Water quality in Walnut Creek watershed: Setting and farming practices

Nonpoint-source pollution has been linked to agricultural practices; howeve r, there is a need for quantitative information describing the effect of sp ecific farming practices on ground and surface water quality, Lack of infor mation at the watershed scale limits our ability to make decisions about th e effect of potential changes in either farming practices or landscape mana gement that would enhance water quality. A multidisciplinary study was desi gned to evaluate the effect of farming practices on subsurface drainage, su rface runoff, stream discharge, groundwater, volatilization, and soil proce sses that influence water quality. Walnut Creek watershed is a 5130-ha inte nsively cropped area in central Iowa on the Des Moines Lobe landform region . Soils within the watershed are in the Clarion-Nicollet-Webster (Typic Hap ludoll-Aquic Hapludoll-Typic Haplaquoll) soil association, and the underlyi ng surficial material is glacial till. Land use is predominantly corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation, Fertilizer use, herbici de application, tillage practices, and crop selection were obtained through surveys of each field operator, Atrazine [6-chloro-N-ethyl-N'-(1-methyleth yl)-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 carb amothioate], and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-meth oxy-1-methylethyl)acetamide] are the primary herbicides used within the wat ershed at rates similar to those for the state, Nitrogen fertilizer was app lied as anhydrous ammonia on 60% of the corn fields at an average rate of 1 53 kg ha(-1) for the 1991-1994 period, but the frequency of corn fields rec eiving <112 kg ha(-1) has increased.