Effects of gullies on space–time patterns of soil moisture in a semiarid grassland

Gullies are pervasive topographic features in many regions, and they can represent an important form of land degradation. Soil moisture is expected to play a significant role in the gullying process because it affects the health of the vegetation cover and the shear strength of the soil. However, the interaction of soil moisture with the gully topography remains poorly understood. The primary objectives of this study are to determine whether the development of a gully affects the spatial pattern of near-surface (0–10 cm) soil moisture and to determine whether the soil moisture dynamics differ between the gully bottoms, sidewalls, and uplands. Three study sites in southeastern Colorado were intensively monitored. One site is ungullied, while the other two sites each contain a gully. Hourly soil moisture observations were collected using time domain reflectometry probes installed along four transects at each site. Each transect contains 6–8 probes that are positioned at the mid-points between breakpoints in topographic slope. Overall, the occurrence of a gully was observed to have little impact on the spatial average soil moisture within the study sites, but it does promote spatial variability in soil moisture. Gully bottoms have lower wind speeds and thus lower reference evapotranspiration rates, and they tend to be wetter than the uplands. Gully sidewalls tend to be drier due to rapid drainage during and after precipitation events. Differences in the insolation of gully sidewalls are also associated with differences in their soil moisture.