Evolution of soil moisture spatial structure in a mixed vegetation pixel during the Southern Great Plains 1997 (SGP97) Hydrology Experiment

Different factors contribute to soil moisture variability at different space scares and timescales, including soil properties, topography, vegetation, land management, and atmospheric forcings, such as precipitation and temperature. Field experiments supported by adaptive geostatistical and exploratory analysis, including categorical elimination of different governing factors, are needed to bring new insight to this important hydrologic problem. During the Southern Great Plains 1997 (SGP97) Hydrology Experiment in Oklahoma, we investigated the within-season (intraseasonal) spatiotemporal variability of surface (0-6 cm depth) soil moisture in a quarter section (800 m x 800 m) possessing relatively uniform topography and soil texture but variable land cover. Daily soil moisture measurements were made between June 22 and July 16 using portable impedance probes in a regular 7 x 7 square grid with 100-m spacings. Initially, the land cover was split between grass and wheat stubble; row tilling on June 27 converted the wheat stubble to bare ground. Geostatistical and median polishing schemes were used to analyze the within-season evolution of the spatial structure of soil moisture. The effects of daily precipitation, variable land cover, land management, vegetation growth, and microheterogeneity including subgrid-scale variability were all visible in the analysis. Isotropic spatial correlation range for soil moisture varied between < 100 m (for nugget and subgrid-scale variability) and > 428 m (for spherical and Gaussian models) within the 4-week-long SGP97 experiment. The findings will be useful for assessing remotely sensed soil moisture data collected;during the SGP97 Hydrology Experiment in mixed vegetation pixels.