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Nutrient removal wetlands have the potential to remove 40-90% of the nitrate in tile drainage, depending on the nitrate load intercepted by the wetland (which varies with watershed size, land use, and precipitation) and the area of the wetland. The Nutrient Removal Wetland siting tool allows the user to sample locations along collective flow pathways for suitability of nutrient removal wetlands. Candidate sites can be ranked based on watershed and wetland areas, and topographic buffers. Further details on wetland siting criteria and discussion of factors impacting prioritization of candidate sites can be found in Tomer et al. (2013b).
Potential impoundment locations (points) are generated along all collective flow paths within the drainage range established for nutrient removal wetlands (> 60 HA (~ 150 acres) --> maximum watershed drainage). A threshold is applied to the input D8 flow accumulation grid to delineate flow paths, which is converted to a polyline. Points are then generated continuously along this polyline at a user-specified distance interval (spacing). Locations are sorted by contributing area and most downstream sites are tested first.
At each location, an impoundment is simulated in the DEM, creating both a pooled area (of user-specified height – measured from the top of the bank) and a vegetated buffer (of user-specified height – measured from the top of the wetland pool). The drainage area to each impoundment is delineated, and descriptive statistics are generated, including the size of the pooled area, the size of the buffer, and the ratio of pooled area to the amount of drainage that it receives. If a site is found to be suitable according to suitability criteria, the site is “kept” and added
Contour buffer (or filter) strips are strips of perennial vegetation planted along topographic contours, which may be alternated with wider cultivated strips that are farmed on the contour. Contour buffer strips are in-field runoff control practices that use permanent vegetation to decrease the length of slopes along which runoff accumulates, and thereby reduce sheet and rill erosion. They are similar yet complementary to grassed waterways because both use grass vegetation, but contour buffer strips are oriented differently by being placed along topographic contours to intercept flows. This practice can be used in combination with grassed waterways, but the types of grass may differ with stiffer stems being preferred in buffer strips. This typically occurs at lower slope (i.e., footslope) positions. This approach is, in essence, based on recent research in Iowa that has documented benefits of reduced runoff volume and improved water quality derived from installation of contour buffer strips, particularly when placed at footslope positions (Zhou et al., 2014; Hernandez-Santana et al., 2013).
Output buffers strips are smoothed using a PAEK algorithm (Polynomial Approximation with Exponential Kernel) to smooth sharp angles and provide results that should better accommodate farming operations. Note output contour location will usually need to be further modified or smoothed to maintain trafficability for farm implements (that is, results should not be viewed as an actual suggested design/layout for the practice in any given field). A field boundary ID (FBndID) and mean slope is attributed to each contour buffer strip included in the output layer.
Hernandez-Santana, V., X. Zhou, M.J. Helmers, H. Asbjornsen, R. Kolka, and M.D. Tomer. 2013. Native prairie filter strips reduce runoff from hillslopes under annual row-crop system, Iowa USA. Journal of Hydrology. 477:94-103.
This practice can reduce sediment and total phosphorus loads, attenuate peak runoff discharge, and reduce risk of gully formation down gradient. The WASCOB tool identifies potential locations for these structures on watersheds from 2 to 50 acres in size. A user-provided embankment height defines the height of the WASCOB (as measured from the bottom center of the drainageway). The WASCOB is a commonly installed practice in much of the Midwest, and information on design and sediment retention can be found in basic texts on engineering of hydrologic structures. However, there is little information in the peer reviewed literature on this practice; Mielke (1985) is one of the rare examples we found. Gassman et al. (2010) evaluated conservation performance in an Iowa watershed following conservation improvements that included WASCOBs.
Potential WASCOB locations (points) are generated approximately every 200 feet along flow paths within the drainage range established for WASCOBS (2 – 50 acres). A threshold is applied to the input D8 flow accumulation grid to delineate this drainage range, which is converted to a polyline.
Gassman, P.W. J.A. Tisl, E.A. Palas, C.L. Fields, T.M. Isenhart, K.E. Schilling, C.F. Wolter, L.S. Seigley, and M.J. Helmers. 2010. Conservation practice establishment in two northeast Iowa watersheds: Strategies, water quality implications, and lessons learned. Journal of Soil and Water Conservation. 65(6):381-392.
Mielke, L.N. 1985. Performance of water and sediment control basins in northeastern Nebraska. Journal of Soil and Water Conservation. 40(6):524-528.
The process involves “burning” each WASCOB into a filled DEM (using the user-defined embankment height for each WASCOB), then determining the sink regions that are created upstream of each WASCOB as a result.
Denitrifying bioreactors typically comprise a buried bed of woodchips that receive a portion of tile drainage flows from an adjoining field. The woodchips provide a carbon source, which combined with the reducing (oxygen limiting) conditions in the saturated subsurface environment, encourage naturally occurring bacteria to reduce nitrate to di-nitrogen gas in a stepwise process (denitrification). A number of recently published studies on this practice are available; Schipper et al. (2010) provided a good overview. Field performance studies (e.g., Christianson 2012) have shown a range of efficiencies. In general, hydraulic retention times between 8 and 18 hrs are needed to reduce nitrate concentrations by half. Assuming a 12 hour hydraulic retention time, Moorman et al. (2015) found that nitrate loads in tile drainage can be reduced by 20-30% by bioreactors of 1 m (3.3 ft) depth and occupying <0.3% of the drainage area. We locate bioreactors sized at 0.5% of the field drainage area to account for construction disturbance and the possibility to allow retention times >12 hrs in the actual design, which may be needed where increased drainage volumes occur under cold conditions that slow microbial processes. Hydraulic retention time requirements in bioreactors are a subject of ongoing research (Moorman et al., 2015).
Christianson, L., A. Bhandari, M. Helmers, K. Kult, T. Stuphin, and R. Wolf. 2012. Performance evaluation of four field-scale agricultural drainage denitrification bioreactors in Iowa. Trans. Am. Soc. Agric. Biol. Eng. 55: 2163–2174.
Moorman, T.B., M.D. Tomer, D.R. Smith, and D.B. Jaynes. 2015. Evaluating the potential role of denitrifying bioreactors in reducing watershed-scale nitrate loads: A case study comparing three Midwestern (USA) watersheds. Ecological Engineering. 75: 441-448.
Schipper, L.A., W.D. Robertson, A.J. Gold, D.B. Jaynes, and S.C. Cameron. 2010. Denitrifying bioreactors – an approach for reducing nitrate loads to receiving waters. Ecol. Eng. 35(11): 1532–1543.
Grassed waterways are installed to reduce the risk of concentrated flow (gully) erosion. This practice may be effective in preventing gully erosion for three reasons. First, the growing grasses can reduce mean velocity of runoff, which discourages soil detachment. Second, grass vegetation subjected to high water velocity may be pushed to lie flat on the surface, and the flattened grass may then provide a physical barrier to prevent gully formation. Third, the fibrous root systems of grasses lead to increased soil strength, which can limit detachment of soil particles that otherwise may be prone to occur with seepage from the soil surface under saturated conditions. Although grassed waterways are among the most common of conservation practices, they remain under-utilized in many of the country’s steeper farmed landscapes, and their capacity to reduce erosion under saturation excess runoff (seepage) conditions may be under-appreciated. Grassed waterways have not been the most frequently evaluated practice in recent conservation-effectiveness research, but several papers by Fiener and Auerswald (2003; 2006) provide a good starting point to learn more. We emphasize that grassed waterways are designed to convey runoff, and are not meant to trap sediment.
The tool interface requires that the user define either a standard deviation threshold (between 2 and 5 standard deviations above the mean SPI value), or a specific SPI value. SPI values that are greater than the value specified will be selected as locations suitable for grassed waterways. SPI values above the selected threshold are first recoded to a value of 1, then smoothed using a majority filter. Values of 1 are expanded by 1 cell to increase overall connectivity between cells, then thinned to a maximum width of 1 cell. Regions are then converted to an output polyline layer. The input stream reach polyline is converted to a raster and serves to remove grid cells corresponding to the stream network. The output is clipped to agricultural field (excluding pasture) as identified by an “isAG” value of “1”. Finally, grassed waterways less than 50 meters in length are excluded from the output.
Fiener, P., and K. Auerswald. 2003. Effectiveness of grassed waterways in reducing runoff and sediment delivery from agricultural watersheds. Journal of Environmental Quality. 32(3):927-936.
Other processes such as denitrification may be enhanced but this has not yet been confirmed by research. Several published studies have evaluated tile discharge and nutrient loads under drainage water management systems (e.g., Williams et al., 2015). The water table is controlled through the use of gate structures that are adjusted at different times during the year. When field access is needed for planting, harvest or other operations, the gate can be opened fully to allow unrestricted drainage. When the gate is used to raise water table levels in the midst of the growing season, this may allow more plant water uptake during dry periods, which can increase crop yields. Crop grain yield increases have been documented with controlled drainage and this has primarily been attributed to the increased availability of soil water (Delbecq et al., 2012).
Controlled drainage may be used on fields with flat topography (typically one percent or less slope), such as in flood plains and on flat fields typical of the large areas of the glaciated Midwest. The practice can be expensive to design and install in areas with slopes steeper than about one percent because of the number of control structures required in a typical field.
A single control gate (depending on its design) can influence the water table in an area of a field that has about a 0.5 meter change in elevation. To identify fields potentially suited to this practice, the Drainage Water Management tool identifies all areas within tile-drained, agricultural fields where a contour interval between 0.3 and 1.5 m (chosen by the user), comprises more than a minimum acreage or a minimum user-defined percentage of the field (must be at least 30% of the field).
Poorly drained and hydric soils are common in these depressions, and to enable cropping of areas subject to surface ponding, drainage has often been improved by installing surface drains (or intakes) as part of in-field tile drainage systems. Conservation practices that may be appropriate in depressions can include filter practices to treat water entering the tile intakes, with impacts on drainage rate that are acceptable. There are several types of intake filter practices including blind (sand-bed) intakes and grass buffers. Wetland restorations may also be feasible where soil wetness in depressions is frequently problematic for crop production. The potential benefits of these practices include reduced sediment and phosphorus loads, and water storage. See Smith and Livingston (2013), and Kessler and Gupta (2015) for further discussion of specific practice options to manage water in topographic depressions.
Locations of depressions in agricultural fields may be suited for several types of conservation practices, including NRCS practice codes: 620 - Underground Outlet, 657 – Wetland Restoration
The Depression identification tool identifies surface depressions in the input DEM. This is performed by performing a “fill” process on the input DEM, then subtracting the input DEM from the filled DEM. Depression regions are then converted to polygons, and polygons are overlaid with the input DEM to extract the range of elevation values within each depression. This range of values represents the maximum depths of ponding that may occur in each depression. Polygons are also overlaid with gSSURGO to determine the mean percent of hydric soils within each depression.
Kessler, A.C., and S.C. Gupta. 2015. Drainage impacts on surficial water retention capacity of a prairie pothole watershed. Journal of the American Water Resources Association. 51(4): 1101-1113.
Smith, D.R., and S.J. Livingston. 2013. Managing farmed closed depressional areas using blind inlets to minimize phosphorus and nitrogen losses. Soil Use and Management. 29:94-102.
This tool should only be run following a manual review of the “Depression” feature class, during which false or inaccurate depressions are manually deleted. Alternatively, the user may interactively select true depressions on-screen rather than delete false depressions. When used as an input to the Depression Watersheds tool, the selection will be honored and watersheds will be found for only those depressions selected.
The output is a Depression Drainage Areas feature class (Depress_Wsheds + inHUC), which can be linked to the depression feature class through a “Depress_ID” field, and contains the following information for each drainage area as attributes.