Mark D. Tomer

HYDROLOGIC EVALUATION OF THE SOIL AND WATER ASSESSMENT TOOL FOR A LARGE TILE-DRAINED WATERSHED IN IOWA

The presence of subsurface tile drainage systems can facilitate nutrient and pesticide transport, thereby contributing to environmental pollution. The Soil and Water Assessment Tool (SWAT) water quality model is designed to assess nonpoint and point source pollution and was recently modified for tile drainage. Over 25% of the nations cropland required improved drainage. In this study, the models ability to validate the tile drainage component is evaluated with nine years of hydrologic monitoring data collected from the South Fork watershed in Iowa, since about 80% of this watershed is tile drained. This watershed is a Conservation Effects Assessment Program benchmark watershed and typifies one of the more intensively managed agricultural areas in the Midwest. Comparison of measured and predicted values demonstrated that inclusion of the tile drainage system is imperative for obtaining a realistic watershed water balance. Two calibration/validation scenarios tested if the results differed in how the data set was divided. The optimum scenario results for the simulated monthly and daily flows had Nash-Sutcliffe efficiency (ENS) values during the calibration/validation (1995-1998/1999-2004) periods of 0.9/0.7 and 0.5/0.4, respectively. The second scenario results for the simulated monthly and daily flows had ENS values during the calibration/validation (1995-2000/2001-2004) periods of 0.8/0.5 and 0.7/0.2, respectively. The optimum scenario reflects the distribution of peak rainfall events represented in both the calibration and validation periods. The year 2000, being extremely dry, negatively impacted both the calibration and validation results. Each water budget component of the model gave reasonable output, which reveals that this model can be used for the assessment of tile drainage with its associated practices. Water yield results were significantly different for the simulations with and without the tile flow component (25.1% and 16.9%, expressed as a percent of precipitation). The results suggest that the SWAT2005 version modified for tile drainage is a promising tool to evaluate streamflow in tile-drained regions when the calibration period contains streamflows representing a wide range of rainfall events.

Quantifying relative contributions from sediment sources in Conservation Effects Assessment Project watersheds

A technique using the relationship between the naturally occurring radionuclide tracers, 7Be and 210Pbxs, was used to differentiate eroded surface soils and channel-derived sediments in the fine suspended sediment loads of runoff events in five Conservation Effects Assessment Project watersheds. A simple two end-member mixing model was used to determine the relative contribution from each source. Results suggest that eroded surface soils were more prevalent in the suspended load early in a runoff event, but channel contributions dominated the suspended load at later stages. The method proved useful for multiple sites due to a constant proportion of the atmospheric deliveries of the two radionuclides globally. Use of only two radionuclide tracers simplifies the differentiation of sediment sources within a watershed but limits precision.

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.

Assessment of the Iowa River's South Fork watershed: Part 1. Water quality

Iowas South Fork watershed is dominated by corn (Zea mays L.) and soybean [Glycine max L. (Merr.)] rotations, and animal feeding operations are common. Artificial subsurface (tile) drainage is extensive; hydric soils cover 54% of the watershed. During spring and early summer, NO3-N concentrations in tile and stream discharge often exceed 20 mg L-1. Total N loads during 2002 to 2005 ranged from 16 to 26 kg NO3-N ha-1 y-1 (14 to 23 lb ac-1 yr-1). Nitrate concentrations increased linearly with log baseflow, effectively a surrogate measure of tile discharge. Phosphorus loads were only 0.4 to 0.7 kg P ha-1 y-1 (0.4 to 0.6 lb ac-1 yr-1), but concentrations commonly exceeded 0.1 mg L-1, a eutrophication-risk threshold. Mean E. coli populations in the stream exceeded 500 cells 100 ml-1 during summer. Statistical comparison of actual nitrate records with independent records generated using regression equations provided modeling efficiencies of 0.91 or less, suggesting performance targets for watershed model validation. Tile drainage is more important in transport of nitrate and dissolved phosphorus than E. coli. Variations in nitrate, phosphorus, and E. coli are uniquely timed, highlighting the complexity of integrated water quality assessments.

Sediment removal by prairie filter strips in row-cropped ephemeral watersheds

Twelve small watersheds in central Iowa were used to evaluate the effectiveness of prairie filter strips (PFS) in trapping sediment from agricultural runoff. Four treatments with PFS of different size and location (100% rowcrop, 10% PFS of total watershed area at footslope, 10% PFS at footslope and in contour strips, 20% PFS at footslope and in contour strips) arranged in a balanced incomplete block design were seeded in July 2007. All watersheds were in bromegrass ( L.) for at least 10 yr before treatment establishment. Cropped areas were managed under a no-till, 2-yr corn ( L.)-soybean [ (L.) Merr.] rotation beginning in 2007. About 38 to 85% of the total sediment export from cropland occurred during the early growth stage of rowcrop due to wet field conditions and poor ground cover. The greatest sediment load was observed in 2008 due to the initial soil disturbance and gradually decreased thereafter. The mean annual sediment yield through 2010 was 0.36 and 8.30 Mg ha for the watersheds with and without PFS, respectively, a 96% sediment trapping efficiency for the 4-yr study period. The amount and distribution of PFS had no significant impact on runoff and sediment yield, probably due to the relatively large width (37-78 m) of footslope PFS. The findings suggest that incorporation of PFS at the footslope position of annual rowcrop systems provides an effective approach to reducing sediment loss in runoff from agricultural watersheds under a no-till system.

Source-Pathway Separation of Multiple Contaminants during a Rainfall-Runoff Event in an Artificially Drained Agricultural Watershed

A watersheds water quality is influenced by contaminant-transport pathways unique to each landscape. Accurate information on contaminant-pathways could provide a basis for mitigation through well-targeted approaches. This study determined dynamics of nitrate-N, total P, Escherichia coli, and sediment during a runoff event in Tipton Creek, Iowa. The watershed, under crop and livestock production, has extensive tile drainage discharging through an alluvial valley. A September 2006 storm yielded 5.9 mm of discharge during the ensuing 7 d, which was monitored at the outlet (19,850 ha), two tile-drainage outfalls (total 1856 ha), and a runoff flume (11 ha) within the sloped valley. Hydrograph separations indicated 13% of tile discharge was from surface intakes. Tile and outlet nitrate-N loads were similar, verifying subsurface tiles dominate nitrate delivery. On a unit-area basis, tile total P and E. coli loads, respectively, were about half and 30% of the outlets; their rapid, synchronous timing showed surface intakes are an important pathway for both contaminants. Flume results indicated field runoff was a significant source of total P and E. coli loads, but not the dominant one. At the outlet, sediment, P, and E. coli were reasonably synchronous. Radionuclide activities of (7)Be and (210)Pb in suspended sediments showed sheet-and-rill erosion sourced only 22% of sediment contributions; therefore, channel sources dominated and were an important source of P and E. coli. The contaminants followed unique pathways, necessitating separate mitigation strategies. To comprehensively address water quality, erosion-control and nitrogen-management practices currently encouraged could be complemented by buffering surface intakes and stabilizing stream banks.

Downstream approaches to phosphorus management in agricultural landscapes: Regional applicability and use

This review provides a critical overview of conservation practices that are aimed at improving water quality by retaining phosphorus (P) downstream of runoff genesis. The review is structured around specific downstream practices that are prevalent in various parts of the United States. Specific practices that we discuss include the use of controlled drainage, chemical treatment of waters and soils, receiving ditch management, and wetlands. The review also focuses on the specific hydrology and biogeochemistry associated with each of those practices. The practices are structured sequentially along flowpaths as you move through the landscape, from the edge-of-field, to adjacent aquatic systems, and ultimately to downstream P retention. Often practices are region specific based on geology, cropping practices, and specific P related problems and thus require a right practice, and right place mentality to management. Each practice has fundamental P transport and retention processes by systems that can be optimized by management with the goal of reducing downstream P loading after P has left agricultural fields. The management of P requires a system-wide assessment of the stability of P in different biogeochemical forms (particulate vs. dissolved, organic vs. inorganic), in different storage pools (soil, sediment, streams etc.), and under varying biogeochemical and hydrological conditions that act to convert P from one form to another and promote its retention in or transport out of different landscape components. There is significant potential of hierarchically placing practices in the agricultural landscape and enhancing the associated P mitigation. But an understanding is needed of short- and long-term P retention mechanisms within a certain practice and incorporating maintenance schedules if necessary to improve P retention times and minimize exceeding retention capacity.

Combining precision conservation technologies into a flexible framework to facilitate agricultural watershed planning

The need to reduce nutrient loads from agricultural watersheds poses a daunting challenge, considering the continental scale of water quality problems in the Gulf of Mexico (Turner et al. 2008), Great Lakes (Joose and Baker 2010), and Chesapeake Bay (Russell et al. 2008). Strategies to address nutrient reduction have suggested that a mix of practices will be required across multiple landscape positions to achieve water quality goals (Iowa Nutrient Reduction Strategy Science Team 2012). Control of both nitrogen and phosphorus may be critical to mitigate eutrophication of freshwaters, estuaries, and marine shelves (Paerl 2009), further emphasizing the need for comprehensive approaches to control agricultural nutrient losses. However, water quality must be improved while agricultural production is becoming more intensified (Lobell et al. 2009). Practices that sustain soil health offer the clearest opportunity to maintain crop production, water supply, and other ecosystem services derived from our agricultural landscapes (Kibblewhite et al. 2008). Our premise is that precision conservation technologies, which can help manage agricultural soils within fields (Delgado and Berry 2008) and place conservation practices below fields (Tomer et al. 2003), could provide the basis for developing watershed-specific strategies to improve environmental conditions and agricultural production with efficiency and flexibility, if…

Nutrient removal by prairie filter strips in agricultural landscapes

Nitrogen (N) and phosphorus (P) from agricultural landscapes have been identified as primary sources of excess nutrients in aquatic systems. The main objective of this study was to evaluate the effectiveness of prairie filter strips (PFS) in removing nutrients from cropland runoff in 12 small watersheds in central Iowa. Four treatments with PFS of different spatial coverage and distribution (No-PFS, 10% PFS, 10% PFS with strips, and 20% PFS with strips) were arranged in a balanced incomplete block design across four blocks in 2007. A no-tillage two-year corn (Zea mays L.) –soybean (Glycine max [L.] Merr.) rotation was grown in row-cropped areas beginning in 2007. Runoff was monitored by H flumes, and runoff water samples were collected during the growing seasons to determine concentrations of nitrate-nitrogen (NO3-N), total nitrogen (TN) and total phosphorus (TP) through 2011. Overall, the presence of PFS reduced mean annual NO3-N, TN, and TP concentrations by 35%, 73%, and 82%, respectively, and reduced annual NO3-N, TN, and TP losses by 67%, 84%, and 90%, respectively. However, the amount and distribution of PFS had no significant impact on runoff and nutrient yields. The findings suggest that utilization of PFS at the footslope position of annual row crop systems provides an effective approach to reducing nutrient loss in runoff from small agricultural watersheds.

Assessment of the Iowa River's South Fork watershed: Part 2. Conservation practices

Documenting the types and extent of conservation practices in a watershed is necessary to determine their water quality impacts. A conservation practice inventory for the South Fork of the Iowa River, 85% in corn (Zea mays L.) and soybean [Glycine max L. (Merr.)] rotations, showed only 7% of cropland was managed using no-tillage. About 30% of cropland receives manure annually, prior to corn. Surface residue following soybean was usually inadequate (<30%), indicating a key management challenge. About 90% of fields with >34% highly erodible land, subject to USDA conservation compliance, indeed had erosion-control practices installed. Grassed waterways and riparian buffers were common edge-of-field practices, and highly erodible land fields near streams often had multiple practices and rotations including third crops. Yet, while most conservation practices are aimed at controlling runoff, tile drainage is the dominant hydrologic pathway. Resource management systems that address tile drainage as the primary route of nutrient loss need to be developed and encouraged. Better targeting of this pathway could include practices such as nutrient removal wetlands.