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Old drains, new challenges: Illinois engineers study base flow in tile-drained watersheds.

Tile-drainage systems are commonly used in many agricultural watersheds in the midwestern United States.

In central Illinois, tile drains and ditches installed in mainly flat watersheds in the early 1850s drain wetlands that once spent much of the growing season under water. Because the land is flat, rainfall forms ponds rather than causing run off. Without the tile-system drainage, most of these fields would be flooded for much of the growing season.

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Adding subsurface drainage systems to this area has created some of the most productive agricultural land in the country. However, with increased agricultural production comes a threat of environmental problems. Concerns about nitrate contamination and pesticide transport have surfaced in many agricultural watersheds. Tile drainage can be a conduit to quickly move dissolved nitrates from field to stream during or after rainfall. This outcome has raised concerns about contamination of drinking water supplies that receive a large quantity of water from tile drainage.

The Little Vermilion River (LVR) watershed in east central Illinois is an example of a watershed with altered hydrology from irregular tile-drainage systems. Researchers at the University of Illinois have been studying the hydrologic and water quality impacts of these tile-drainage networks. ASAE members Richard Cooke, J. Kent Mitchell and Mike Hirschi, faculty in the department of agricultural engineering, have contributed to this research.

The LVR watershed drains into the Georgetown Reservoir, which supplies drinking water to the local community. A continuous monitoring program on the reservoir has found nitrate-N and atrazine levels that often exceed drinking water standards. Extensive irregular-patterned subsurface-tile drains and ditches in the LVR watershed may inadvertently transport pollutants to the reservoir. Surface runoff rarely occurs in this watershed and most of the soil water is removed by the tile drainage systems. Classical surface-runoff models fail to represent the watershed's true hydrologic response.

Tile flow is the groundwater removed by the tile drainage system. Base flow is the groundwater that seeps through walls of an open channel or ditch. Because surface runoff is rare in the LVR watershed, these drainage components combine to form the total channel flow. Although a large amount of base flow infiltrates the ditches during a rainfall, the exact proportion of base to tile flow is unknown.

The irregular nature of the tile-drainage spacing complicates computational methods for determining the base and tile flow volumes on a field or watershed scale. If an irregular tile-drain network were unearthed and viewed from overhead, it would look like a tree branch sprouting outward from an open channel. Conventional overland flow models cannot represent the hydrologic impacts of irregular networks. Field-scale subsurface drainage models also cannot be applied to watershed-scale drainage networks. New approaches are needed to model these areas.

Hydrograph separation is a technique that could simplify hydrologic modeling of irregular drainage networks. Hydrograph separation has been used to divide base flow from surface runoff. However, no technique has been developed to partition base and tile flow components from total channel flow in a watershed with no surface runoff. A pilot study has been conducted to develop a hydrograph separation technique for this purpose.

Base flow estimation

University of Illinois researchers selected a study site by analyzing a remotely sensed tile-drainage map and chose a section of drainage channel with no tile outlet. They conducted a field investigation to verify that tile drainage made no contribution to channel flow within the chosen channel section. They also aimed to determine if the channel banks were high enough to prevent surface runoff into the channel.

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Two weirs installed 623 feet (190 meters) apart measured flow in and out of the channel section. A data logger and two depth probes at each weir recorded flow depth above the weir crest every 15 minutes. Tile flow from three tile-main outlets entering the channel origin 1,165 feet (355 meters) from the upstream location was monitored with stream flow. Rainfall was also recorded on site. From October 1999 through August 2000, 11 storms produced measurable hydrographs.

The researchers used total channel flow hydrographs produced by the storms to estimate average base flow per unit length of the selected channel section. They subtracted upstream from downstream flows and divided that by the distance between the upstream and downstream weirs to produce an average unit base flow. The assumptions that no surface runoff enters the channel section and evaporation is negligible simplify the hydrograph computation.

Base flow from the storms ranged from 66 to 100 percent of total channel flow with an average base flow value at 86 percent of total flow. These values mirror the 64 to 80 percent estimates of total flow values reported earlier in a Canadian study.

Hydrograph separation

The research led to a hydrograph separation technique that isolates base flow from total channel flow from rainfall. The method can estimate tile and base flow components from the total channel flow measurements in a tile-drained watershed.

The first step in developing the hydrograph separation technique was to choose a simple method that provided a best fit to the rising and falling limb of estimated base flow hydrographs. Researchers analyzed the estimated base flow hydrograph shapes for the 11 storms and determined that a triangular hydrograph would best suit the estimated values.

To match the average 86 percent base flow composition, the triangular hydrograph's apex was set to 86 percent of total flow. The hydrograph's remaining vertices were connected to the origin of the rising limb and a terminal point on the falling limb. This method can be applied to a continuous data set of channel flow measurements if a more precise estimate of the base-flow volume is desired.

The hydrograph separation technique must be validated with additional flow measurements at other locations in the watershed and in similar watersheds. However, the method may be used to separate tile and base flow contributions from a channel-flow hydrograph for a location when neither tile- nor base-flow measurements are available.

The method can estimate pollutant loads associated with tile- or base-flow components in a tile-drained watershed and help define criteria for limiting total maximum daily loads to water resource systems.

Ongoing research and benefits

Current research at multiple sites in the LVR watershed and a similar nearby watershed incorporate spatial variability of soil and hydraulic and land use conditions. The tests aim to provide an understanding of base flow under a range of land use conditions. These studies involve monitoring more water quality parameters including pH, specific conductance and dissolved organic carbon to assist in partitioning stream flow between base and tile flow.

Flow-based and chemical tracer-based separation will be studied for watershed-scale water quality analysis. The base flow estimation and separation method could be beneficial in simplifying water quality modeling in watersheds where base and tile flow are the only components of channel flow hydrographs.

This approach to water quality modeling may be an alternative to developing more complex models. The effort level required for data collection is minimized as channel flow data are readily available from various sources. Also, the modeling process can be simplified.

Once this hydrograph separation technique has been refined, it could be a useful addition to the water quality modeling toolbox.

ASAE member Kristopher Lander is a water resources engineer at Camp Dresser and McKee Inc., 9200 Ward Parkway, Suite 500, Kansas City, MO 64114, USA; 816-444-8270, fax 816-444-8232, landerks@cdm.com.

ASAE member Prasanta Kalita is an assistant professor in the agricultural engineering department at University of Illinois, 338 AESB, 1304 W. Pennsylvania Ave., Urbana, IL 61801, USA; 217-333-3570, fax 217-244-0323.
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Author:Lander, Kristopher; Kalita, Prasanta
Publication:Resource: Engineering & Technology for a Sustainable World
Geographic Code:1USA
Date:May 1, 2002
Words:1257
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