Delineation of Areas Potentially Drained by Tile Drains and Updating of Streamflow-Trend Statistics from the Elkhorn River Basin in Eastern Nebraska

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The Elkhorn River Basin is located in northeastern Nebraska and is situated within two distinct landforms—the migrating sand dunes to the west and the windblown loess deposits to the east. The topography is characteristic of many basins in Nebraska, with a relatively flat surface near the stream transitioning to rolling hills and dunes or loess deposits at the basin margins. The sustainability of water resources in the Elkhorn River Basin, Nebraska is a critical issue. The understanding of streamflow trends is important to the determination of sustainable use of surface water and groundwater in the basin.  

For this study, we are looking to answer two primary questions:

1. Can remote sensing methods be used to map tile drains in the Elkhorn River Basin?
2. What affect do tile drains have on streamflow trends during irrigations seasons and periods of low flows in the Lower Elkhorn River? 

To answer these questions, we will

1. Determine if the use of remote sensing methods is a practical option to map tile drains in the Elkhorn River Basin by testing the method on a small area of the basin.
2. Update the streamflow statistics and trends from 2005 to 2009 at 8 streamflow-gaging stations: Platte River near Duncan (06774000), Platte River at North Bend (06796000), Elkhorn River at Neligh (06798500), Logan Creek near Uehling (06799500), Maple Creek near Nickerson (06800000), Elkhorn River at Waterloo (06800500), Salt Creek at Greenwood (06803555), and Platte River at Louisville (06805500).
3. Determine and demonstrate a method to measure the quantity of water that flows in tile drains to streams in the basin.

Use of Remote-Sensing Methods to Map Tile Drains

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The first step in this study was to use a decision tree classification (DTC) to delineate areas that are likely to be drained by tile drains following the guidelines described by Naz and Bowling (2008). A DTC along with Geographic Information System (GIS) coverages of land use, soil-drainage class, surface-slope data, and a set of three rules was used to determine the areas potentially drained by tile drains in the Elkhorn River Basin. Areas highlighted by the DTC were investigated for the presence of tile drains.  In addition, area farmers, LENRD employees, Conservation and Survey Division employees, and other knowledgeable individuals in the areas highlighted by the DTC process were surveyed to determine if any known tile drains are present.  

We used aerial and satellite photography, along with an edge-detection technique to map tile drains within the areas highlighted by the DTC. The edge-detection technique achieves the best results with 1-meter resolution photography collected within several days of a rainfall event of at least 1 inch.  The timeliness of the photography is important because soils above the tile drains dry more quickly than soils in the surrounding areas and will be displayed as different colors of soil. These color differences can be detected in aerial photography and mapped out using an edge-detection filter and GIS.  Once the best-suited photography was selected, an edge-detection filter was applied to map the tile drains.  

Streamflow Statistics and Trends

The streamflow trends in the report “Trends in Streamflow Characteristics of Selected Sites in the Elkhorn River, Salt Creek, and Lower Platte River Basins, Eastern Nebraska, 1928-2004, and Evaluations of Streamflows in Relation to Instream-Flow Criteria, 1953-2004” were updated with data from water years 2005-2009. Mean-annual discharges were determined for each of the past 5 years for each gage and mean-monthly discharges were updated for June, July, August, and September of each complete water year of gage operation. Daily values were reformatted and converted to watershed data management (WDM) format using the program Input and Output for a Watershed Data Management (IOWDM).  The program Surface Water Statistics (SWSTAT) was used to analyze data for trends. Kendall’s tau test was used to test for the presence of temporal trends in mean-annual discharge, and mean-monthly discharge for June, July, August, and September. A p value threshold of 0.01 was used to determine whether possible trends are significant.

Low-Flow Trends

SWSTAT was used to update the 1- and 7-day low-flow discharge magnitude and time period for each water year at each gage based on mean-daily discharge data.  Kendall’s tau test was used to determine the presence of trends in low-flow data.  Histograms were generated to summarize the number of water years that 1- and 7-day lowest flow for the year occurred in each calendar month.  Flow duration curves were generated for June, July, August, and September.  Additionally, duration hydrographs for each gage were generated in SWSTAT depicting the flow exceedance percentiles (for example, 90th percentile) and related flow for each day of the year at each gage to identify low-flow seasons over the long term at each gage.

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Measuring Water that Flows in Tile Drains to Streams

USGS and the LENRD determined and demonstrated a method to measure the quantity of water that flows in tile drains to streams in the basin. This method helps with understanding how tile drains are affecting the streamflow trends in the Elkhorn River Basin. We focused on investigating mechanisms other than precipitation and groundwater withdrawals that may affect streamflow, particularly during low-flow period, by reviewing literature, contacting other agencies that have experience measuring tile-drain discharge, and testing different methods for effectiveness.