Agricultural drainage is a double-edged sword: helping farmers achieve everincreasing
crop yields to meet consumers’ demands, while providing a short-circuit
through the soil profile for excess water and nutrients. Drainage ditches are an
important pathway as water moves downstream in headwater landscapes. As low
order streams, ditches have the potential to remove and assimilate nutrients. In order
to operate at their maximum nutrient removal potential, ditches should be healthy,
self-sustaining ecosystems that function similarly to natural streams.
The two-stage agricultural drainage ditch is an innovative solution for creating
drainage ditches that behave more like natural streams. A low-flow channel provides
sediment transport during low-flow periods, while benches within the ditch allow for
overbank flow and energy dissipation during high-flow periods. The larger crosssectional
area increases surface contact between water and the ditch at certain flow
depths, which likely enhances nutrient removal.
In this study, a two-stage agricultural drainage ditch was designed and then
constructed in southern Minnesota, USA in the autumn of 2009. Extensive monitoring
of the ditch has been conducted following construction; efforts have focused on
establishing an understanding of the geomorphic, water source, and water quality
aspects of the ditch. Analysis of field measurements from August 2010 show that between 10 and 15 percent of nitrate N entering the ditch was removed within the ditch reach. A slight increase in average channel thalweg elevation has been
measured, while increased pool-riffle sequencing has also been observed. Channel
cross-sectional surveys have showed slight changes in low-flow channel dimensions.
Economic analyses have been performed to measure the feasibility of two-stage ditch
construction. There are situations where predicted cost reductions in periodic ditch
maintenance provide enough savings to offset two-stage channel construction costs.
In other cases, subsidies may be required to economically justify a two-stage system.
An analysis was performed to estimate the cost of additional nitrogen (N) removal
($/kg N removed) in two-stage ditches, using increased N removal as a basis for
subsidies. Results show situations where N removal costs is less than $3 to $4 kg-1 of
N removed, which is competitive with other Best Management Practices.