The extensive development of surface and subsurface drainage systems to facilitate agricultural production throughout North America has significantly altered the hydrology of landscapes compared to historical conditions. Drainage has transformed nutrient and hydrologic dynamics, structure, function, quantity, and configuration of stream and wetland ecosystems. In many agricultural regions, more than 80% of some catchment basins may be drained by surface ditches and subsurface drain pipes (tiles). Natural channels have been straightened and deepened for surface drainage ditches with significant effects on channel morphology, instream habitats for aquatic organisms, floodplain and riparian connectivity, sediment dynamics, and nutrient cycling. The connection of formerly isolated wetland basins to extensive networks of surface drainage and the construction of main channel ditches through millions of acres of formerly low-lying marsh or wet prairie, where no defined channel may have previously existed, have r...

Effects of Agricultural Drainage on Aquatic Ecosystems: A Review

Riverine nitrate N in the Mississippi River leads to hypoxia in the Gulf of Mexico. Several recent modeling studies estimated major N inputs and suggested source areas that could be targeted for conservation programs. We conducted a similar analysis with more recent and extensive data that demonstrates the importance of hydrology in controlling the percentage of net N inputs (NNI) exported by rivers. The average fraction of annual riverine nitrate N export/NNI ranged from 0.05 for the lower Mississippi subbasin to 0.3 for the upper Mississippi River basin and as high as 1.4 (4.2 in a wet year) for the Embarras River watershed, a mostly tile-drained basin. Intensive corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] watersheds on Mollisols had low NNI values and when combined with riverine N losses suggest a net depletion of soil organic N. We used county-level data to develop a nonlinear model of N inputs and landscape factors that were related to winter-spring riverine nitrate yields for 153 watersheds within the basin. We found that river runoff times fertilizer N input was the major predictive term, explaining 76% of the variation in the model. Fertilizer inputs were highly correlated with fraction of land area in row crops. Tile drainage explained 17% of the spatial variation in winter-spring nitrate yield, whereas human consumption of N (i.e., sewage effluent) accounted for 7%. Net N inputs were not a good predictor of riverine nitrate N yields, nor were other N balances. We used this model to predict the expected nitrate N yield from each county in the Mississippi River basin; the greatest nitrate N yields corresponded to the highly productive, tile-drained cornbelt from southwest Minnesota across Iowa, Illinois, Indiana, and Ohio. This analysis can be used to guide decisions about where efforts to reduce nitrate N losses can be most effectively targeted to improve local water quality and reduce export to the Gulf of Mexico.

http://www.epa.state.il.us/water/nutrient/presentations/sources_of_nitrate_yields.pdf

Sources of nitrate yields in the Mississippi River Basin.

Phosphorus (P) loss from agricultural fields and watersheds has been an important water quality issue for decades because of the critical role P plays in eutrophication. Historically, most research has focused on P losses by surface runoff and erosion because subsurface P losses were often deemed to be negligible. Perceptions of subsurface P transport, however, have evolved, and considerable work has been conducted to better understand the magnitude and importance of subsurface P transport and to identify practices and treatments that decrease subsurface P loads to surface waters. The objectives of this paper were (i) to critically review research on P transport in subsurface drainage, (ii) to determine factors that control P losses, and (iii) to identify gaps in the current scientific understanding of the role of subsurface drainage in P transport. Factors that affect subsurface P transport are discussed within the framework of intensively drained agricultural settings. These factors include soil characteristics (e.g., preferential flow, P sorption capacity, and redox conditions), drainage design (e.g., tile spacing, tile depth, and the installation of surface inlets), prevailing conditions and management (e.g., soil-test P levels, tillage, cropping system, and the source, rate, placement, and timing of P application), and hydrologic and climatic variables (e.g., baseflow, event flow, and seasonal differences). Structural, treatment, and management approaches to mitigate subsurface P transport-such as practices that disconnect flow pathways between surface soils and tile drains, drainage water management, in-stream or end-of-tile treatments, and ditch design and management-are also discussed. The review concludes by identifying gaps in the current understanding of P transport in subsurface drains and suggesting areas where future research is needed.

Phosphorus transport in agricultural subsurface drainage: a review.

The re-eutrophication of Lake Erie: Harmful algal blooms and hypoxia.

Mitigation options to reduce phosphorus losses from the agricultural sector and improve surface water quality: a review.

Outlined is a practical approach to size and modify agricultural drainage channels to two-stage geometry to maintain drainage function and capacity while increasing channel stability. Two-stage channel systems consist of an inset channel and small floodplain (benches) within the ditch confines. The two-stage channel sizing procedure includes nine steps: (1) project identification; (2) data collection; (3) data analysis; (4) hydrologic evaluation; (5) conceptual channel system sizing; (6) project assessment; (7) design and/or final sizing; (8) construction; and (9) monitoring and assessment of performance. Channel width and depth dimensions are determined based on a weight-of-evidence approach that considers geomorphology measurements at the project site and throughout the watershed. The authors have developed spreadsheet tools to aid in evaluating the geomorphology of one and two-stage channels. Constructing a two-stage channel requires more excavation than traditional ditch maintenance, but benefits include improved conveyance capacity, a channel geometry that will be more self-sustaining, and improvement to in-stream habitat.

Two-stage channel systems: Part 1, a practical approach for sizing agricultural ditches

Measured data were used to evaluate whether bankfull discharges were related to effective discharges for large rivers in Ohio. The frequency and sediment transport associated with these channel-forming discharges was also examined. Rural watersheds in the Midwest region of the U.S. are dominated by agricultural land uses that incorporate subsurface drainage improvements. Bankfull discharges were determined by measuring fluvial features at each USGS gage and then relating these features to the rating curve and historic daily discharge data for each gage. Effective discharges were determined by using suspended sediment data obtained at the gages, the Wolman-Miller method for calculating geomorphic work, and bin sizes based on stage intervals to group sediment and discharge data. There was good agreement between the effective discharge and bankfull discharge estimates. Bankfull and effective discharges were primarily related to flows that transported the middle 50% of the total sediment load. Recurrence intervals of the bankfull and effective discharges ranged from 0.3 to 1.4 years. These recurrence intervals are more frequent than generally reported in the literature. The duration of daily discharges that equaled or exceeded the channel-forming discharge ranged from 1 to 24 days annually, with mean values of 9 and 11 days for the bankfull discharge and effective discharge, respectively. Common methods for determining the recurrence interval are inadequate for frequent channel-forming discharges, and better insight is obtained by determining the number of days on which these flows are exceeded annually.

Evaluating channel-forming discharges: a study of large rivers in ohio

This paper presents case studies with two-stage channel geometries that were sized based on geomorphic principles. Geomorphic data were collected at the project site and throughout the watershed. Watershed specific regional curves were developed for each project. Factors considered in sizing each system were the widths and depths of the inset channels that were associated with the channel forming discharge, bench widths, and the side slopes of the banks of the second stage. The channel-forming discharges for the inset channels corresponded with appropriate recurrence intervals for the region. The approach leaves the preconstruction inset channel intact and constructs, or widens, benches at elevations consistent with channel-forming and regional curve concepts. Two-stage channel system construction was discovered to be easier than clean out operations for traditional trapezoidal channel maintenance. Overall, two-stage channel projects cost more than traditional channel maintenance, but they have an anticipated improved design life. Since construction, cross-section surveys were periodically made for two case studies. No elevation changes were found in the benches, but the inset channels have narrowed.

Two-stage channel systems: Part 2, case studies

Collaborated efforts exist to develop a practical procedure that can be used throughout the
Midwest to correctly size the fluvial channel and minimum bench widths for stable effective
discharge features in agricultural drainage ditches. The fluvial channel design dimensions are
determined by measuring the effective discharge features at the project site and making regional
curve measurements. Benches are the natural result of fluvial processes in most streams. The
bench acts as a floodplain within the ditch to dissipate energy, reduce the erosive potential of
high flow volumes, and reduce the shear stress on the bank toe. The construction of two-stage
ditches requires capital investment to create a wider surface width and more earth moving.
However, it is anticipated that two-stage ditches will be more self-sustaining, will create and
maintain better habitat, and will improve water quality. A case study is presented for a recently
constructed two-stage ditch in Michigan. An overview is provided of the design procedures that
included developing and then using a regional curve for a tri-state (Indiana, Michigan, and Ohio)
region. A spreadsheet procedure for evaluating two-stage agricultural ditches has been developed
by the Ohio Department of Natural Resources and the Ohio State University in cooperation with
county, state and federal agencies, faculty at other institutions in Ohio, Illinois and Minnesota,
and watershed conservancy groups.

Designing two-stage agricultural drainage ditches

Stream physical condition is increasingly a priority for resource managers. Assessment, monitoring and restoration techniques continue to be developed and standardized. A suite of spreadsheet tools, the STREAM Modules, has been developed by the Ohio Department of Natural Resources and Ohio State University to meet the technical demand. This ongoing project began in 1998 and includes the following modules that are available at no cost: 1) Reference Reach Spreadsheet for reducing channel survey data and calculating basic bankfull hydraulic characteristics, 2) Regime Equations for determining the dimensions of typical channel form, 3) Meander Pattern that dimensions a simple arc and line best fit of the sine-generated curve, 4) Two-Stage Geometry that sizes a two-stage channel system based on regional curves and provides estimates of earthwork requirements to construct various two-stage geometries, 5) Sediment Equations which includes expanded and condensed forms of critical dimensionless shear, boundary roughness and common bed load equations, 6) Effective Discharge that, based on historic data from USGS stream gages, develops a Wolman-Miller geomorphic work plot and contains several methods for calculating the geomorphic work and the effective discharge, 7) Lane Balance educational tool, and 8) Contrasting Channels that computes hydraulic and bed load characteristics in a side-by-side comparison of two channels of different user defined forms. An overview of each module will be presented together with example applications.

G. E. Powell

Papers

Two-stage channel systems: Part 1, a practical approach for sizing agricultural ditches

Evaluating channel-forming discharges: a study of large rivers in ohio

Two-stage channel systems: Part 2, case studies

Designing two-stage agricultural drainage ditches

Spreadsheet Tools for River Evaluation, Assessment, and Monitoring: The STREAM Diagnostic Modules