Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2005
Publication Date: 1/10/2006
Citation: Endale, D.M., Fisher, D.S., Steiner, J.L. 2006. Hydrology of a zero-order Southern Piedmont watershed through 45 years of changing agricultureal land use. Part 1. Monthly and seasonal rainfall-runoff relationships. Journal of Hydrology. 316:1-12. Interpretive Summary: The effectiveness of various agricultural land uses for reducing runoff needs to be determined at field and watershed scales to identify those practices that protect the land and its resources. Runoff-induced soil erosion is a serious and costly problem in the Southern Piedmont, a 41.2 million acres region that extends from Virginia to Alabama. Few studies have recorded runoff from small agricultural watersheds over sufficiently long periods to allow such determinations. The USDA Agricultural Research Service, J. Phil Campbell Sr. Natural Resource Conservation Center has been monitoring rainfall and runoff from a 19.2 acres watershed since 1940 near Watkinsville, GA, in the Southern Piedmont. Four different land uses were evaluated for runoff generation potential: row cropping, kudzu, grazed kudzu mixed with rescuegrass, and grazed bermudagrass. The data from 45 years of research showed that row cropping produced the greatest runoff and peak flow rates. The other land uses provided the soil protective cover throughout the year and reduced runoff and peak flow rates significantly, especially from highly intense spring and summer storms. These long-term data demonstrate that the best way to protect Southern Piedmont farmlands is to have vegetative cover all year around. The information is useful for landowners, local, state and federal resource conservation agents, and for citizens concerned about degradation of our natural resources.
Technical Abstract: Few studies have recorded runoff from small agricultural watersheds over sufficiently long time so that the effect of different cover types on runoff can be explored. The goal of this paper is to quantify, based on long-term hydrologic monitoring, seasonal monthly runoff from a small agricultural watershed in response to temporal variation in monthly rainfall and agricultural land use, and develop predictive capabilities for potential use in land management and conservation measure decision making. We analyzed 45-yrs of monthly, seasonal and annual rainfall-runoff characteristics of a small (7.8 ha) typical zero-order Southern Piedmont watershed in the southeastern United States (US) that is part of the US Dept. of Agriculture, Agricultural Research Service network watersheds. Agricultural land use varied as follows: 1. Row cropping (5-yr); 2. Kudzu (Pueraria lobata; 5-yr); 3. Grazed kudzu mixed with rescuegrass (KR)(Bromus catharticus; 7-yr); and 4. Grazed bermudagrass (Cynodon dactylon; 28-yr). Agricultural land use and rainfall variability influenced runoff characteristics. Row cropping produced the greatest runoff amount, percentage of the rainfall partitioned into runoff, and peak flow rates. Kudzu reduced spring runoff amount and almost eliminated summer runoff amount, as did a mixture of kudzu and rescuegrass compared to the crop-phase. Winter runoff amount increased slightly under kudzu. Peak flow rates were also reduced during the kudzu and KR phases. Runoff amount under bermudagrass was lower than during row cropping. Peak flow rates increased under bermudagrass but were lower than during row cropping. A single, two parameter exponential model fitted the data best, which was able to correlate monthly runoff with monthly rainfall with coefficient of determination (R2) of 0.6 or better for 12 of 16 agricultural land use-season pairs. The model was then tested on independent data and was found to over predict runoff during drought periods. Sustained hydrologic monitoring is essential to understanding long-term rainfall-runoff relationships in agricultural watersheds.