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

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

Technical Note: The Potential of Municipal Yard Waste to be Denitrification Bioreactor Fill

The use of denitrification bioreactors to mitigate nitrate in agricultural drainage has recently gained much interest in the Midwestern United States and in similarly drained agricultural regions. However, as the number of bioreactor installations has increased, questions have been raised about the supply and consistency of denitrification carbon source material. In selecting such material, there is an important balance between optimal media properties (e.g., hydraulic properties, chemical composition), practicality, and material cost. The use of free material such as municipal yard waste may help minimize the cost of this voluntary water quality improvement strategy in the Midwestern United States, but may not provide other sufficient media properties. To investigate this, pilot-scale bioreactors were used to compare hardwood chips with free, chipped municipal yard waste in terms of nitrate removal potential and changes in the media. Sampling of bioreactor influent and effluent over a range of retention times showed the yard waste had higher removal efficiencies at a given retention time and higher removal rates than the woodchips. However, buried carbon media bags revealed the yard waste lost weight to a greater extent and more consistently than the woodchips meaning the woodchips had a half-life over two times greater than the yard waste. This, combined with the low carbon-to-nitrogen ratio of the yard waste, indicated yard waste material is not ideal for bioreactor installations that are intended to be low maintenance for at least ten years

Nitrification Inhibitor and Nitrogen Application Timing Effects on Yields and Nitrate-Nitrogen Concentrations in Subsurface Drainage from a Corn-Soybean Rotation

Excess precipitation in Iowa and many other agricultural production areas is removed artificially via subsurface drainage systems that intercept and usually divert it to surface waters. Nitrogen, either applied as fertilizer or manure and derived from soil organic matter, can be carried as nitrate with the excess water in quantities that can cause deleterious effects downstream. A four-year, five-replication, field study was initiated in the fall of 1999 in Pocahontas County, Iowa on 0.05 ha plots that are predominantly Nicollet, Webster, and Canisteo clay loams with 3-5% organic matter. The objective was to determine the influence of seasonal N application and the use of nitrapyrin [inhibitor; 2-chloro-6 (trichloromethyl) pyridine] on flow-weighted nitrate-nitrogen concentrations and yields in a corn-soybean rotation, combined on single plots. Six aqua-ammonia nitrogen treatments (168 and 252 kg/ha at planting and in late fall, and 168 kg/ha at planting and late fall with nitrapyrin) were imposed on subsurface drained, continuous-flow-monitored plots. Combined fall 1999 and spring 2000 precipitation was 42% of normal average. Subsequently, normal precipitation was recorded for both fall and spring periods (after fall application, and before spring application) until spring and fall 2002 (51% and 73% of normal, respectively). Spring 2003 precipitation was again only 51% of normal average. Four-year average, flow-weighted nitrate-nitrogen concentrations ranked in highest to lowest order: spring-252(22.9 mg/L;a) \u3e fall-252(18.1 mg/L;b) \u3e spring-168 w/inhibitor(17.7 mg/L;bc) \u3e fall-168 w/inhibitor(16.0 mg/L;bcd) \u3e spring-168(14.8 mg/L;cd) \u3e fall-168(14.2 mg/L;cd). Spring application plots had significantly greater soybean yield the following season compared to fall applications. Greatest corn yields were observed for the spring-252 and fall-168 rates, but were only significantly different than the spring-168 rate for yield. Therefore, under slightly dry to normal precipitation conditions, corn yields and nitrate-nitrogen concentrations in subsurface drainage were not significantly different between seasonal timing or inhibitor use treatments at the 168 kg/ha nitrogen rate

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

Water Balance Investigation of Drainage Water Management in Non-Weighing Lysimeters

Artificial subsurface drainage systems are often used throughout the upper Midwest to remove excess precipitation and improve crop production. However, these drainage systems export nitrate-nitrogen (NO3-N) to downstream water resources. Management practices are needed to reduce this export of NO3-N with subsurface drainage water. One such practice being considered is the use of drainage water management where subsurface water is held in the soil profile during portions of the year. Previous research has shown that drainage water management has potential to reduce subsurface drainage volume but there is still a need to understand the performance of the practice and the pathways of water flow under varying conditions. The objectives of this study, therefore, were to quantify the pathways of water movement for conventional or free drainage (FD) and drainage water management (DWM) during the growing season. In this study, six non-weighing lysimeters (0.92 × 2.30 m) with a depth of 120 cm were monitored over a 3-yr period under natural and simulated rainfall conditions. The objectives were performed to measure the effects of drainage water management (DWM) on surface runoff, subsurface drainage, and crop yield. The in-season data from natural rainfall conditions showed that DWM reduced subsurface drainage by approximately 14%. The simulated rainfall data showed that DWM increased surface runoff by 54% when the water table was established at 90 cm below the soil surface, and by 87% when the water table was established at 60 cm below the soil surface. Overall DWM was found to have the potential to reduce subsurface drainage but there is the potential that at least a portion of this reduction may be reflected in an increase in surface runoff

ISU Extension Offers Ag Drainage School

Agricultural drainage is becoming increasingly important due to the critical role it plays for Iowa\u27s bio-economy. Drainage systems that are properly designed and operating are essential to achieving maximum agricultural production capability. These issues will be addressed at the Iowa Drainage School Aug. 23-25 at the Borlaug Learning Center on the Northeast Research and Demonstration Farm near Nashua, Iowa

Impact of Fertilizer Application Timing on Drainage Nitrate Levels

Nitrate loss from drainage systems in Iowa and other upper Midwestern states is a concern relative to local water supplies as well as the hypoxic zone in the Gulf of Mexico. As a result, there is a need to quantify how various nitrogen management practices impact nitrate loss. One practice that is commonly mentioned as a potential strategy to reduce nitrate loss is to vary fertilizer application timing and specifically apply nitrogen as close to when the growing crop needs it as possible. At a site in Gilmore City, Iowa, a number of fertilizer timing and rate schemes within a corn soybean rotation were used to study the impacts on nitrate leaching. Timing schemes include nitrogen application in the fall and an early season sidedress in the spring with each scheme having four replicates for both corn and soybeans. Fertilizer application rates investigated are 84 and 140 kg/ha (75 and 125 lb/ac) in the fall and 84 and 140 kg/ha (75 and 125 lb/ac) in the spring. The timing and rates have been practiced since 2005 with contrasting weather conditions each year. Overall, an annual basis there was not significant differences in nitrate concentrations or loss exiting the drainage system between the application rates or between the fall and spring application. In addition, there was not a yield penalty to the corn crop when fertilizer as applied in the fall versus the spring

Internal hydraulics of an agricultural drainage denitrification bioreactor

Denitrification bioreactors to reduce the amount of nitrate-nitrogen in agricultural drainage are now being deployed across the U.S. Midwest. However, there are still many unknowns regarding internal hydraulic-driven processes in these engineered treatment systems. To improve this understanding, the internal flow dynamics and several environmental parameters of a denitrification bioreactor treating agricultural drainage in Northeastern Iowa, USA were investigated with two tracer tests and a network of bioreactor wells. The bioreactor had a trapezoidal cross section and received drainage from approximately 14.2 ha at the North East Research Farm near Nashua, Iowa. It was clear from the water surface elevations and the continuous pressure transducer data that flow was attenuated within the bioreactor (i.e., reduction in peak flow as the hydrograph moved down gradient). Over the sampling period from 17 May to 24 August 2011, flow conditions and internal parameters (temperature, dissolved oxygen, oxidation reduction potential) varied widely resulting in early samplings that showed little nitrate removal ranging to complete nitrate removal (7–100% mass reduction; 0.38–1.06 g N removed per m3 bioreactor per day) and sulfate reduction at the final sampling event. The bioreactor\u27s non-ideal flow regime due to ineffective volume utilization was a major detriment to nitrate removal at higher flow rates. Regression analysis between mass nitrogen reduction and theoretical retention time (7.5–79 h) suggested minimum design retention times should be increased, though caution was also issued about this as increased design retention times and corresponding larger bioreactors may exacerbate detrimental by-products under low flow conditions. Operationally, outlet structure level management could also be utilized to improve performance and minimize detrimental by-products

Simulating Long-Term Impacts of Winter Rye Cover Crop on Hydrologic Cycling and Nitrogen Dynamics for a Corn-Soybean Crop System

Planting winter cover crops into corn-soybean rotations is a potential approach for reducing subsurface drainage and nitrate-nitrogen (NO3-N) loss. However, the long-term impact of this practice needs investigation. We evaluated the RZWQM2 model against comprehensive field data (2005-2009) in Iowa and used this model to study the long-term (1970-2009) hydrologic and nitrogen cycling effects of a winter cover crop within a corn-soybean rotation. The calibrated RZWQM2 model satisfactorily simulated crop yield, biomass, and N uptake with percent error (PE) within ±15% and relative root mean square error (RRMSE) \u3c30% except for soybean biomass and rye N uptake. Daily and annual drainage and annual NO3-N loss were simulated satisfactorily, with Nash-Sutcliffe efficiency (NSE) \u3e0.50, ratio of RMSE to standard error (RSR) \u3c0.70, and percent bias (PBIAS) within ±25% except for the overestimation of annual drainage and NO3-N in CTRL2. The simulation in soil water storage was unsatisfactory but comparable to other studies. Long-term simulations showed that adding rye as a winter cover crop reduced annual subsurface drainage and NO3-N loss by 11% (2.9 cm) and 22% (11.8 kg N ha-1), respectively, and increased annual ET by 5% (2.9 cm). Results suggest that introducing winter rye cover crops to corn-soybean rotations is a promising approach to reduce N loss from subsurface drained agricultural systems. However, simulated N immobilization under the winter cover crop was not increased, which is inconsistent with a lysimeter study previously reported in the literature. Therefore, further research is needed to refine the simulation of immobilization in cover crop systems using RZWQM2 under a wider range of weather conditions