Soil moisture is an important state variable in the hydrologic cycle, and its spatiotemporal distribution depends on many geophysical processes operating at different spatial and temporal scales. To achieve a better accounting of the water and energy budgets at the land-atmosphere boundary, it is necessary to better understand the spatiotemporal variability of soil moisture under different hydrologic and climatic conditions and at different hierarchical space scales and timescales. During the Southern Great Plains 1997 (SGP97) Hydrology Experiment the 0–6 cm soil water content was measured on consecutive afternoons at 400 locations in a small, gently sloping range field (Little Washita field site 07). The soil moisture measurements were made using portable impedance probes. Spatiotemporal data analyses of the two sampling events showed a significant change in the field variance but a constant field mean, suggesting moisture was redistributed by (differential) base flow, evapotranspiration, and condensation. Among the different relative landscape positions (hilltop, slope, valley) the slope was the largest contributor to the temporal variability of the soil moisture content. Using a sequential aggregation scheme, it was observed that the relative position influencing the field mean and variance changed between the two sampling events, indicating time instability in the spatial soil moisture data. Furthermore, high-resolution (impedance probe) sampling and limited (gravimetric) sampling gave different field means and variances.

Different factors contribute to soil moisture variability at different space scales 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×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 7×7 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.