Jonathan D. Witter

Floodplain restoration enhances denitrification and reach‐scale nitrogen removal in an agricultural stream

Streams of the agricultural Midwest, USA, export large quantities of nitrogen, which impairs downstream water quality, most notably in the Gulf of Mexico. The two-stage ditch is a novel restoration practice, in which floodplains are constructed alongside channelized ditches. During high flows, water flows across the floodplains, increasing benthic surface area and stream water residence time, as well as the potential for nitrogen removal via denitrification. To determine two-stage ditch nitrogen removal efficacy, we measured denitrification rates in the channel and on the floodplains of a two-stage ditch in north-central Indiana for one year before and two years after restoration. We found that instream rates were similar before and after the restoration, and they were influenced by surface water NO3- concentration and sediment organic matter content. Denitrification rates were lower on the constructed floodplains and were predicted by soil exchangeable NO3- concentration. Using storm flow simulations, we found that two-stage ditch restoration contributed significantly to NO3- removal during storm events, but because of the high NO3- loads at our study site, < 10% of the NO3- load was removed under all storm flow scenarios. The highest percentage of NO3- removal occurred at the lowest loads; therefore, the two-stage ditchs effectiveness at reducing downstream N loading will be maximized when the practice is coupled with efforts to reduce N inputs from adjacent fields.

Effects of geomorphology, habitat, and spatial location on fish assemblages in a watershed in Ohio, USA

In this paper, we evaluate relationships between in-stream habitat, water chemistry, spatial distribution within a predominantly agricultural Midwestern watershed and geomorphic features and fish assemblage attributes and abundances. Our specific objectives were to: (1) identify and quantify key environmental variables at reach and system wide (watershed) scales; and (2) evaluate the relative influence of those environmental factors in structuring and explaining fish assemblage attributes at reach scales to help prioritize stream monitoring efforts and better incorporate all factors that influence aquatic biology in watershed management programs. The original combined data set consisted of 31 variables measured at 32 sites, which was reduced to 9 variables through correlation and linear regression analysis: stream order, percent wooded riparian zone, drainage area, in-stream cover quality, substrate quality, gradient, cross-sectional area, width of the flood prone area, and average substrate size. Canonical correspondence analysis (CCA) and variance partitioning were used to relate environmental variables to fish species abundance and assemblage attributes. Fish assemblages and abundances were explained best by stream size, gradient, substrate size and quality, and percent wooded riparian zone. Further data are needed to investigate why water chemistry variables had insignificant relationships with IBI scores. Results suggest that more quantifiable variables and consideration of spatial location of a stream reach within a watershed system should be standard data incorporated into stream monitoring programs to identify impairments that, while biologically limiting, are not fully captured or elucidated using current bioassessment methods.

Implementing Innovative Drainage Management Practices in the Mississippi River Basin to Enhance Nutrient Reductions

In the Mississippi River Basin (MRB), practices that enhance drainage (e.g., channelization, tile drainage) are necessary management tools in order to maintain optimal agricultural production in modern farming systems. However, these practices facilitate, and may speed the delivery of excess nutrients and sediments to downstream water bodies via agricultural streams and ditches. These nonpoint sources contribute to elevated nutrient loading in the Gulf of Mexico, which has been linked to widespread hypoxia and associated ecological and economic problems. Research suggests agricultural drainage ditches are important links between farm fields and downstream ecosystems, and application of new management practices may play an important role in the mitigation of water quality impairments from agricultural watersheds. In this article, we describe how researchers and producers in the MRB are implementing and validating novel best management practices (BMPs) that if used in tandem could provide producers with continued cropping success combined with improved environmental protection. We discuss three BMPs — low-grade weirs, slotted inlet pipes, and the two-stage ditch. While these new BMPs have improved the quality of water leaving agricultural landscapes, they have been validated solely in isolation, at opposite ends of the MRB. These BMPs have similar function and would greatly benefit from stacked incorporation across the MRB to the benefit of the basin as a whole.

A decade later: the establishment, channel evolution, and stability of innovative two-stage agricultural ditches in the midwest region of the united states

Much of the landscape in the Midwest region of the United States has been converted to agricultural use and with this conversion has come greatly altered hydrologic functioning. The elimination of wetland storage and installation of subsurface drainage systems and agricultural ditches has caused water to drain from agricultural watersheds at greatly accelerated rates. In some cases, these hydrologic alterations have led to severe water quality problems, including stream bank erosion, sedimentation, and inadequate processing of nutrients, each of which pose dire consequences for aquatic biota. Research by the authors has led to the modification of some trapezoidal agricultural ditches to two-stage geometries that are sized based on geomorphic concepts. A procedure for sizing these systems has been developed by the authors. Most of these innovative systems are located in Indiana, Michigan and Ohio. The main objective of the paper is to present details on how these systems have evolved since construction. The paper addresses issues that require further consideration. Channel evolution, determined by the assessment and analysis of physical condition, includes tracking changes in form by repeated surveys of channel dimension, pattern and profile. Pre-construction and post-construction properties are compared. Channel dimensions are also compared to regional curves. Analysis will include computing the hydrology and hydraulics for the range of recurrence intervals using the computer simulation models HEC-HMS, HEC GeoHMS, and HEC-RAS. All systems that have been studied have been stable, exhibited small adjustments on the constructed floodplains (benches), and have required little or no maintenance.

Ecological Services of Constructed Two-Stage Agricultural Ditches

Data collection occurred on geomorphic evolution, turbidity and denitrification potential at 8 constructed two-stage ditch sites in Ohio, Indiana, and Michigan. Systems ranged in age from one to nine years. Objectives were to: 1) demonstrate that two-stage channels maintain a stable geometry over time without excessive aggrading or degrading; 2) quantify the capacity of agricultural ditches using a two-stage management approach to reduce stream water turbidity and sediment export; 3) quantify if the two-stage ditch improves stream bottom habitat for macroinvertebrates, fishes, and mussels; 4) determine whether two-stage project sites of varying age differ in nitrogen removal capacity via microbial denitrification. All systems studied have been very stable, exhibited small adjustments on the constructed floodplains (benches), and have required little or no maintenance. Turbidity and sediment export decreased in the majority of two-stage ditches, compared to upstream control reaches. Results showed that older two-stage ditches (= 4 yrs) have proportionally more large substrates compared to conventional trapezoidal ditches, but younger two-stage sites (= 3 yrs old) do not. Therefore, the two-stage ditch may improve in-stream habitat by revealing larger substrate, but improvement in stream habitat may take several years to occur. Soil denitrification rates were highest on mature benches (>5 yrs). When both in-stream and floodplain bench rates were scaled to areal rates, N removal in a two-stage ditch was 2-14 times higher compared to conventional trapezoidal ditches. Two-stage designs enhanced the ability and capacity for ditches to permanently remove Nitrate-N from stream water via microbial denitrification.

BEYOND THE FIELD: A LOOK AT AGRICULTURAL DITCH FLOODPLAINS AS A WATER QUALITY BMP

Agricultural fields, subsurface drainage, and a network of modified headwater systems dominate the landscape in the Midwest region of the United States. These systems are often the main conduits that export sediment and nutrients downstream, but very little is known about how these systems function. While most agricultural Best Management Practices (BMPs) focus on landscape measures to reduce sediment loss and water pollution little research has been performed on in-stream processes and channel system floodplains as agricultural BMPs. An important step in quantifying nutrient reduction capacities on floodplains is to predict discharges occurring on floodplains. This paper evaluates methods to predict the recurrence interval of discharges at ungaged sites and the annual exceedances of different discharge thresholds. Also presented is a study to evaluate the benefits of modifying channels to two-stage geometries that provide connection to floodplains and more bank storage. Preliminary results indicate that benches (small floodplains) should be located at elevations associated with about 25-35% of the 2-year discharge and they will usually flood about 10-60 days annually. Nitrate-N removal, in systems with flooded width ratios of 4 to 5 times the bankfull width, might be 5-20% of exports from fields if the treatment area (surface of the benches and inset channel) is about 1% of the watershed area. Results of this study are being used to develop a tool that quantifies the reduction in nitrate exports associated with different floodplain geometries and linear extent of the floodplains.

Consideration of Hydrology, Hydraulics, and Stream Morphology to Effectively Meet State and National Water Quality Standards

Assessing stream assemblages in systems that were historically designed and managed without regard for ecological criteria will serve as a benchmark for restoration potential in highly modified watersheds. The challenge for modern management is to identify the best measures of sustainability and manage to those objectives in state and national water quality/quantity programs. Our objectives in this study were to: (1) Evaluate the factors associated with the formation of bankfull features in modified streams; and (2) Measure how spatial position within the watershed and habitat availability affects the structure and organization of aquatic communities. At 30 sites within a modified central Ohio watershed we collected data on fish, macroinvertebrates, instream habitat, geomorphology, hydrology, and spatial location. Key ecological drivers for macroinvertebrate communities in modified central Ohio headwater systems primarily related to stream size and gradient. Key ecological drivers for fish communities in these systems are quality of in-stream habitat (i.e., the presence of riffles, pools, and diverse substrates); however, proximity to a larger receiving system, stream size, and presence of water year-round also were important. Larger sites tended to support more aquatic biota and more diverse assemblages either as primary habitat or as conduits to travel or take refuge between higher quality upstream or downstream locations. Our dataset had low variability between sites, which affected multivariate analyses. We contend that strategies to improve water quality, aquatic biology, and ecology will have a higher likelihood of success if in-stream processes and stream geomorphology are considered in national assessment standards.

The Sustainability and Nitrate Nitrogen Reduction Potential of Two-Stage Agricultural Ditches

Much of the landscape in the Midwest region of the United States has been converted to agricultural use and with this conversion has come greatly altered hydrologic functioning. Elimination of wetland storage and installation of subsurface drainage systems and agricultural ditches has caused water to drain from agricultural watersheds at accelerated rates. Agricultural fields, subsurface drainage, and a network of modified headwater systems now dominate the landscape in the Midwest region of the United States. These systems are often the main conduits that export sediment and nutrients downstream, but very little is known about how they function. Research by the authors has led to the modification of some trapezoidal agricultural ditches to two-stage geometries that are sized based on geomorphic concepts. Most of these innovative systems are located in Indiana, Michigan and Ohio. A procedure for sizing these systems has been developed by the authors. The main objective of the paper is to present details on how these systems have evolved since construction. The paper addresses issues that require further consideration. All systems that have been studied have been stable, exhibited small adjustments on the constructed floodplains (benches), and have required little or no maintenance. Also presented is a tool for evaluating the nitrate-nitrogen reduction potential of modifying channels to two-stage geometries that provide connection to floodplains and more bank storage. Preliminary results indicate that benches (small floodplains) should be located at elevations associated with about 25-35% of the 2-year discharge, and nitrate-nitrogen removal in systems with flooded width ratios of 4 to 5 times the bankfull width might be 5-20% of exports from fields if the treatment area (surface of the benches and inset channel) is 1%-2% of the watershed area.