Michael G. Dosskey

Methods to prioritize placement of riparian buffers for improved water quality.

Agroforestry buffers in riparian zones can improve stream water quality, provided they intercept and remove contaminants from surface runoff and/or shallow groundwater. Soils, topography, surficial geology, and hydrology determine the capability of forest buffers to intercept and treat these flows. This paper describes two landscape analysis techniques for identifying and mapping locations where agroforestry buffers can effectively improve water quality. One technique employs soil survey information to rank soil map units for how effectively a buffer, when sited on them, would trap sediment from adjacent cropped fields. Results allow soil map units to be compared for relative effectiveness of buffers for improving water quality and, thereby, to prioritize locations for buffer establishment. A second technique uses topographic and streamflow information to help identify locations where buffers are most likely to intercept water moving towards streams. For example, the topographic wetness index, an indicator of potential soil saturation on given terrain, identifies where buffers can readily intercept surface runoff and/or shallow groundwater flows. Maps based on this index can be useful for site-specific buffer placement at farm and small-watershed scales. A case study utilizing this technique shows that riparian forests likely have the greatest potential to improve water quality along first-order streams, rather than larger streams. The two methods are complementary and could be combined, pending the outcome of future research. Both approaches also use data that are publicly available in the US. The information can guide projects and programs at scales ranging from farm-scale planning to regional policy implementation.

A design aid for determining width of filter strips

Watershed planners need a tool for determining width of filter strips that is accurate enough for developing cost-effective site designs and easy enough to use for making quick determinations on a large number and variety of sites. This study employed the process-based Vegetative Filter Strip Model to evaluate the relationship between filter strip width and trapping efficiency for sediment and water and to produce a design aid for use where specific water quality targets must be met. Model simulations illustrate that relatively narrow filter strips can have high impact in some situations, while in others even a modest impact cannot be achieved at any practical width. A graphical design aid was developed for estimating the width needed to achieve target trapping efficiencies for different pollutants under a broad range of agricultural site conditions. Using the model simulations for sediment and water, a graph was produced containing a family of seven lines that divide the full range of possible relationships between width and trapping efficiency into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering field length and cover management, slope, and soil texture. Relationships for sediment-bound and dissolved pollutants are interpreted from the modeled relationships for sediment and water. Interpolation between lines can refine the results and account for additional variables, if needed. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. This design aid strikes a balance between accuracy and utility that fills a wide gap between existing design guides and mathematical models.

A design aid for sizing filter strips using buffer area ratio

Nonuniform field runoff can reduce the effectiveness of filter strips that are a uniform size along a field margin. Effectiveness can be improved by placing more filter strip where the runoff load is greater and less where the load is smaller. A modeling analysis was conducted of the relationship between pollutant trapping efficiency and the ratio of filter strip area to upslope contributing area, i.e., buffer area ratio. The results were used to produce an aid for designing filter strips having consistent effectiveness along field margins where runoff load is nonuniform. Simulations using the process-based Vegetative Filter Strip Model show that sediment and water trapping efficiencies of a filter strip increase nonlinearly as the buffer area ratio gets larger. Site characteristics, including slope, soil texture, and upslope soil cover management practices, help to define this relationship more accurately. Using the Vegetative Filter Strip Model simulation results, a graphical design aid was developed for estimating the buffer area ratio required to achieve specific trapping efficiencies for different pollutants under a broad range of agricultural site conditions. A single graph was produced showing simulation results for seven scenarios as a family of lines that divide the full range of possible relationships between trapping efficiency and buffer area ratio and into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering slope, soil texture, and field cover management practices. Relationships for sediment-bound and dissolved pollutants are interpreted from the Vegetative Filter Strip Model results for sediment and water. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. The use of this design aid will enable a more precise fit between filter size and runoff load where runoff from agricultural fields is nonuniform.

Modeling sediment trapping in a vegetative filter accounting for converging overland flow

Vegetative filters (VF) are used to remove sediment and other pollutants from overland flow. When modeling the hydrology of VF, it is often assumed that overland flow is planar, but our research indicates that it can be two-dimensional with converging and diverging pathways. Our hypothesis is that flow convergence will negatively influence the sediment trapping capability of VF. The objectives were to develop a two-dimensional modeling approach for estimating sediment trapping in VF and to investigate the impact of converging overland flow on sediment trapping by VF. In this study, the performance of a VF that has field-scale flow path lengths with uncontrolled flow direction was quantified using field experiments and hydrologic modeling. Simulations of water flow processes were performed using the physically based, distributed model MIKE SHE. A modeling approach that predicts sediment trapping and accounts for converging and diverging flow was developed based on the University of Kentucky sediment filtration model. The results revealed that as flow convergence increases, filter performance decreases, and the impacts are greater at higher flow rates and shorter filter lengths. Convergence that occurs in the contributing field (in-field) upstream of the buffer had a slightly greater impact than convergence that occurred in the filter (in-filter). An area-based convergence ratio was defined that relates the actual flow area in a VF to the theoretical flow area without flow convergence. When the convergence ratio was 0.70, in-filter convergence caused the sediment trapping efficiency to be reduced from 80% for the planar flow condition to 64% for the converging flow condition. When an equivalent convergence occurred in-field, the sediment trapping efficiency was reduced to 57%. Thus, not only is convergence important but the location where convergence occurs can also be important.

Buffers and Vegetative Filter Strips

First paragraph: This chapter describes the use of buffers and vegetative filter strips relative to water quality. In particular, we primarily discuss the herbaceous components of the following NRCS Conservation Practice Standards

Improved indexes for targeting placement of buffers of Hortonian runoff

Targeting specific locations within agricultural watersheds for installing vegetative buffers has been advocated as a way to enhance the impact of buffers and buffer programs on stream water quality. Existing models for targeting buffers of Hortonian, or infiltration-excess, runoff are not well developed. The objective was to improve on an existing soil survey–based approach that would provide finer scale resolution, account for variable size of runoff source area to different locations, and compare locations directly on the basis of pollutant load that could be retained by a buffer. The method couples the Soil Survey Geographic database with topographic information provided by a grid digital elevation model in a geographic information system. Simple empirical equations were developed from soil and topographic variables to generate two indexes, one for deposition of sediment and one for infiltration of dissolved pollutants, and the equations were calibrated to the load of sediment and water, respectively, retained by a buffer under reference conditions using the process-based Vegetative Filter Strip Model. The resulting index equations and analytical procedures were demonstrated on a 67 km2 (25.9 mi2) agricultural watershed in northwestern Missouri, where overland runoff contributes to degraded stream water quality. For both indexes, mapped results clearly mimic spatial patterns of water flow convergence into subdrainages, substantiating the importance of size of source area to a given location on capability to intercept pollutants from surface runoff. A method is described for estimating a range of index values that is appropriate for targeting vegetative buffers. The index for sediment retention is robust. However, the index for water (and dissolved pollutant) retention is much less robust because infiltration is very small, compared to inflow volumes, and is relatively insensitive to the magnitude of inflow from source areas. Consequently, an index of inflow volume may be more useful for planning alternative practices for reducing dissolved pollutant loads to streams. The improved indexes provide a better method than previous indexes for targeting vegetative buffers in watersheds where Hortonian runoff causes significant nonpoint pollution.

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 Green- Ampt 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 (Kfs) 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 Kfs input values than with the field-measured input Kfs 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 Kfs 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 Kfs 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.

Green-Ampt Infiltration Parameters in Riparian Buffers

Riparian buffers can improve surface water quality by filtering contaminants from runoff before they enter streams. Infiltration is an important process in riparian buffers. Computer models are often used to assess the performance of riparian buffers. Accurate prediction of infiltration by these models is dependent upon accurate estimates of infiltration parameters. Of particular interest here are Green-Ampt infiltration parameters, satiated hydraulic conductivity (Ko) and wetting front suction (hf). The objectives of this research were to (i) modify the Smith sorptivity procedure so that it can be used to estimate Green-Ampt infiltration parameters, (ii) Determine the relative closeness of Ko estimated by the inverse sorptivity and inverse Green-Ampt procedures and hf estimated by Rawls and Brakensiek (1985) to the laboratory-determined standards and (iii) Compare Ko estimates of the inverse sorptivity and inverse Green-Ampt procedures to those estimated by pedotransfer functions. This project was conducted at six sites in Nebraska, at which soil type and land use varied. The results of this study suggest that the inverse Green-Ampt procedure can be used to provide Ko estimates, even in the presence of macropores. Generally, pedotransfer function predictions did not estimate Ko well. Finally, hf as predicted by pedotransfer function was lower than laboratorydetermined hf. These predicted infiltration parameters were used in the Green-Ampt infiltration equation to illustrate their effect on cumulative infiltration. Cumulative infiltration based on the inverse Green-Ampt procedure parameters resulted in the closest match to cumulative infiltration prediction from laboratory-based infiltration parameters at five of the six sites.

Modeling Beaver Dam Effects on Ecohydraulics and Sedimentation in an Agricultural Watershed

Populations of North American beaver (Castor canadensis) have increased in recent decades throughout the agricultural Midwestern U.S., leading to an increase in the frequency of beaver dams in small streams. The impact of beaver dams on channel structure in this region is not known. Our field observations indicate that beaver dams are too dynamic and their affects on channel structure occur over longer time frames than is practical to study with field measurements. Modeling is therefore needed to determine if beaver dams will help stabilize and aggrade incised streams. The objective of this paper is to determine how a channel evolution model (CONCEPTS) might be used to predict the impact of beaver dams on channel structure.

Modeling Vegetative Buffer Performance Considering Topographic Data Accuracy

Riparian buffers are a promising tool in efforts to reduce sediment contribution to streams. Models that predict the capacity of buffers to trap sediment have recently been developed. A number of parameters that are required to conduct such modeling efforts are derived from the topography of the site. In this study, three topographic data sources were used to generate the model input for an agricultural field with a riparian buffer. The runoff and sediment transport in the system was then simulated for three years. As a result, the area that contributed runoff and sediment to the buffer was substantially different for each of the topographic data sources. In addition, the predicted runoff and sediment loss from the field was different for each case. Finally, the predicted sediment delivered to the stream was substantially affected by the accuracy of the topographic data source.