Estimation of net physical transport and hydraulic residence times for a coastal plain estuary using box models

A box model based on salinity distributions and freshwater inflow measureme nts was developed and used to estimate net non-tidal physical circulation a nd hydraulic residence times for Patuxent River estuary, Maryland, a tribut ary estuary of Chesapeake Bay. The box model relaxes the usual assumption t hat salinity is at steady-state, an important improvement over previous box model studies, yet it remains simple enough to have broad appeal. Average monthly 2-dimensional net non-tidal circulation and residence times for 198 6-1995 are estimated and related to river flow and salt water inflow as est imated by the box model. An important result is that advective exchange at the estuary mouth was not correlated with Patuxent River flow, most likely due to effects of offshore salinity changes in Chesapeake Bay. The median r esidence time for freshwater entering at the head of the estuary was 68 d a nd decreased hyperbolically with increasing river flow to 30 d during high flow. Estimates of residence times for down-estuary points of origin showed that, from the head of the estuary to its mouth, control of flushing chang ed from primarily river flow to other factors regulating the intensity of g ravitational circulation.