Influence of flow concentration on parameter importance and prediction uncertainty of pesticide trapping by vegetative filter strips

SummaryFlow concentration is a key hydrologic factor limiting the effectiveness of vegetated filter strips (VFS) in removing pesticides from surface runoff. Numerical models, such as VFSMOD-W, offer a mechanistic approach for evaluating VFS effectiveness under various hydrological conditions including concentrated flow. This research hypothesizes that the presence of concentrated flow drastically alters the importance of various hydrological, sedimentological, and pesticide input factors and the prediction uncertainty of pesticide reduction. Using data from a VFS experimental field study investigating chlorpyrifos and atrazine transport, a two-step global sensitivity and uncertainty analysis framework was used with VFSMOD-W based on (1) a screening method (Morris) and (2) a variance-based method (extended Fourier Analysis Sensitivity Test, FAST). The vertical, saturated hydraulic conductivity was consistently the most important input factor for predicting infiltration, explaining 49% of total output variance for uniform sheet flow, but only 8% for concentrated flow. Sedimentation was governed by both hydrologic (vertical, saturated hydraulic conductivity and initial and saturated water content) and sediment characteristics (average particle diameter). The vertical, saturated hydraulic conductivity was the most important input factor for atrazine or chlorpyrifos trapping under uniform sheet flow (explained more than 46% of the total output variance) and concentrated flow (although only explained 8% of the total variance in this case). The 95% confidence intervals for atrazine and chlorpyrifos reduction ranged between 43% and 78% for uniform sheet flow and decreased to between 1% and 16% under concentrated flow. Concentrated flow increased interactions among the system components, enhancing the relative importance of processes that were latent under shallow flow conditions. This complex behavior warrants the need for process-based modeling to be able to predict the performance of VFS under a wide range of specific hydrological conditions.