Modeling Water and Sediment Trapping by Vegetated Filters Using VFSMOD: Comparing Methods for Estimating Infiltration Parameters

The vegetated filter strip model (VFSMOD) was used to investigate the effect of GreenAmpt infiltration parameters (found with different estimation techniques) on sediment and water trapping in vegetated filters of varying soil types. Field-measured and empirically-estimated infiltration parameters were compared. Field saturated hydraulic conductivity (Kk) values were calculated with an inverse Green-Ampt equation using infiltration data measured in three vegetated filter plots located near Mead Nebraska. Also, three pedotransfer functions (PTFs) were used to empirically generate average Kfs values for each plot, based on percent sand, percent clay, and bulk density. Pedotransfer functions underestimated Kfs (10 to 99 percent) compared to field-measured values. Using VFSMOD to replicate actual field scenarios, more runoff (up to 62 percent) from the filter was predicted with the PTF Kr, input values than with the field-measured input Kf, values. These results were compared to data from overland flow studies performed on these plots in July 2004. Using the field-measured Kfs values resulted in the closest match for model water trapping predictions (in 2 of the 3 plots). Water trapping was more sensitive to Kt, than was sediment trapping, even at a higher sediment loading rate. Neither water trapping nor sediment trapping was sensitive to changes in wetting front suction or initial water content. One reason PTFs may underestimate Kf, and thus infiltration, is that they do not account for preferential flow (e.g. macropore flow). Vegetated filters may have a substantial number of preferential flow pathways. Tension infiltrometers were used on these three plots to measure infiltration rates and determine if macropores contributed significantly to flow in these soils. We found that 45-47 percent of the saturated flow was through pores larger than 0.1 cm in diameter indicating that macropores may significantly impact (increase) the infiltration rates and thus the field saturated hydraulic conductivities at our site. The inverse Green-Ampt method, being based on field measured data, may implicitly account for preferential flow and may better approximate field saturated hydraulic conductivity than PTFs.