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.
KeywordsConservation planning Conservation practices Non-point pollution Soil survey Terrain analyses
Establishment of riparian buffers has been encouraged and financially supported by agricultural policies in the US, partly because riparian vegetation has the potential to improve water quality. Many field-scale studies have shown buffers can improve water quality, and this literature is well reviewed (e.g., Fennessy and Cronk 1997; Dosskey 2001). Yet at watershed scales, where public concern about water quality is focused, the water quality impacts of conservation practices (such as buffers) are difficult to establish. Therefore, efforts are underway to document benefits from practices supported by public funds (Mausbach and Dedrick 2004). This will be difficult, largely because the efficacy of riparian buffers in controlling non-point pollution depends on location. A number of soil and landscape processes influence the movement of water across or beneath riparian zones towards a stream or river, and these processes all vary in time and space. Riparian buffers are installed to modify these processes in a way that can improve water quality, most typically by slowing water movement, trapping sediment, encouraging infiltration, increasing nutrient uptake and storage, increasing transpiration, and promoting denitrification in the shallow subsurface. However opportunities to alter these processes through management are not the same everywhere.
Buffer design and species selection are influenced by environmental and other management objectives including wildlife habitat or agroforestry production. This paper is focused on environmental benefits. Buffers intended to trap sediment and associated pollutants from runoff typically should include grass (Lyons et al. 2000), perhaps as part of a multi-species buffer with trees (Lee et al. 2000). Including trees in buffers can influence shallow groundwater flow through increased transpiration, even in temperate climates (Komor and Magner 1996; Wagner and Bretschko 2003). Also riparian trees help reduce or denitrify groundwater nitrate (Haycock and Pinay 1993), and provide a range of benefits to aquatic ecosystems (Harper et al. 1999). Studies on environmental effects of harvesting trees in riparian buffers are also published (Hubbard and Lowrance 1997; Liquori 2006).
This paper is focused on prioritizing locations for installing riparian buffers on agricultural landscapes for water quality benefits. If buffers are to be installed where they will have the greatest impact on water quality, then managers need techniques to help them identify these locations. The idea of targeting conservation practices to optimize their effectiveness is not new, and has been discussed in the literature for at least 20 years (Maas et al. 1985). Although examples in the research literature are rare, these types of assessments have been successfully applied at scales ranging from national (Johansson and Randall 2003) to individual landscapes (Bren 1998). However, methods to prioritize locations for buffer establishment using publicly available data across broad areas are still needed. In this paper, we present two techniques for using soil survey and digital terrain data to identify priority locations for attaining water quality benefits of riparian buffers. Location obviously influences buffer design; for this discussion we abridge these considerations by assuming that buffers intercepting surface runoff will include a grass strip (Lee et al. 2000), and that buffers intended to influence shallow groundwater will include trees.
Soil survey technique
Soil surveys map the locations of various soil types across agricultural landscapes. The US Department of Agriculture’s National Soil Survey contains data on soil and topographic characteristics that are important determinants of a buffer’s capacity to filter pollutants from agricultural runoff. This technique applies a simple model to rank each soil type for the capacity of a buffer located on it to trap sediment delivered in surface runoff from a cultivated field. Then a map is prepared to highlight the soil types where buffers will perform relatively better. This method ranks and maps all farmable soil types across the landscape, including riparian zones. The rankings of soil types could therefore be applied to riparian and other vegetative practices such as contour buffers, field borders, and filter strips that function to filter surface runoff from cropped fields. Slope, soil texture, and soil erodibility are the key soil attributes used in this technique.
A two-step model was developed for sediment trapping by buffers. First, an empirical equation calculates a factor for each soil map unit based on soil and slope information contained in a soil survey. Then, a calibration equation converts the empirical factor into an estimate of sediment trapping efficiency of a buffer placed in that soil map unit (Dosskey et al. 2006).
Values for median particle diameter (D50) used for calculating the sediment factor in Eq. 1, estimated based on soil texture (from Muñoz-Carpena and Parsons 2000)
Soil texture class
Silty clay loam
Sandy clay loam
Very fine sandy loam
Fine sandy loam
Coarse sandy loam
Loamy very fine sand
Loamy fine sand
Loamy coarse sand
Very fine sand
This technique is used by computing one value for sediment trapping efficiency (STE) for each soil-survey map unit in the area of interest using Eqs. 1 and 2. A difference between soil map units reflects intrinsic soil and slope conditions that affect sediment trapping by a buffer. These results can be used to base different recommendations for management in each soil map unit (Dosskey et al. 2006).
Terrain analysis technique
The National Elevation Database (USGS 2004) is a 30-m raster topographic map for the entire United States. These digital elevation model (DEM) data are derived from digitized quadrangle maps, which are typically at 1:24000 scale, similar to soil survey maps. USGS (2004) provide metadata on map sources, and Tomer et al. (2003) summarize source-map implications for data quality. Digital terrain analyses (Moore et al. 1991) can be applied to determine a range of landform parameters such as slope, aspect, upslope contributing area, and others that are defined below. Mapping these parameters provides images that reveal pathways of water movement and areas of water accumulation on the landscape. These maps can be classified and interpreted to identify priority sites for riparian buffers. These analyses have been applied to identify priority stream reaches (Burkart et al. 2004), and specific riparian zones for field-level planning (Tomer et al. 2003).
The factor 1000 simply converts the proportion to units of per mille (‰). Simply interpreted, larger values of this index occur where riparian forest buffers are likely to measurably impact water quality in the stream.
Flat areas with large upslope contributing areas are associated with large W values. Buffers in these areas can remove contaminants from shallow groundwater, and/or filter surface runoff. Filtering of surface runoff can occur where it slows and infiltrates in flat areas below hillslopes. Also, flat riparian areas tend to have shallow groundwater. In both situations, permanent riparian vegetation can benefit water quality. In some instances, however, shallow ground water approaches the surface and limits infiltration of runoff, therefore benefits for surface and subsurface flows may not accrue at all locations with large W values. Again, grass buffers should work best to remove sediment from runoff, and trees should most effectively influence shallow groundwater.
Advantages and limitations
Similar advantages and limitations apply to both types of methods. Both provide a standardized basis for comparing locations across watersheds, states, and regions in the eastern US Soil-survey map units can be one hectare or less, and individual DEM grid-cells represent 0.09 ha. Therefore, both techniques can provide detailed spatial resolution. Optimal locations for installing buffers can be located easily by displaying computed results in maps. Calculations and mapping for large areas are readily accomplished using digitized databases for soil survey (USDA Natural Resources Conservation Service 1994) and topography (USGS 2004) in a geographic information system (GIS). Both data sources are freely available to the public. The methods can also be applied at multiple scales, by varying the soil survey data source (i.e., STATSGO or SSURGO), or shifting the focus from individual riparian zones to stream reaches for DEM analyses.
Because simplifying assumptions are used in both methods, and because spatial data sources are not always of uniform quality, these techniques should be used only as a general guide for locating buffers. The soil survey method applies only to controlling sediment runoff from cultivated cropland. For terrain-modeling results, field review is needed to determine whether surface runoff or groundwater may be most influenced by buffers at specific locations (Tomer et al. 2003). This difference has implications for buffer design and management, including tree species selection to meet multiple management goals. Scientific validation of these methods would be challenging in terms of experimental design, but could perhaps be undertaken by a synoptic survey across a range of sites. Results are probably best used as an interpretive tool to target locations where water-quality benefits are likely to accrue, and avoid locations where they are likely to be minimal. Conservation planning inherently involves human judgment; these techniques can inform that judgment but should not supersede it.
Soil survey data can be used to identify locations where buffers can function better to trap sediment and associated pollutants from surface runoff. In general, better locations for buffers are those where slope and soil conditions lead to greater runoff and sediment generation.
Terrain analyses can show where buffers will intercept more runoff. Maps generated using terrain analyses have been found interpretively useful for conservation planning. In general, better opportunities to intercept runoff and/or baseflow occur along first order streams than along larger streams.
Detailed maps of riparian zones can indicate specific locations best suited for buffers, and can be applied to field-scale planning.
Both the soil survey and terrain analysis techniques can be applied at varying scales. General availability of data also allows application in most areas in the United States.
Results depicted in map form, while visually compelling, should only be used as an interpretive aid. Conservation planning requires human judgment, and these decision support tools should only inform that judgment, which must consider site-specific management objectives and design alternatives.
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