Water quality response times to pasture management changes in small and large watersheds

Abstract

To interpret the effects of best management practices on water quality at a regional or large watershed scale, likely response times at various scales must be known. Therefore, four small (≤1 ha [≤2.5 ac]) watersheds, in rotational grazing studies at the North Appalachian Experimental Watershed near Coshocton, Ohio, were used to study management impacts on water quality and response times. Surface runoff was sampled on an event basis; groundwater discharge was sampled monthly from springs developed where a perching clay layer outcropped at the soil surface. In four large watersheds ranging from 18 to 123 ha (44 to 303 ac), base flow was over 50% of annual stream flow and approximately 20% of annual precipitation. Nitrate-N loads in base flow were 31% to 59% of total annual NO₃-N load in stream flow. When the N fertilization rate in a “medium fertility” area that contains two small watersheds was increased from 56 to 168 kg ha^-1 y^-1 (50 to 150 lb ac^-1 yr^-1), NO₃-N concentrations in groundwater discharge responded little in four years. Then NO₃-N levels in groundwater discharge increased for 10 years. With discontinuation of N fertilization, NO₃-N concentrations in groundwater discharge returned to pre-N increase levels after six years. In a “high fertility” grazing area with a similar perched water table, 224 kg N ha^-1 (200 lb ac^-1) was applied annually. Concentrations of NO₃-N increased to >10 mg L^-1 (ppm) after five years. Legumes were then interseeded into the grass forage, and mineral N fertilization was discontinued. Nitrate-N concentrations in groundwater discharge returned to their pre-fertilization levels after about five years. This multi-year response of groundwater discharge quality to management change in small watersheds indicates that the response time for measurable change in multi-square-mile watersheds will be equally long, if not longer, and trends will be muted.