Water quality in Walnut Creek Watershed: setting and farming practices

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 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, groundwater, 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 (Typic Hapludoll-Aquic Hapludoll-Typic Haplaquoll) soil association, and the underlying surficial material is glacial till. Land use is predominantly corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation. Fertilizer use, herbicide application, tillage practices, and crop selection were obtained through surveys of each field operator. Atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine- 2,4-diamine], cyanazine [2-[[4-chloro-6-(ethylamino)- 1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile], EPTC [S-ethyl dipropyl carbamothioate], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1- methylethyl)acetamide] 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% 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