CHESAPEAKE BAY WATERSHED MODEL SENSITIVITY TO OBSERVED CLIMATE CHANGE

The Chesapeake Bay (CB) Total Maximum Daily Load (TMDL) program drives water quality policy and management in parts of six states — Delaware, Maryland, New York, Pennsylvania, Virginia, and West Virginia — along with the District of Columbia to achieve water quality standards in the Bay through reductions in nutrient and sediment pollution. The Phase 5.3.2 (P532) CB Watershed Model was used as an accounting tool in the development of the TMDL to track progress and guide implementation of best management practices. Although the potential effects of climate change have not been taken into account in the CB TMDL, they will be examined in the 2017 Midpoint Assessment in accordance with the CB Executive Order (2009) which mandates assessment of the impacts of climate change.

Research shows that precipitation has increased in the US during the 20^th century, and rainfall events have become more intense and frequent in the upper percentiles of the distribution, especially over the last three to five decades (Karl and Knight 1998; Melillo et al. 2014). The Northeast has experienced a greater recent increase in extreme precipitation than any other region in the United States (Melillo et al., 2014). Projections from global climate models suggest that these trends are anticipated to continue over the next century.

In this research, historical precipitation data in the CB watershed over the last three decades was analyzed. Seasonal trends of precipitation intensity in the upper 10^th percentile were statistically significant and the slopes of trends were steepest for the summer and fall seasons. Regression slopes were used to develop rainfall projections and used as forcing data in the P532 model. Simulated runoff and sediment loss were incorporated into Annual Phosphorous Loss Estimator (APLE) model to estimate the model sensitivity to rainfall projections. This study will be used in the 2017 Midpoint Assessment to guide management actions related to climate change.