content. For the unusually wet experimental period, results at the watershed scale show that LWREW stations 133 and 134 have very high non-zero temporal stability at all depths indicating that they could be used as representative watershed sites provided a constant offset value is used to acquire a watershed mean soil water content value. In general, the deeper depths exhibited higher soil moisture spatial variability, indicative of higher standard deviations. At the field scale, measured average profile soil moisture was higher in the winter wheat fields than the rangeland fields with the majority of the winter wheat depth intervals having high non-zero temporal stability. Field scale temporal stability analysis revealed that 4 of the 16 sampling sites in the rangeland fields and 3 of the 16 sampling sites in the winter wheat fields did not maintain stability at the 0-5 and 0-60 cm depth intervals. This finding is significant in terms of soil moisture ground-truth sampling for calibrating and validating airborne remotely sensed soil moisture products under extremely wet conditions. In addition, identification of temporal stable sites at the watershed and field scales in the LWREW provide insight in determining future measurement station locations and field scale ground sampling protocol, as well as providing data sets for hydrologic modeling.