Impact of Urbanization on the Hydrology of the Pocono Creek Watershed: A Model Study
Final Report (69 pp, 1,957 Kb) September 2006
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The Pocono Creek watershed located in Monroe County, PA, is threatened by high population growth and urbanization. Of concern specifically is the potential impact of future developments in the watershed on the reduction of base flow and the consequent risk of degradation of wild brown trout habitats in Pocono Creek. Anticipated increase in imperviousness, on the other hand, is expected to elevate flood risk and the associated environmental damage. A watershed hydrology based modeling study was initiated by the U.S. EPA in collaboration with the U.S. Geological Survey and the Pennsylvania Fish and Boat Commission to assist Monroe County in planning for sustainable future development in the Pocono Creek watershed.
The Soil and Water Assessment Tool (SWAT) is selected to model the impact of projected future build out in the Pocono Creek watershed on the hydrologic response thereof. The model is successfully calibrated and validated for two sources of precipitation data, raingauge and Next Generation Weather Radar (NEXRAD) hourly precipitation data. The results clearly show that NEXRAD is an effective and economic alternative source of spatiotemporal precipitation, and that future modeling studies in ungauged watersheds may benefit from the use of NEXRAD rainfall data.
Ensemble model forecast is constructed using time series analysis and Monte Carlo (MC) simulations to evaluate model predictive uncertainty. The MC simulations over a 20-year long period yielded an ensemble of rating curves of which the median and 95% confidence band of daily streamflows are plotted. These plots allow for the construction of the 95% confidence band for design flows corresponding to any given recurrence or return period. SWAT simulated daily streamflow rates in the range 2 to 11 (m3/s) show the least uncertainty. Computed daily streamflow rates below 2 m3/s have the greatest uncertainty, whereas for flows higher than 11 m3/s uncertainty is moderate.
MC simulations over a 20-year period predict that, on the average, daily base flow would be reduced by 31% based on the projected build out in the watershed. However, predicted change in the average of daily streamflow (averaged over the entire simulation period) appears to be negligible. The computed low-flow index, 7Q10, is expected to decline by 11%, and the monthly median daily flow is expected to be reduced by 10% on the average. The monthly peak of simulated daily flows and annual maximum daily flow on the average are predicted to increase by 21% and 19%, respectively. Watershed-averaged groundwater recharge is predicted to decline by 31% due to the projected land use changes. The median of the MC simulated flow duration curves shows that in general the likelihood that the watershed will experience high and low streamflows will increase with the projected urbanization.
An index methodology is developed to rank seven subwatersheds composing the modeled portion of the Pocono Creek watershed based on their relative impact on watershed response to anticipated land developments. The first index, a, signifies the absolute impact of a particular catchment area on the watershed response. The second index, ß, is a normalized by the percentage area of the sub-catchment, and therefore describes the impact per area of land use changes. With a few exceptions, a and ß indices produce similar rankings among the 7 catchment areas for 7Q10, monthly median of daily flow, and annual maximum daily flow. These ranking results may be related to groundwater recharge, area, topographic features, and proximity to the streamflow gauge station. The very downstream catchment area 7 ranked first in terms of impact on annual maximum daily flows, and second in terms of impact on 7Q10 and monthly median daily flows. Catchment area 4 associated with the highest groundwater recharge was ranked first and second for impact on 7Q10 based on a and ß indices, respectively. Areas characterized by steep topography and intense wetlands ranked low, some times the lowest, with respect to impact on the three design flows.
The results of this model study point toward significant changes in low as well as high flow regimes, should the Pocono Creek watershed experience land use changes consistent with the projected build out in the watershed. Management measures may be taken in the future to minimize the predicted changes in the watershed hydrology.