Changes in Center Time of Spring Discharge and Precipitation in Chesapeake Bay Watershed, 1927-2012

Abstract:

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 a temporal difference in runoff, with data from 1927 through 1969 (early) behaving differently than data from 1970 through 2012 (late). Spring center time (CT) of runoff or precipitation is defined as the flow-weighted timing or “center of mass” of water. We analyzed spring CT for 18 north watersheds (watershed areas of 303-62,418 km²) and nine south watersheds (watershed areas of 572-24,996 km²) and compared the results of the two time intervals. CT of runoff shifted by 1.4 days earlier in the north but 1.6 days later in the south during 1970-2012 relative to 1927-1969. In contrast, CT of precipitation shifted by 1.2 days (north) and 0.9 day (south) later for the same time intervals.

The percent of annual precipitation that fell as snow in the north was less in the later interval (56%) relative to the early interval (62%). In addition, air-temperature data for the north indicates that mean air temperature for January through May was warmer in the later interval (2.2 degrees C) relative to the early interval (2.0 degrees C). As a consequence of less snowfall and warmer air temperature in the north, CT of runoff has shifted to earlier in the season; had CT of precipitation not been later for the same time interval, CT of runoff likely would have been even earlier. In contrast, in the south, lacking the confounding influence of snowpack, CT of both runoff and precipitation has shifted to later in the season. These seasonal changes in the timing of precipitation and stream runoff have potential implications for aquatic biota and water quality in streams in the CB watershed.