A Geophysical Study Of The Impact Of Agricultural Management Practices On The Hydrologic Properties Of A Texas Vertisol

We integrate geoelectrical measurements with in-situ soil-moisture determination using core sampling to assess changes in vadose zone properties induced by agricultural management practices in a Texas Vertisol. Two sites were established at the USDA research station, Riesel, Texas, where the underlying soils are cultivated soil for crop production and a soil in grass pasture. Center spread multielectrode resistivity data were collected on each site along the same profiles on February 22, May 7, and September 12, 2007. Data analysis involved inversion using a rapid least-square technique and temperature corrections. The results were calibrated against the in-situ soil-moisture values to generate apparent soil-moisture sections. Using the van Genuchten pedotransfer function, we used the apparent soil-moisture data to compute apparent distribution of pressure heads in the soil. The results for successive sections were then related to compute apparent soil-moisture flux over the intervals between each field survey. Significant variations in resistivity and soil-moisture dynamics between the two sites were observed in the upper 2 m of the soil, especially within the root zone. We show that, within the limitation posed by the problem of equivalence in data interpretation, management-induced changes in soil hydrologic properties can be characterized using the electrical resistivity method. Introduction Land management practices impact soils’ physical and chemical properties. Changes in physical properties (such as porosity and bulk density) manifest as changes in percolation rates and rooting depths of plants, and variations in depths to the water table (among others). Quantifying the effects of these practices is a major component of many ongoing conservation programs, e.g. Total Maximum Daily Load (TMDL) model (Torbert et al. 2005; Harmel et al. 2004). In the USDA Grassland Soil and Water Research Station (GSWRS), Riesel, Texas (Figure 1), hydrologic data have been collected since 1937. These data have been valuable in analyzing hydrologic properties of the soils for evaluating the effects of various management practices at the site (Torbert et al. 2005; Harmel et al. 2004, Harmel et al., 2006). For example, Harmel et al. (2006) used a comprehensive analysis of these data to establish a link between soil loss and erosion rate, and agricultural land use in the area. The authors also made quantitative comparisons of the earlier (pre-1964) and relatively recent (1982-2002) hydrologic data. The results show that conservation practices in recent times have helped minimize the impact of cultivated agriculture on the hydrologic properties of the soil. Geophysical surveying offer a potential alternative for quantifying the impact of agricultural land use on soils. Electrical-resistivity technique in particular can be useful in collecting spatial and temporal data to complement results of hydrological models. This method has the advantage that surveys can be carried out in 2D and 3D, and at various scales, which may be as small as centimetric scales (Samouelian et al. 2003). Besson et al. (2004) used this method to analyze structural heterogeneity of a tilled soil layer. The authors correlated resistivity distribution to variation in bulk density in the soil.