Soil water regime in space and time in a small Georgia Piedmont catchment under pasture

Authors: Endale, Dinku; Fisher, Dwight; Schomberg, Harry

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2005
Publication Date: 12/2/2005
Citation: Endale, D.M., Fisher, D.S., Schomberg, H.H. 2005. Soil water regime in space and time in a small Georgia Piedmont catchment under pasture. Soil Science Society of America Journal. 70:1-13.

Interpretive Summary: Despite the importance of soil water information(water held in the soil)for private citizens and private and government agencies involved in agriculture and water resource areas such as weather forecasting, climate analysis, runoff, flood, erosion or water quality control, there is very little measurement of soil water done regularly or continuously. Knowledge and understanding of how soil water varies over time and across landscapes is of fundamental importance to understanding many processes that impact our environment. Scientists at the USDA-ARS J. Phil Campbell Sr., Natural Resource Conservation Center measured soil water over 3 years in a 19 acre pasture near Watkinsville, GA in the Georgia Piedmont. The depth where the highest clay concentration in the subsoil (Bt horizon) begins controlled the soil water content of the soil above in a major way by restricting water movement through this zone. Soils higher up in the landscape where the Bt was closer to the surface due to historical erosion proved generally wetter than soils lower in the landscape where the Bt occurred deeper in the soil profile. This differentiation of high and low soil water content zones was observed also just prior to runoff events, suggesting that the upper landscape was a likely zone for runoff generation because the soil was more saturated before a rainfall event. Researchers have begun to seriously study this 'variable source area' concept of runoff generation across landscapes in recent times. Over 86% of the Piedmont, a 41 million acre region extending from Virginia to Alabama in an approximately 60 to 190 mile wide zone east of the Appalachian Mountains, is classified as eroded and would be a candidate for identification of the location of the Bt horizon. This could lead to better estimation of the distribution of soil water across the landscape and assist action agencies and citizens toward improved land use decisions, better water resources management for improved water quality, and better control of erosion and floods.

Technical Abstract: Soil water influences hydrological, biological and biogeochemical processes that determine on- and off-site response of landscapes from differing agricultural use in a major way. There are relatively little detailed spatial and temporal soil water measurements to validate the several approaches of representing spatial and temporal soil water variability. We measured soil water over 3 years at 12 locations to 1.2 m depth in a 7.8 ha watershed in the Georgia Piedmont in long-term pasture use. A Maholanobis distance and multidimensional scaling (MDS) analysis showed that a single dimension would explain the Maholanobis distances between measurement sites (r = 0.99). The depth to the Bt horizon then factored as a primary variable that could explain the variation of soil water across measurement sites (r = 0.69). We found that those sites where the Bt was close to the surface, up the landscape, were generally wetter than those sites where the Bt was deeper, which happen to lie in the lower part of the watershed. We conclude from soil water data prior to runoff events that those sites where the Bt is close to the surface could be primary sources of runoff generation. Generally soil water content was greatest in winter (22 to 30 % average) and least in summer (8 to 30% - higher value periodic and associated with summer precipitation). Transitional periods occur in spring and fall. In order to fully understand the soil water dynamics of Piedmont landscapes, it is important to know the spatial distribution of the Bt horizon. Eroded landscapes are an integral part of the Piedmont and the Bt could lie at or near the surface or deeper. Improved understanding of the soil water dynamics could lead to improved land use decisions, better management of water resources, improved erosion and flood controls, as well as aid in weather forecasting and climate analysis.