Assessment of Gilgai Microtopography Based On Elevation.

Gilgai microtopography associated with subsurface features developed in Vertisols affects spatial and temporal variability of soil cracking and re-distribution of water in a landscape. In the literature, it has been shown that gilgai microtopography can be mapped based on surface elevation data. Our objectives of this project were to 1) quantify gilgai microtopography of our study site, and 2) estimate water capturing capacity of gilgai depressions while cracks closed. The study site (45-m x 40-m) was located at USDA-NRCS Grassland Soil and Water Research Laboratory in Riesel, Texas. The field was grazed native prairie vegetation. The soil was Houston Black (Fine, smectitic, thermic Udic Haplusters) with circular gilgai on gently sloping landscape. The data set consisted of elevation data points measured by using survey quality GPS in continuous measurement mode (12/21/2010). A DEM of the study site was created with 0.25-m cell size using thin plate spline interpolation in ArcGIS. Slope, profile curvature and smoothing techniques were used to identify gilgai microhigh, microslope and microlow microtopography. To estimate volume of water capturing capacity of gilgai depressions (assuming no water infiltration), ArcScene was applied at 10 randomly selected local gilgai basins. Topographic Wetness Index (TWI) was calculated for each cell. Using TWI, a ridge-mask was determined to determine elevation values of pour points of each basin by applying zonal statistics. Our findings indicated that microhigh, microslope and microlow covered 46 %, 41 %, and 13 % of the study area, respectively. We estimated a rate of 0.0243 m3/m2 water capturing capacity of gilgai basins that could result in 43.74 m3 of rain and runoff water captured at the 1800 m2 study site assuming the same gilgai microtopography. Consequently, under the same conditions, no runoff would be expected following a rain of 24.3 mm/day intensity over a 1-km2 area.