Changes in Duration, Frequency, and Magnitude of Low Flows in the Chesapeake Bay Watershed

Critical to nearly all water management decisions is the need to quantify and understand variability and change in low streamflows over time; yet, few studies have examined changes to low streamflows over regional to continental scales in the United States. A regional analysis of trends in various low streamflow statistics was applied to the Chesapeake Bay watershed, which has a diverse set of stakeholder and regulatory interests spanning nearly three decades. Trend analysis was performed on low streamflow statistics derived from mean-daily streamflow records at U.S. Geological Survey gaging stations with nearly complete record between 1940-2014. In this study, low streamflows were defined as streamflows in the entire record that are less than or equal to the 2-percent streamflow value calculated during a base period between 1940 and 1970. Streamflows below this threshold were used to derive an annual time series of the days below the threshold, average deficit, and scaled average deficit. The Mann-Kendall test for monotonic trend was applied to the above variables as well as to the annual 1-day and 7-day minimum flows. A regional Kendall test was then applied to the scaled average annual deficit and the scaled 7-day minimum flow to account for spatial correlation that may be present in the annual time series of low streamflow statistics across the region. Preliminary results generally show wetter low flow conditions in the northern upland part of the Chesapeake Bay watershed and dryer conditions in the southern lowlands of the watershed; however, assessing the drivers of change in low streamflows is complicated by the diverse physiography, land use, climate and population distribution within this watershed.