FLOOD RESPONSE TO CHANGING CLIMATE AND LAND COVER: DELAWARE RIVER BASIN

Climate change and urban expansion are expected to affect flood magnitude and frequency in the Delaware River Basin (DRB). Such impacts were investigated for five case-study sub-watersheds representing a range of scale, topography, land cover, and climate: Tremper Kill, NY; Pennypack Creek, PA; White Clay Creek, DE; and the Lehigh River at Walnutport and at Bethlehem, PA. The HEC-HMS rainfall-runoff model was calibrated for each watershed over a range of historic flood magnitudes using the curve number loss method and Muskingum routing. Model parameters and precipitation time series were then adjusted to reflect climate projections for 2030, 2060, and 2090. Future climate conditions included estimates of changes in rainfall and snowmelt based on two different assemblages of global climate models (moderate and high-emissions scenarios) and previous models of future Atlantic hurricane precipitation. Land cover projections included forecasts of land conversion from agriculture and forest to low-, medium-, and high-intensity developed land generated using the SLEUTH model. Flood model results across the full range of possible future conditions indicate that the magnitude of future floods could increase or decrease, with most future scenarios suggesting substantial increase in flood magnitude as watersheds respond to greater precipitation and expansion of urban land cover. Changes in flood magnitude vary with watershed scale, geographic location, flood recurrence interval, emissions scenarios, patterns of urban development, and time frame.