ANALYSIS OF LONG-TERM HYDROLOGIC RECORDS IN CHESAPEAKE BAY WATERSHED

The Chesapeake Bay (CB) watershed, with an area of 165,759 square kilometers (km²), extends from New York to Virginia. Previous work in the watershed identified a difference in stream runoff north of the Maryland-Pennsylvania border (north) relative to that south of that border (south). Previous work also identified that runoff from 1927 through 1969 (early) was different than runoff from 1970 through 2012 (late). For this study, daily stream discharge data for 27 streamgages in and near the Chesapeake Bay watershed were further analyzed for temporal and spatial patterns. To identify changes in temporal patterns, the 86-year discharge record was split into two 43-year intervals; for spatial patterns, the dataset was split at the previously identified north-south boundary. Daily discharge data for each watershed for each time interval were summed and binned by flow deciles, i.e., 90^th – <100^th, 80^th – <90^th, etc. The percent difference in each flow decile was calculated to compare the late interval relative to the early interval. The percent change for each flow decile for the north streamgages was compared to that of the south streamgages. Almost all percent differences were positive, i.e., flow was greater in the later interval relative to the early interval. For the three highest deciles (90^th – <100^th, 80^th – <90^th, 70^th – <80^th), the mean percent difference between the time intervals was larger in south than north. For the 60^th – <70^th decile, the mean percent difference between time intervals was equal in north and south. For all deciles less than the 60^th, the mean percent difference was greater in north than south. The spatial and temporal changes in flow deciles suggest that stormflow in the south has increased, whereas base flow in the north has increased in the later interval relative to the early interval. Results of this detailed analysis of historical flows in the CB watershed have implications for changes in aquatic biota and water quality over time.