Paper No. 5
Presentation Time: 9:00 AM-6:00 PM
ASSESSING THE HYDROLOGIC IMPACT OF TILE DRAINAGE IN IOWA USING A PHYSICALLY BASED, COUPLED SURFACE WATER/GROUNDWATER MODEL
Recent large-scale flood events in Iowa demonstrate a need to understand how drainage alteration, specifically through tile drainage and ditches related to intensive agriculture, affect peak flow events and streamflow hydrology. We are investigating these relationships in the South Fork watershed in north-central Iowa using the physically based, coupled surface water/groundwater model, HydroGeoSphere. The model will represent all flow partitioning mechanisms, simulate the effect of agricultural drainage on peak flow events, and quantify spatial and temporal heterogeneous water fluxes by simultaneously solving the flow and transport equations in surface, tile drain, and groundwater flow pathways and the exchanged fluxes between these systems. In addition to more standard calibration techniques (i.e., hydraulic head, streamflow), stable isotopes of water (δ2H and δ18O) will be used to estimate the contributions of tile drainage and groundwater to streamflow under varying flow conditions. Contributions to peak flow by tile drainage will be estimated by hydrograph separation using the simulated hydrographs produced by the model. Preliminary results for September 2011 to March 2012 suggest δ18O ranges of -13.91‰ to -8.95‰ for tile drainage water, -10.20‰ to -7.09‰ for groundwater, and a mean precipitation value of -7.99‰. The slope of the local meteoric water line (LMWL) from precipitation is δ2H = 7.85•δ18O + 6.21. The drought of 2012 has also affected stream water isotopic composition. Samples from June and July 2012 trend away from the LMWL along a slope of 4.5 – indicative of evaporation. The goal of this research is to describe, through modeling of the watershed pre- and post-alteration of drainage, how the hydrologic behavior of the South Fork watershed has been altered by drainage systems. The results will inform water resource and land management practices and suggest potential control measures for future peak flow events.