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Title: Water quantity and quality from a small Georgia Piedmont pasture during 1998-2009: Impact of drought

item Endale, Dinku
item Fisher, Dwight
item Jenkins, Michael
item Schomberg, Harry
item STEVENS, C - Environmental Protection Agency (EPA)

Submitted to: Georgia Water Resources Conference
Publication Type: Proceedings
Publication Acceptance Date: 3/10/2011
Publication Date: 4/11/2011
Citation: Endale, D.M., Fisher, D.S., Jenkins, M., Schomberg, H.H., Stevens, C.L. 2011. Water quantity and quality from a small Georgia Piedmont pasture during 1998-2009: Impact of drought. Georgia Water Resources Conference. CDROM.

Interpretive Summary: The Southern Piedmont of the United States has approximately 4.4 million acres (11%) of grasslands. This land resource is typically used for cow-calf beef production with limited inputs. Cattle manure can mobilize nutrients from grasslands and increase concerns for water quality. To effectively manage plant nutrients in grasslands we need datasets collected over long periods of time that encompass contrasting weather patterns. These types of data are lacking for the Southern Piedmont. The objective of our research is to relate 11 years of rainfall and runoff, and 10 years of nitrogen (N), phosphorus (P), total organic carbon (TOC), iron (Fe) and aluminum (Al) in runoff water from a small (19 acre) Georgia Piedmont pasture to rotational grazing by cattle during a period of highly variable weather. Our work was conducted from 1999 to 2009 at the USDA-ARS J. Phil Campbell Senior Natural Resource Conservation Center near Watkinsville, GA. During this time we utilized between 20 and 225 head of cattle and grazed the pasture 69 times for periods ranging from 1 to 71 days. Animals were grazing the pasture approximately a third of the time. Annual rainfall was 7 to 18 inches below the long-term average of 49 inches in 7 of the 11 years. Only 20 runoff events were recorded during 86 months of below average rainfall compared to 54 events during the 46 months when rainfall was at or above long-term means. Across runoff events we found the N concentration in runoff to be well below the maximum allowable drinking water concentration. However, the P concentrations were several orders of magnitude above the standard set to limit risk of eutrophication in surface water. The application rates of P fertilizer were limited and we did not use poultry litter as fertilizer. Consequently, the source for the P in runoff was likely cattle manure. Nutrient losses were 3 to 6 times greater during periods of average or above average rainfall than during periods of drought. We found that the estimated nutrient mass lost in runoff during the 11 years of study was approximately 0.23 lbs/ac/yr nitrate-N, 0.19 lbs/ac/yr ammonium-N, 1.13 lbs/ac/yr total N, 3.26 lbs/ac/yr total organic C, 0.68 lbs/ac/yr phosphate-P, 0.69 lbs/ac/yr total P, 0.10 lbs/ac/yr Fe, 0.04 lbs/ac/yr Al. The Environmental Protection Agency encourages refinement of water quality criteria by states and these results should be of interest to a large spectrum of stakeholders involved in water quality assessment, control and improvement.

Technical Abstract: The water quality impact of pasture grazing in the Piedmont, which generally occurs under low-input management, is not well studied. Cattle, hydrologic and water quality data were collected from 1999 to 2009 from a rotationally grazed 7.8-ha pasture near Watkinsville Georgia. Grazing occurred during 69 time periods, with 20 to 225 head of cattle grazing 1 to 71 days each period. Mean cattle days (head of cattle x days spent) was 182.4 ha-1 grazing-period-1. Drought occurred with 7 of the 11 years having below average annual rainfall. Runoff events were limited to 20 during 86 months of below average rainfall (deficit period) compared with 54 during 46 months of the non-deficit period. Instrument problems limited sample collection to 43-47 out of possible 67 events from 2000-2009. Across all data, mean event flow weighted concentration (FWC) in mg L-1 was < 1.0 for nitrate-nitrogen (NO3-N) and ammonium-nitrogen (NH4-N), 3.7 for total nitrogen (TN), 9.1 for total organic carbon (TOC), 2.0 for ortho-P (PO4-P), 2.4 for total P (TP), 0.23 for iron (Fe), and 0.06 for aluminum (Al). Nutrient loads in kg ha-1 event-1 averaged 0.04 for NO3-N, 0.03 for NH4-N, 0.19 for TN, 0.54 for TOC, 0.11 for PO4-P, and TP, 0.02 for Fe, and 0.01 for Al. Peak nutrient concentrations and loads occurred during calving season and/or when monthly rainfall was above average. Total load was 3 to 6 times greater from non-deficit than deficit periods. Concentrations of N were well below drinking water standards. Nevertheless observed N and P losses could pose risk of eutrophication because it can be stimulated at low concentrations. Such long-term data are needed to help states set or refine water quality standards.