Design, Construction, and Assessment of a Self-Sustaining Drainage Ditch.

摘要

Agricultural drainage is a double-edged sword: helping farmers achieve ever-increasing 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 cross-sectional 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 (