Quality of Runoff From a Small Piedmont Pasture with Periods of Drought.

Approximately 11% of the Piedmont (1.8 million ha) is used as pastures and hay fields, mostly under low-input management. Animal manure can lead to enrichments of surface soils with nutrients leading to elevated water quality concerns.  We present 11 years (1999-2009) of hydrologic and water quality data from a 7.8 ha zero-order catchment with mix of bermudagrass over-seeded with winter annuals used as a rotational grazing pasture near Watkinsville in the Georgia Piedmont. Mean cattle days (CD = number of cattle x number of days) was least in December (75) and highest in Feb and Aug (1000-1400), and 350-950 for the remaining months. More cattle grazed the pasture in 2002 to 2004 (CD 11500-13000) than the rest of the years (3900-11500). Rainfall was below the 1250-mm annual average (166 to 397 mm) in 8 of the 11 years, and below the monthly normal in 65.3% of the months. Under these conditions, the statistics (mean ± standard error and range) for runoff causing rainfall, runoff and water quality were: Rainfall-mm (44.6 ± 2.9, 8-129); runoff-mm (5.3 ± 1.0, 0.01-41.1); percent runoff (8.7 ± 1.3, 0.01-65.5); flow weighted concentration (mg/L) - NO₃-N (1.1 ± 0.2, 0.04-4.9); NH₄-N (1.7 ± 0.7, 0.07-16.87); TN (4.4 ± 0.7, 0.72-15.63); TOC (8.9 ± 0.5, 2.74-15.8); PO₄-P (2.3 ± 0.2, 0.83-7.07); TP (2.8 ± 0.3, 0.78-7.59); Fe (0.2 ± 0.02, 0.02-0.55); and Al (0.02 ± 0.01, 0.00-0.17); nutrient load (kg/ha)- NO₃-N (0.05±0.01, 0.00-0.22); NH₄-N (0.05 ± 0.01, 0.00-0.29); TN (0.22 ± 0.06, 0.00-1.26); TOC (4.66 ± 4.18, 0.00-134); PO₄-P (0.13 ± 0.03, 0.00-0.64); TP (0.13 ± 0.03, 0.00-0.51); Fe (0.02 ± 0.02, 0.00-0.11); and Al (0.00 ± 0.00, 0.00-0.002). Only the P-release was, therefore, of concern, most of which must have come from the animal manure and not from management input.