Simulating the Impacts of Projected Climate Change on Streamflow Hydrology for the Chesapeake Bay Watershed

A gridded hydrologic model was developed to simulate the hydrology of the Chesapeake Bay watershed (CBW), the largest estuary in the United States. The Bay has one of the most productive ecosystems in the world, supporting over 2700 plant and animal species and producing over 227 million kg of seafood per year. The watershed that provides freshwater to the Bay is 166,534 km2, encompasses parts of 6 states and has over 100,000 creeks, streams, and rivers. More than 17 million people live in the CBW and urban and agricultural land uses occupy significant areas of the CBW. Consequently, point and nonpoint source water pollution is one of the major issues facing the Bay.

The hydrologic model consists of 1128 grid cells (1/8° resolution) and operates on a monthly time step from 1950-2099. The model is driven by temperature and precipitation inputs and produces outputs of rainfall, snowfall, snow water storage, snowmelt, potential and actual evapotranspiration, subsurface moisture, and runoff. Gridded runoff is aggregated by subwatershed to simulate streamflow. The model was validated using historical hydrologic observations from 21 sub-watersheds covering a range of elevations, latitudes, and sizes. Of the 273 index of agreement (D) values that were calculated for the validation, 87% had a value above 0.90 and 69% had a value above 0.95.

To assess the impact of projected climate change on the CBW, 346 downscaled CMIP3 and CMIP5 climate projections encompassing a range of emission scenarios and concentration pathways were used to drive the hydrologic model to assess changes in future streamflow and watershed-wide hydrology. Annual average temperature for the CBW is projected to increase 1.9 to 5.4°C by 2080-2099 compared to 1950-1999. The warming is projected to be greatest during the summer and fall. More warming is projected to occur in the northern part of the CBW and this north-south gradient is most pronounced during winter. Annual total precipitation for the CBW is projected to increase between 53.8 mm (5.2%) and 157.4 mm (15.2%) by 2080-2099. The largest increases generally occur during winter and, to a lesser extent, spring. Large spatial variability in precipitation across the watershed exists depending on the climate projection and season. Based on the temperature and precipitation projections, average potential evapotranspiration, actual evapotranspiration and rainfall values across the CBW generally increased over the coming decades, while snowfall, snow water storage, and snowmelt decreased. Subsurface moisture decreased during the warmer months when atmospheric demand is greatest and remained relatively unchanged and near saturation during the cooler months. The time to recharge subsurface moisture to capacity increases and by 2080-2099 under the highest concentration pathways never actually occurs.

Average percent change in runoff for the different emission scenarios and concentration pathways ranges from -1.1 to 3.3% (0% average) for 2020-2039, -6.6 to 4.4% (-1.5% average) for 2050-2069, and -13.5 to 3.9% (-5.1% average) for 2080-2099. Assuming equal weight to each climate projection, there is a 48%, 52%, and 60% chance that annual runoff for the entire CBW will be less than 1950-1999 values in the years 2020-2039, 2050-2069, and 2080-2099 respectively. For 2020-2039 the distribution of projected runoff changes is nearly equally distributed between increases and decreases. By 2080-2099, extreme decreases in runoff are much more likely than extreme increases. Averaged over all climate scenarios, 17% of the model predictions show decreases in runoff greater than 20%. This rate is over 3 times greater than the chance that runoff will increase by over 20%. From August to December, runoff generally decreases. From January to March, runoff generally increases. From April to July, change in runoff varies depending on scenario. The most apparent spatial pattern in runoff is the increase in winter runoff in the northern and higher elevations of the watershed.

Given the Chesapeake Bay's size and economic importance as well as the historic and projected future concerns about its health, it is critical to assess both the historic and future hydrology of the CBW. Nutrient and sediment loads carried to the Bay via streamflow have been and will continue to be one of the major environmental issues associated with the region. The present study provides insight into future streamflow conditions that may be evaluated when considering the implementation and modification of future Total Maximum Daily Loads for the Bay. Additionally, the gridded nature of the hydrologic model used in the study allows for a higher resolution examination of activities (e.g. forestry, agriculture, resource extraction, tourism) that may be impacted by changing hydrologic conditions across the watershed.