Pond Flushing and Residence Time
Pond Flushing and Residence Time:
These are two of many terms used to describe the exchange of water between an estuary and the ocean. The rate of exchange of water is crucial to the way a pond responds to nutrients entering the system from precipitation, stream flow, runoff and groundwater input. The underlying assumption is that the dissolved nutrients and even the living and dead organic particulate matter moves with the water and exits to the ocean at the same rate as the actual water itself.
In the strictest definition, flushing time defines the amount of time required to exchange the fresh water in an estuary for new fresh water. The fresh water entering the estuary on day 1 would move outward into the main body of the estuary moving in with the flood tide and out with the ebb tide until it is removed by an ebb tide into the ocean and not pushed back into the pond by the following flood tide. Formulas for calculating flushing time are probably not appropriate for use in our coastal ponds as they have so little stream input.
Residence time is the average age of a given water particle in an estuary. It is very similar to turnover time that is the amount of time needed to remove 63 percent of the water in a pond. Sixty three percent is the equivalent to 1/e1 that is an exponential term meant to capture the nature of the tidal exchange where there is mixing of new water and old water pushed back up into the pond by a flood tide. During each turnover time period, 63% of the water entering the pond on day 1 is removed. In the next turnover time unit, 63% of the remaining 37% that wasn’t removed in the first turnover is removed. At the end of three turnover time periods, 95% of the water starting on day 1 has been removed.
Ponds with short residence times are more likely to able to flush out nitrogen entering the system from sources such as groundwater discharge, streamflow, runoff and rainfall. In coastal systems where nitrogen is in short supply, phytoplankton growth may be suppressed by lack of available nitrogen. Short residence times not only flush out inorganic nitrogen entering the pond but they also remove nitrogen in the form of living and dead phytoplankton as well as their organic waste products. Where the residence time is longer, the inorganic nitrogen that enters the system contributes to the growth of not only one generation of phytoplankton but many as the nitrogen is passed on as waste products and by the decay of the earlier generations.
In general, pond systems with short residence times tend to be clearer as the phytoplankton production is lower and they have fewer wrack algae. Assuming equal nitrogen inputs, pond systems with long residence times will have more phytoplankton and be less transparent and may have more trouble with the growth of epiphytes and wrack algae.
Factors that effect the residence time include the tide range, the amount of freshwater input, the depth of the pond and its volume. In its simplest form, if a pond averages 3 feet deep and each tide brings in one foot of water, you might conclude that after three tides, the tide volume would have equaled the volume of the pond and all water at the start should have been removed. In reality, the circulation is messy, complicated by eddies and gyres as well as by a vertical separation between the new water flooding in and the old water. The result is that a good deal of the water in the pond at day one may begin to exit the pond with the ebb tide but, before it can exit, the tide turns and this water is pushed back up into the pond. This sloshing back and forth with the changing of the tides causes the water to exit the pond in a two steps forward and one step back pattern. This is why only about 63% of the water in the pond at the start is removed by the time one residence time has passed.
Calculated Residence Times: The process for estimating residence time is based on a 30-day tidal elevation data set that includes precise measurements at 10-minute intervals. From this record, we can calculate the average tide range. The average range multiplied by the area of the pond at mid-tide yields the volume of a tide or the tidal prism. The mid-tide volume of the pond must be determined as closely as possible. By dividing the tidal prism volume into the mean volume of the pond we calculate the number of days required for the tides to carry in and remove the total volume of the pond. Due to incomplete mixing, this number is not equal to the residence time. We estimate that 95 percent of the original water in the pond is removed to the ocean after three times this period. This number is then inserted into a formula prepared by the Buzzard’s Bay Program that yields a nitrogen-loading limit.
The residence times for our tidal coastal ponds are as follows: Menemsha Pond 3.2 days Tashmoo Pond 3.2 days Lagoon Pond (main arm) 8.8 days Sengekontacket Pond 2.3 days Cape Pogue 5.4 days Squibnocket Pond 354 days
The residence times for our south shore great ponds vary depending on whether they are connected to the ocean or closed to it. Where we know it, the two residence times are given: Tidal Time between inlets Edgartown Great 13 days 30 to 60 days Chilmark Pond 14.9 days 30 to 60 days Tisbury Great 12.6 days 30 to 60 days