Soil Moisture and Rainfall Intensity Thresholds for Runoff Generation In Southwestern Wisconsin Agricultural Basins.

Identifying time periods when land application of manure is likely to contribute to surface runoff contamination is important for making proper management decisions and reducing the risk of surface water contamination.  The goal of this study was to improve our understanding of the factors that influence runoff generation in agricultural watersheds during non-frozen ground periods.  Six small basins (ranging from 6 to 17 ha) within two southwestern Wisconsin farm sites were instrumented and surface runoff continuously monitored from 2004 to 2007 by the University of Wisconsin Discovery Farms Program (DFP) and the University of Wisconsin – Platteville Pioneer Farm (PF).  The soils in all basins were silt loam.  A direct-plant management strategy and corn-soybean crop rotation were utilized within basins at the first farm site (DFP).  The second farm site (PF) utilized a conventional tillage system (chisel plow in the fall followed by soil finisher in the spring) and a corn-oat-alfalfa crop rotation within the basins.  Tillage differences between the farm sites influenced the amount of surface runoff generated during the non-frozen ground period.  At PF, the amount of precipitation leaving the landscape as surface runoff (2%) was approximately two times greater compared to DFP (0.9%), indicating that the direct-plant management system was better at retaining precipitation than the chisel plow/soil finisher system.  An antecedent soil moisture (ASM) threshold of 0.39 cm³cm^‑3 for runoff generation was determined for all six basins.  Below this threshold, runoff coefficients (runoff depth divided by precipitation depth) were near zero.  Above this threshold, runoff coefficients increased with ASM.  Maximum 30 minute rainfall intensity (I30) thresholds for runoff generation increased as ASM decreased and as crop cover increased.  Avoiding manure application during time periods when soil moisture is near or above a critical soil moisture threshold would decrease the risk of surface water contamination.