Soil Water Regime in Space and Time in a Small Georgia Piedmont Catchment under Pasture

Soil water influences hydrological, biological, and biogeochemical processes that determine onand off-site response of landscapes under different agricultural uses. There are relatively little detailed spatial and temporal soil water measurements to validate current representations of spatial and temporal soil water variability. Soil water was measured over 3 yr at 12 sites to a 1.2-m depth in a 7.8-ha pasture catchment in the Georgia Piedmont in southeasternUSA. TheMahalanobis statistical difference was estimated between all pairs of measurement sites for soil water. Multidimensional scaling of the Mahalanobis differences showed that only a single statistical dimension was separating the observed soil water variations at measurement sites (r 5 0.99). This statistical dimension was then found to be most closely correlated with the depth to the Bt and the depth of the Ap horizons for explaining the observed variation in soil water between sampling sites (r5 0.69 and r5 0.8, respectively). Those sites where the Bt, an argillic horizon, was close to the surface, even when higher up the landscape, were generally wetter than those in which the Bt was deeper, even in the lower part of the landscape. The depth to the Bt horizon may serve as an indicator of the portions of the watershed most likely to be primary sources of runoff in association with the depth of overlying coarse-textured soil. Volumetric soil water contentwas generally greatest in winter (22 to 30% average) and least in summer (8 to 12% except when influenced by intense summer storms). To fully understand the soil water dynamics of Piedmont or similar landscapes, it is important to know the spatial distribution of the depth to the Bt horizon. Improved understanding of the soil water dynamics could lead to improved land use decisions, erosion control, andmanagement of water resources. This should be of interest to many researchers across many disciplines. UNDERSTANDING the spatial and temporal characteristics of soil water across landscapes is important for evaluation of the influence of agricultural land use on water resources. Interactions among landscape and soil water are apparent in the partitioning of precipitation to surface and subsurface water, plant water availability, heat and energy exchange between land surface and the atmosphere, and fate and transport of chemical and biological soil constituents (Loague, 1992; Western et al., 1998; Robock et al., 2000). Soil water varies spatially and temporally because of differences in soil properties, vegetation, topography, weather, and land use. Historically, relatively few detailed spatial and temporal soil water measurements have been made. As a result, spatial variability has been estimated using geostatistical techniques, deterministic models, and from remote sensing methods (O’Loughlin, 1986; Western et al., 1999a). Water balance models of different complexities have also been adapted for spatial representations. Validating the applicability of these approaches has been hindered by lack of suitable soil water data. Environmental problems have created a demand for reliable agro-hydrologic models (Beven, 1989; Grayson et al., 1992;Western et al., 1999a) that require testing with extensive datasets. According to the National Research Council (NRC, 1999), site-specific research and models will always be necessary for watershed management because regional variation in physical hydrology, ecology, and human impacts significantly affect the functioning of watersheds.Weiler andMcDonnell (2004)make the point that simply observing outflow from a catchment is a weak test of a model or process conceptualization. The objective of this research was to determine the variation of soil water by seasons and landscape position while testing formechanisms thatmight explain the variation observed in a small pastured (28 yr) catchment in the Georgia Piedmont of southeastern USA. The Georgia Piedmont is part of the 16.7 million ha (41 million acres) Southern Piedmont region. The Southern Piedmont is approximately 100 to 300 km wide, immediately east of the Appalachian Mountains, and extends from Alabama to Virginia (Radcliffe and West, 2000). Pastures have reduced acute problemsof erosion as they gradually replaced D.M. Endale, D.S. Fisher, and H.H. Schomberg, USDA-ARS, J. Phil Campbell Sr. Natural Resource Conservation Center, 1420 Experiment Station Road, Watkinsville, GA 30677. Received 4 Apr. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:1–13 (2006). Soil Physics and Soil & Water Management & Conservation doi:10.2136/sssaj2005.0106 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: MD, Mahalanobis distance; MDS, multidimensional scaling; TDR, time domain reflectometry. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1 Published online December 2, 2005