|Lerch, Robert - Bob|
Submitted to: USDA-CSREES National Water Quality Conference
Publication Type: Abstract Only
Publication Acceptance Date: 1/15/2010
Publication Date: 2/21/2010
Citation: Shea, P.J., Milner, M., Willett, C.D., Lerch, R.N., Bernards, M.L., Barnes, P.L., Dhakal, K. 2010. Accounting for the Impact of Impermeable Soil Layers on Pesticide Runoff and Leaching in a Landscape Vulnerability Model. In: 2010 Land Grant & Sea Grant National Water Conference, February 21-25, 2010, Hilton Head, South Carolina. Available: http://www.usawaterquality.org/conferences/2010/PDF's/Shea.pdf Interpretive Summary:
Technical Abstract: A regional-scale model that estimates landscape vulnerability of pesticide leaching and runoff (solution and particle adsorbed) underestimated runoff vulnerability and overestimated leaching vulnerability compared to measured data when applied to a gently rolling landscape in northeast Missouri. Many of the soils in this watershed have naturally occurring thick clay layers (clay pans) at or near the soil surface that limit infiltration and/or soil water storage capacity. Because of this, runoff events are common whenever the soil profile above the claypan is saturated prior to a rain event (capacity limited), or when the rain event is heavy and intense (rate limited). In addition, subsurface lateral flow along the top of the clay pan that subsequently emerges from seeps or along stream banks may prolong elevated pesticide levels in stream flows following a runoff event. In our model we use NRCS’s Index of Surface Runoff (ISRO) as a component to identify leaching and runoff vulnerable areas in the landscape. ISRO is derived from slope and the soil’s minimum saturated conductivity (Ksat) and is modified based on depth to free water. To better account for reduced water storage capacity due to restrictive clay layers in the Missouri watershed, we replaced the free water ISRO adjustments with a function based on available water storage capacity above a restrictive layer. However, claypans are not identified in the SSURGO (Soil Survey Geographic) data base. Consequently, we developed a claypan definition for the model based on a clay intensity function for the Bt horizon(s). The depth to restrictive layer adjustment could also be applied to soils with fragic, cemented or other restrictive horizons. When the modification was added to the model, it better represented the expected runoff vulnerability relative to leaching vulnerability in a claypan watershed.