WATER QUALITY IMPROVEMENT FROM MANAGEMENT PRACTICES IN AGRICULTURAL WATERSHEDS
Location: Agroecosystems Management Research Unit
Title: Drainage water management for reducing nitrate losses from tile drained fields
Submitted to: Meeting Abstract
Publication Type: Other
Publication Acceptance Date: January 26, 2011
Publication Date: October 1, 2010
Citation: Jaynes, D.B., Malone, R.W., Thorp, K.R. 2010. Drainage water management for reducing nitrate losses from tile drained fields. In: Rudebusch, L., (ed.) Getting into soil and water: 2010. Iowa Water Center, Ames, IA. p. 28-30.
Excessive nitrate in surface waters increases the costs of water treatment for domestic use and each summer drives the formation of a hypoxic or low oxygen zone in the Northern Gulf of Mexico. Much of the nitrate in surface waters comes from artificially drained row crop land in the Midwest. There are over 7.8 million ac of drained lands in Iowa alone with an additional 43.8 million acres in the Midwest. Research in Iowa and surrounding states over the past 10 yr has clearly demonstrated that N fertilizer management alone cannot reduce nitrate losses from drained fields sufficiently to meet water quality goals, thus additional methods to reduce nitrate losses are needed. Drainage water management (DWM) is a promising technology for reducing nitrate losses from artificially drained fields. While there is an extensive history for the practice in North Carolina, little is known about the efficacy or cost effectiveness of the practice under Midwest conditions.
DWM differs from conventional free artificial drainage in that a control structure such as a flashboard riser is installed at the drainage outlet allowing the farmer to manage the field’s drainage. By setting the elevation of the riser, the depth of the water table can be adjusted whenever drainage is occurring. When using DWM, the drain outlet is typically set just below the soil surface during the winter or off-season when a high water table within the field would not hinder agricultural activity or crop growth. During planting and harvesting, the outlet is set to the depth of the tile drain to give maximum drainage for good trafficability and seed bed tilth. The option also exists to manage the water table during the growing season by raising the outlet within a few feet of the surface to retain some water in the field that would otherwise drain and have the water available for crop uptake – potentially increasing crop yields. DWM is best suited for fields that are flat, with slopes less than 0.5% so that a control structure can control the water table within 1 or 2 feet of elevation for at least 20 acres and where pattern drainage systems have been installed.
In earlier studies, DWM has been found to primarily reduce the annual amount of water discharged at the drain outlet rather than lower the concentration of nitrate in the drainage. The reduction in discharge also reduces the loss of agricultural chemicals such as nitrate dissolved in the water. Reductions observed outside of the Midwest have ranged from 30 to 50% and it has been estimated that DWM is being used on as much as 1.98 million acres in the U.S. However, little is known of the potential for this practice to reduce nitrate contamination of Midwest Rivers.
With the support of an NRCS Conservation Innovation Grant, the effectiveness of DWM is being investigated across Ohio, Indiana, Illinois, Minnesota, and Iowa. Results from one Iowa farmer’s field with parallel tiles with and without control structures illustrate the potential benefits of DWM. The nitrate losses in the tiles using DWM were numerically lower than in conventionally drained tiles in every year. The differences were not statistically significant (P = 0.05) in any year, but across all four years DWM significantly reduced nitrate losses in tile drainage by more than 40 kg/ha compared to conventional drainage.
Average yields for the DWM treatment were higher in 2006, 2007, and 2009 than for the conventional drainage (CNV). However, only in the soybean years (2007 and 2009) were the yield differences by drainage significant (P = 0.05). In 2008, DWM actually resulted in about a half a bushel per acre lower yield on average than CNV drainage. Relatively wet weather during the 2008 growing season may have negated any advantage DWM would have had for conserving water.
Based on these results and previous field studies, we have investigated the water quality potential for DWM if widely adopted across the Midwest. Using the comprehensive agronomic model Root Zone Water Quality Model, we estimated the potential for DWM to reduce nitrate in streams across the range of climate and agronomic practices of the Midwest. Using STATSGO soils data and the National Land Cover Database land cover information, we showed that DWM might be suitable on about 4.8 million ha of land used to grow corn within the Midwest. If DWM were adopted on all of this land, nitrate losses in tile drainage could be reduced by approximately 83 million kg/yr. For comparison the entire Mississippi River transports about 813 million kg of nitrate each year so while DWM could potentially remove a sizeable amount of the annual nitrate load, it would be only a fraction of the load being transported by the Mississippi River.
Drainage water management appears to be a viable practice of reducing nitrate losses from field tiles entering surface waters in the Midwest in some landscapes. However, research to date does not show a sufficient yield benefit to warrant by itself adoption by farmers. Wide spread adoption of the practice for water quality improvements will only result if the practice is cost shared using public monies. DWM is currently part of the cost share program for the USDA’s Mississippi River Basin Healthy Watersheds Initiative which should help spur the installation of the practice across the Midwest.