1a. Objectives (from AD-416):
Three specific research objectives are proposed: (1) to determine the magnitude of specific hydrologic pathways of streamflow under runoff and baseflow periods at various spatial and temporal scales in an agricultural claypan watershed, (2) to understand how the hydrologic pathways control the transport of nutrients and herbicides to streams at those temporal scales and quantify the contributions of nutrients and herbicides from various hydrologic pathways (i.e. source waters) to streamflow, and (3) to develop and test a quantitative model for evaluating the impact of changes in land use and catchment scales on nutrient and herbicide transport.
1b. Approach (from AD-416):
To achieve the research objectives, two field experiments are needed to characterize the hydrologic pathways and their impact on contaminant transport. One experiment supports Objective 1 through a combination of meteorological measurements and source water samples to characterize the chemical contents of stream water, soil water, groundwater and precipitation. A key outcome of Objective 1 will be the development of a mixing model that accurately captures the relative contribution of the different hydrologic pathways (runoff, interflow, and groundwater recharge). This model will be then be used in conjunction with the Soil Water Assessment Tool model to look at the impact of scale and changing land use on contaminant transport. The other field experiment supports Objective 2 by specifically investigating the role of interflow on contaminant transport by directly measuring the quantity and quality of interflow. For Objective 3, the results of mixing models will help to develop a conceptual framework for SWAT and calibrate some of its parameters. The sensitivity analysis of the Soil and Water Assessment Tool model (SWAT) helps validate the hydrologic controls of nutrients and pesticides in stream water. SWAT will then be used to simulate stream water quality (nutrients and herbicides) with changes in crops and catchment scales.
3. Progress Report:
In FY13, a suitable site for monitoring interflow was found. The site has a periodically flowing seep spring in which water diffusely resurges through an existing cattail wetland. Upland areas to the south and west of the site, presumed to be the recharge area to the spring, are cropped to corn and soybean. The field immediately adjacent to the site was planted to corn in 2013. The landowners granted permission to conduct the work, and monitoring of the site began in February 2013 with the collection of bi-monthly grab samples from the spring. Surface monitoring equipment for automated sampling of the spring has been installed at the site, and samples will be collected on an equal time interval basis. A small flume and stilling well were installed to permit measurement of the spring flow. A rain gage was also installed near the flume. Nine time domain reflectrometry (TDR) soil moisture probes were installed to a depth of 10 cm around the site. Seven of the TDR probes encompass the cattail wetland area, one is near the center of the wetland, and another is located about 100 m to the south. The probes measure volumetric water content and temperature at 5-minute intervals. Currently, soil cores are being collected at the site in preparation for installing piezometers and wells. The cores are being collected along a catena sequence from the shoulder to the toe slope positions at the edge of the adjacent corn field. The cores will be used to determine the depth to the claypan and for general characterization of the soils at the site. Ten to 15 piezometers will be installed and screened to just above and below the claypan to isolate interflow and characterize its water quality across the landscape. Two to three deeper wells (~3 m) will be installed to allow for sampling of water that may resurge into the wetland along deeper groundwater flow paths. On-going collection of source water samples from groundwater, rain water, soil-water, and runoff have continued in FY13. Sites include the ARS Centralia Field 1 site (ground, rain, runoff, and soil-water samples), eight other raingages throughout Goodwater Creek Experimental Watershed, and the three Goodwater Creek weir sites. Samples have been analyzed for commonly occurring cations (calcium, magnesium, potassium, etc…) and analyses of stable hydrogen and oxygen isotopes has begun. With the installation of the interflow site described above, samples from the seep spring, interflow piezometers, and groundwater wells at this site will also be included in these analyses. Preliminary results based on samples collected in 2011 and 2012 estimated that 40-50% of the flow in Goodwater Creek was derived from surface runoff, 15-20% was interflow, and the remaining 30-45% was from groundwater recharge. These results were presented at the American Water Resources Association meeting on Agricultural Hydrology and Water Quality held in St. Louis, MO from March 25-27, 2013. Additional samples collected under a broader range of flow conditions in FY13 plus inclusion of the interflow site will help to refine the source water estimates.