Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/1998
Publication Date: N/A
Interpretive Summary: The effect of farming practices on water quality has become a great public concern over the past 10 years. There are many perceptions about the potential role of different farming practices on surface and ground water quality and the atmospheric transport of agricultural chemicals from farming to non-farming regions. Unfortunately, there is little information nthat has been collected from a number of farms within a watershed to bette understand the impact of farming on water quality. The watershed selected was the Walnut Creek watershed located just south of Ames, Iowa, in an area that is characterized by gently rolling fields and poorly drained soils. This study began in 1991 with the installation of a series of monitoring equipment to measure the flow of water in different segments of the water cycle and the amount of herbicides and nitrate-nitrogen being transported by the water. There are several different segments of this study to evaluate water quality because of the complexity of the problem. We found that farming practices have changed from 1991 through 1995 in terms of a reduction in corn herbicide use even though the area in corn has not increased. There has been a reduction in the amount of tillage that farmers do because of the changes in the planting equipment used for soybean production. Farming practices are dynamic and assumed to have a large role in nonpoint source pollution. These results define for the first time the complexities of farm practices within an area and provide a common basis for decision making by farmers and policy-makers.
Technical Abstract: Nonpoint source pollution has been linked to agricultural practices; however, there is a need for quantitative information describing the effect of specific farming practices on ground and surface water quality. Lack of information of the integrated effect of farming practices at the watershed scale limits our ability to make decisions about the effect of potential changes in either farming practices or landscape management that would enhance water quality. A multidisciplinary study was designed to evaluate the effect of farming practices on subsurface drainage, surface runoff, stream discharge, ground water, volatilization, and soil processes that influence water quality. Walnut Creek watershed is a 5130 ha intensively-cropped area in central Iowa on the Des Moines Lobe landform region. Soils within the watershed are in the Clarion-Nicollet-Webster soil association, and the underlying surficial material is glacial till. These soils are high in organic matter and have a high water holding capacity. Land use within the watershed is predominantly corn-soybean rotation. Fertilizer use, herbicide application, tillage practices, and crop selection were obtained through surveys of each field operator. Atrazine, cyanazine, EPTC, and metolachlor are the primary herbicides used within the watershed at rates similar to those for the state. Nitrogen fertilizer was applied as anhydrous ammonia on 60 percent of the corn fields at an average rate of 153 kg ha**-1 for the 1991-1994 period, but the frequency of corn fields receiving less than 112 kg ha**-1 has increased. Changes show that farming practices are continuing to change as farmers reduce herbicide, fertilizer, and tillage inputs.