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Well-managed grazing systems can provide valuable ecosystem services, such as reducing sediment and phosphorus (P) loading to nearby waterways. However, the available long-term data to fully support this hypothesis are limited. In this article, we describe flow-weighted concentrations (FWCs) and loads for dissolved reactive P (DRP), total P (TP), iron (Fe), and aluminum (Al) over 11 years (1999-2009) from a 7.8 ha rotationally grazed pasture (W1) near Watkinsville, Georgia. The region is characterized by Fe and Al rich and acidic Ultisols. Cattle numbering 21 to 224 (mean 91) grazed W1 on 69 occasions for 1 to 71 d (mean 19.2). Of 74 runoff events, 20 occurred when the monthly rainfall was below the long-term average (deficit period) and 54 occurred during non-deficit periods. Samples were collected from 43 of 74 runoff events for nutrient analyses. Event FWC (mg L-1) ranged from 0.38 to 7.07 for DRP (mean 1.91), from 0.36 to 7.60 for TP (mean 2.43), from 0.03 to 0.55 for Fe (mean 0.23), and from 0.43 to 553 μg L-1 for Al (mean 65 μg L-1). Event load (kg ha-1) ranged from 0.00 to 0.45 for DRP (mean 0.10), from 0.00 to 0.55 for TP (mean 0.12), from 0.00 to 0.11 for Fe (mean 0.02), and from 0.00 to 0.10 for Al (mean 0.01). The total load (kg ha-1) was 4.12 for DRP, 5.12 for TP, 0.71 for Fe, and 0.25 for Al. DRP accounted for 80% of the TP FWC and load. Cattle presence increased sediment load, but the difference was not statistically significant. There was high correlation between Fe and DRP loads (r = 0.87), a likely indicator of erosion-induced losses due to cattle treading. Cattle presence increased FWCs but not loads for DRP and TP. The FWCs for DRP and TP were not different between deficit and non-deficit periods, but mean loads were 3-fold to 4-fold greater during non-deficit periods. Means from the six largest P loss events were 3-fold greater for FWC and 7-fold greater for load than the remaining 37 events. These six large events accounted for 53% of the total P load. Less than 1% of the inorganic P applied and redeposited through manure was lost in runoff. The study demonstrated that hydrologic transport processes were the dominant drivers of pollutant fluxes and highlighted the possible mitigation of pollutant fluxes through grazing management that includes maintenance of good grass cover, effective rotational grazing, and limited fertilization. [ABSTRACT FROM AUTHOR]

The curve number (CN) method for estimating direct runoff from a rainfall event is commonly used in hydrologic investigations, but currently accepted CNs for no-till cropping might need revising. We derived CNs from ranked rainfall-runoff data gathered from 1972 to 2010 from a 2.7 ha, zero-order watershed (P1) near Athens, Georgia. The watershed was initially managed under conventional tillage for 2.5 years, followed by double-cropped continuous no-till rotations. During conventional tillage, on average 19% of the rainfall was partitioned into runoff (median 13%). The mean and median CN was 82 and 85, respectively. Least square fitting of CN versus rainfall produced an asymptotic CN of 81 (R² = 0.61). In contrast, the mean and median runoff during no-till was 7% and 0.6% of the rainfall, respectively. The mean CN equaled the median CN of 62. The asymptotic CN was 58 (R² = 0.78), compared with 72 from standard tables. When estimated using CN of 72, the mean runoff during no-till was 142% greater than the measured mean runoff. These results support the hypothesis that CNs for no-till cropping systems should be smaller than currently accepted values for comparable watersheds in the region. This has implications for hydrologic modeling for water quantity and quality where the use of the CN method is ubiquitous. [ABSTRACT FROM AUTHOR]

Widespread implementation of conservation tillage systems during cotton (Gossypium hirsutum L.) and peanut (Arachis hypogea L.) production in the Atlantic Coastal Plain region of the U.S. has substantially reduced erosion and sediment loss. However, the benefits of conservation tillage in these cropping systems are being threatened by weather shifts that include increased frequency of high-intensity rainfall, policies that encourage the removal of crop residues as cellulosic feedstocks for energy production, and increased herbicide resistance in weeds. Long-term integrated studies are needed to quantify potential impacts. We evaluated runoff and sediment loss from six 0.2 ha fields located on a gently sloping hillslope in a Southern Atlantic Coastal Plain landscape over ten years (2000-2009) during rotational cotton-peanut production, with a rye (Secale cereale) winter cover crop. Half of the fields were in conventional tillage (CT; inversion tillage with cover crop residue incorporation prior to planting), and half were in strip tillage (ST), which is a commonly used conservation tillage practice in the region. Fields were laid out as contrasting tillage pairs at upper, middle, and lower landscape positions and irrigated as needed. Runoff and sediment load from CT fields was significantly greater than from ST fields at each landscape position. Over the 10-year study period, mean annual total sediment load was 1823 kg ha-1 year-1 from CT fields and 237 kg ha-1 year-1 from ST fields (an 87% difference), and runoff was reduced by 41%. A soil tolerance value (T) of 2,200 kg ha-1 year-1 was exceeded in three out of ten years in CT fields and never exceeded in ST fields. On a monthly basis, extreme events (>90th percentile; 10%) accounted for 61% to 72% of the 10-year total sediment load for the CT system (341 to 2,828 kg ha-1 month-1) and 73% to 84% for the ST system (49 to 593 kg ha-1 month-1). The rainfall and irrigation total was above the normal monthly rainfall in 83% of these extreme monthly periods (mean 179 mm, range 81 to 316 mm). Sediment load was greatest in summer and spring, and during cotton production. The study results improve estimates of the increase in rainfall event size tolerated by CT vs. ST cropping systems, demonstrate the benefits of conservation tillage management during cotton and peanut production, and are expected to guide management decisions that focus on reducing sediment loss in this landscape. Specifically, the results point to the use of cover crops and strip tillage as essential best management practices for this purpose. [ABSTRACT FROM AUTHOR]