Abstract
The exchange between estuaries and the coastal ocean is a key dynamical
driver impacting nutrient and phytoplankton concentrations and
regulating estuarine residence time, hypoxia, and acidification.
Estuarine exchange flows can be particularly challenging to monitor
because many systems have strong vertical and lateral velocity shear and
sharp gradients in water properties that vary over space and time,
requiring high-resolution measurements in order to accurately constrain
the flux. The Total Exchange Flow (TEF) method provides detailed
information about the salinity structure of the exchange, but requires
observations (or model resolution) that resolve the time and spatial
co-variability of salinity and currents. The goal of this analysis is to
provide recommendations for measuring TEF with the most efficient
spatial sampling resolution. Results from three realistic hydrodynamic
models were investigated. These model domains included three estuary
types: a bay (San Diego Bay), a salt-wedge (Columbia River), and a fjord
(Salish Sea). Model fields were sampled using three different mooring
strategies, varying the number of mooring locations (lateral resolution)
and sample depths (vertical resolution) with each method. The exchange
volume transport was more sensitive than salinity to the sampling
resolution. Most ($>$90$\%$) of the
exchange flow magnitude was captured by three to four moorings evenly
distributed across the estuarine channel with a minimum threshold of 1-5
sample depths, which varied depending on the vertical stratification.
These results can improve our ability to observe and monitor the
exchange and transport of water masses efficiently with limited
resources.