Riverine Discharges to Chesapeake Bay: Analysis of Long-Term (1927-2014) Records and Implications for Future Flows in the Chesapeake Bay Basin

Abstract:

The Chesapeake Bay (CB) Basin is under a total maximum daily load (TMDL) mandate to reduce nitrogen, phosphorus, and sediment loads to the bay, and identifying shifts in the hydro-climatic regime may help explain observed trends in water quality. To identify potential shifts, hydrologic data (1927-2014) for 27 watersheds in the CB basin were analyzed to determine the relationships among long-term precipitation and stream- discharge trends. The amount, frequency, and intensity of precipitation increased from 1910-1996 in the eastern U.S., with the observed increases greater in the northeastern U.S. than the in southeastern U.S. The CB Basin spans the north-to-south gradient in precipitation increases, and hydrologic differences have been observed in watersheds in the north relative to watersheds south of the Pennsylvania—Maryland (PA-MD) border. Time series of monthly mean precipitation data specific to each of 27 watersheds were derived from Precipitation-elevation Regression on Independent Slopes Model, and monthly mean stream-discharge data were obtained from U.S. Geological Survey streamgages. All annual precipitation log-linear trend slopes in the 18 watersheds north of the PA-MD border were greater than or equal to those of the nine south of that border; these results confirm and refine the location of the precipitation trends previously observed in the eastern U.S. The magnitude of the trend slopes in both precipitation and discharge over the period of record decreased in a north-to-south pattern. Distributions of the monthly precipitation and discharge datasets were assembled into percentiles for each year for each watershed. Correlation matrices of precipitation and discharge percentiles indicated no strong associations between them in any individual watershed. Examination of regional-scale average behaviors of the trends in the distribution of precipitation and discharge annual percentiles revealed differences between the northern and southern watersheds. In general, the linkage between precipitation and discharge was weak, with the linkage weaker in the northern watersheds compared to those in the south. On the basis of simple linear regression, 26 of the 27 watersheds are projected to have higher annual mean discharge in 2025, the target date for implementation of the TMDL for the CB Basin.