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

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

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

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

Corn Harvest and Nutrient Management Systems Impacts on Phosphorus Loss with Surface Runoff

Cellulosic biomass is being promoted for use in future bioenergy production systems as a better alternative to current grain-based systems. Cropping systems and partial or total corn biomass removal in addition to grain harvest changes crop P needs, crop residue, and P recycling.Both sediment and water losses may also be altered and these changes could result in increased P loss from fields. Livestock production results in the generation of large quantities of manure that is a valuable nutrient source for producing high biomass yield. Manure can be used to minimize use of inorganic fertilizers and enhance production efficiency. Therefore, this study evaluated the impacts of several crop and corn biomass harvest systems on P loss with surface runoff as affected by management based on fertilizers or liquid swine manure

Crop Yields and Phosphorus Loss with Surface Runoff as Affected by Tillage Systems and Phosphorus Sources

Excess sediment, phosphorus (P) and/or nitrogen (N) impair many Iowa lakes and streams. Most of the sediment and P originate from fields and stream banks. Previous research was based on rainfall simulations. This technique provided useful information about potential differences between management practices, but estimated poorly long-term differences and total losses. Little work has been conducted elsewhere in Iowa to study P loss with surface runoff based on natural rainfall and large-scale plots. Therefore, a long-term study was established to investigate effects of corn and soybean production, tillage, and fertilizer or manure P management system on crop yield and loss of soil and P with runoff

Impacts of crop, biomass harvest systems, and nutrient management on field and subsurface drainage water quality

Grain-crop biomass and perennial grass biomass are of particular interest for their use in bioenergy production systems. Nutrient needs, particularly nitrogen and phosphorus, change with varying cropping systems, harvest systems, and rates of fertilizer application. Furthermore, manure generated from livestock production can be a viable nutrient source for cropping systems, reducing the need for commercial fertilizers. The primary focus of this study was to investigate nutrient loss, primarily nitrate-nitrogen loss, in subsurface drainage water under a variety of cropping, nutrient management, and harvest scenarios. Overall crop yields and biomass production were also evaluated