MODELING THE IMPACTS OF DEVELOPMENT AND CLIMATE CHANGE ON GROUNDWATER DISCHARGE TO A STREAM

Many rural areas are subject to development for residential, commercial, and industrial land uses, with an increase in impervious area and a reduction in recharge. The impact of these land use changes on groundwater flow can be simulated using numerical models, but the model results are only as good as the input data. Recharge is particularly difficult to quantify because until recently, models to estimate the spatial and temporal variability of recharge were not readily available. We used a Soil Water Balance (SWB) model to estimate spatially variable rates of groundwater recharge in a 138 km² watershed in central Wisconsin. The SWB model incorporates topography, soil properties, and land use to simulate recharge rates. The recharge rates were used as input into a calibrated groundwater flow model to study how base flows and heads respond to changes in land use and climate. We simulated the effects of different percentages of residential and commercial land use, increased precipitation and drought.

Our results indicated that a large percentage (75%) of the watershed could sustain development before substantial changes in discharge to the stream occurred. However, considerable decreases in head occurred when as little as 25% of the watershed was developed. As expected, model results also showed that as precipitation increased, groundwater discharge to the stream increased and that drought conditions resulted in decreased discharge to the stream. The low impact of development on discharge was unexpected and may be due to how the SWB model routes water across the land surface. In the SWB model, precipitation that falls in developed (i.e. impervious) cells may infiltrate in adjacent undeveloped cells. This might not happen in the real world, where water would be captured in storm sewers and not infiltrate. Although simulating the impacts of development on groundwater flow can be a valuable tool for planners and regulators, using the SWB model to simulate recharge for this purpose may lead modelers to underestimate the effects of development.