Multi-scale temporal stability analysis of surface and subsurface soil moisture within the Upper Cedar Creek Watershed, Indiana

Soil moisture plays a significant role in determining the amount of energy exchange between the atmosphere and the earthʹs surface and is highly variable in space and time. Temporal stability analysis (TSA) is a statistical approach for describing the persistence of spatial patterns and characteristic behavior of soil moisture. Using TSA, this study is aimed at determining the adequacy of long term point-scale surface and subsurface soil moisture (θv) measurements in representing field and watershed scale averages that serve as in situ ground truth locations for remotely sensed soil moisture calibration and validation programs, as well as applications for hydrologic modeling. In two agricultural fields, twenty temporary frequency-domain reflectometry (FDR) soil moisture sensors, spaced 70 m apart, were installed at depths of 5 and 20 cm in each field with measurements transmitted every 30 min from June 29 through September 21, 2010. Soil moisture data were also obtained from FDR sensors permanently installed at depths of 5 and 20 cm at seven sites located within the USDA, Upper Cedar Creek Watershed (UCCW) monitoring network in northeastern Indiana. Additionally, meteorological data (i.e., rainfall, air temperature, humidity) were obtained from existing UCCW network weather stations. Spatiotemporal analysis revealed persistent patterns in surface soil moisture and identified sites that were temporally stable at both study scales. However, soil water patterns differed between preferred states (wet/dry) and were primarily controlled by lateral and vertical fluxes. At the field scale, locations that were optimal for estimating the area-average water contents were different from the permanent sensor locations. However, minimum offset values could be applied to the permanent sensor data to obtain representative field average values of surface θv. TSA of 20 cm θv showed little correlation with surface θv TSA results in terms of comparable stable sites or vertical transferability at either scale. The results are of relevance for the interpretation, scaling, or in describing the variability of coarser resolution soil moisture data such as that retrieved from remotely sensed active and passive microwave platforms and in terms of modeling field and watershed scale soil moisture based on point measurements.