A Physically Based Surface/ Subsurface Flow Model to Assess the Impacts of Climate Change Extremes on the Hydrology of an Upper Midwest U.S. Watershed

Abstract:

Climate change is already affecting the Midwest U.S. Occurrence and intensity of extreme events such as heat waves, droughts and floods are expected to increase in the next few decades. It is the climate extremes, not averages, that have the greater impact on crop and livestock productivity which are vital for the State’s economy. Accordingly, potential changes in the hydrologic cycle under prospective climate conditions need to be addressed at the watershed scale for the Midwestern agricultural region to develop better management and adaptation solutions. For this purpose, the 3-D finite element model, HydroGeoSphere has been applied to and calibrated for a representative watershed in north-central Iowa, Tipton Creek watershed. The conceptual model for the watershed consists of all the elements of the hydrologic cycle from the ground surface through the Quaternary aquitard and into the underlying Mississippian limestone aquifer. Extreme wet and dry conditions derived from statistically downscaled climate model scenarios have been used as input to the basin model to simulate the impacts on streamflow and groundwater flow. The model accomplishes integrated hydrologic analysis by the coupled solution of the diffusion wave equation governing 2-D (areal) surface water flow and the Richards’ equation governing 3-D unsaturated/ saturated subsurface flow. Thus, actual evapotranspiration is calculated internally as a function of the soil moisture at each node of the defined evaporative zone at each time step and interdependent processes like recharge that are critical for climate change can be handled more accurately. Preliminary results for HadCM3 scenario combined with two SRES projections, A2 and A1fi predict more remarkable increases in stream levels in response to wet periods than the decreases in flows for dry periods in comparison to control (contemporary) period simulations. The impacts on the water table levels seem to be more prominent, in the range of ±4 m for dry and wet period simulations than for the potentiometric surface map changes that are 0-2.6 m of declines for the drought and up to 1.7 m of rise for the wet scenarios. The variations in the projected changes depend on the location throughout the model domain and the climate change scenario analyzed.