Tuesday, November 14, 2006 - 10:00 AM

Source Water Effects on Runoff Composition under Simulated Rainfall.

Nancy Lea Bohl1, Christopher Baxter2, Todd Andraski1, and Larry Bundy3. (1) Univ Wisconsin, Madison, 1107 150th Ave, Lu Verne, IA 50560, (2) University of Wisconsin-Platteville, UW Platteville, 1 University Plaza Tower 208, Platteville, WI 53818-3001, (3) Univ Wisconsin, Madison, Dept of Soil Science, 1525 Observatory Dr, Madison, WI 53706-1299

Rainfall simulations are used to study phosphorus (P) and soil loss in cropping systems.  Frequently, well water is used instead of rainwater to perform these simulations.  This study was conducted to determine whether the source water used in simulated rainfall studies affects runoff amount and composition.  Rainfall simulation studies were conducted in southwest Wisconsin in a corn [Zea mays(L.)] and alfalfa field [Medicago sativa (L.)] on a Tama silt loam (fine-silty, mixed, superactive, mesic Typic Argiudoll) in spring and fall 2004.  No significant differences in runoff composition were found between the well and deionized (DI) source waters in the spring rainfall simulation study in corn.  The fall rain simulation in the alfalfa showed significantly higher runoff volumes and dissolved P (DP) concentrations with DI water than with well water.  Laboratory experiments to determine water extractable P (WEP) in soil were conducted comparing DI, rain, and well water at two soil to water ratios (1:10 and 1:50).  Well water extracted less P than DI water and increasing the soil to water ratio from to 1:50 decreased the P concentration in the extracting solution.  The rain source water extracted similar amounts of P as the DI source water.  Using the same source water in laboratory WEP procedures and in rainfall simulations yielded good relationships between simulated rain runoff and soil extract DP concentrations.  Natural runoff P concentrations measured in the same treatments as those used in the simulated rainfall studies were much greater than those found in any simulated rainfall runoff, but the relative treatment effects on runoff P concentrations were the same.  These results emphasize that simulated rainfall data can provide a relative comparison of treatment effects on runoff composition, but results from simulated rainfall experiments are not likely to duplicate natural runoff composition, regardless of the source water used.