Effects of Antecedent Soil Moisture On Vegetative Buffers Trapping Efficiency.

Edge-of-field vegetative buffers have been proposed as a means of reducing sediment and nutrient loadings in streams by minimizing cropland runoff. Buffers trapping efficiencies are usually reported as annual values, but little is known about the event efficiency. Unlike most of the previous research, this paper describes and analyzes runoff and sediment trapping efficiencies event by event during a three-year field study, collecting rainfall and snowmelt runoff. With this purpose, two sites were selected at different locations in Wisconsin to obtain differences of field size and shape, slope, soil texture, and agricultural practices. Site 1 had a 0.03-ha contributing area planted the first with corn and later with soybean, with rows up and down the slope and a second area in series planted in grass as a vegetative buffer. In this field, the slope varied from 0.5 to 9.0% (5% average slope). Site 2 had a 0.10-ha contributing area with a 10% average slope, planted in corn during the first year and with alfalfa later, with row direction across the slope on sandy loam soil. This site also had an area in series planted in grass as a buffer. At each site rainfall rate was recorded, and runoff and sediment were collected before and after the buffer with a multislot divisor system specially designed for this purpose. A total of 44 and 66 rainfall/snowmelt runoff events were recorded in Site 1 and 2 respectively.  At annual base, results show that buffers are an effective way to reduce the edge-of-field water pollution, but when the event efficiency is analyze, this is not so clear. Some of this situation is explained by the grass condition in the buffer, but most is controlled by the antecedent soil moisture condition. This paper discusses these relations and presents an overview and evaluation of this long-term study.