Hydrodynamic modeling of salinity habitat changes with reduced submarine groundwater discharges in a spring-fed estuary

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Effects of reduced submarine groundwater discharge (SGD) on salinity habitats were studied for a narrow spring-fed estuary. A laterally averaged model was used to simulate circulations, salinity transport processes, and thermal dynamics in the estuary. It was calibrated and verified against measured water elevations, salinities, and temperatures at five stations during November 2014–August 2017, before it was used to evaluate effects of SGDs on salinity habitats during a 125-month period. Salinity habitats considered and analyzed here included water volumes, bottom areas, and shoreline lengths for salinity ≤ 1, 2, 3, 5, 10, and 15 psu.It is found that the long-term (hourly) and short-term (monthly or longer) responses of salinity habitats to the SGD reduction are different in the estuary. Long-term average salinity habitats have a linear relationship with the SGD reduction. Except those for salinity ≤ 1 psu, long-term oligohaline salinity habitats in the estuary are more sensitive to the SGD than salinity habitats with mesohaline salinity zones being included. Long-term average water volumes and bottom areas of ≤2 psu are most sensitive to the SGD and they can be reduced by 1.38% for every 1% of SGD reduction. The short-term response of salinity habitats is greatly affected by special physical features in the estuary, and the sensitivity of a salinity habitat to the SGD varies with the location of the isohaline in relationship with these physical features. For example, when the 2 and 3 psu isohalines pass a narrow transect in the upstream area of the estuary, ≤2 and 3 psu salinity habitats become much more sensitive to the SGD than other locations of the isohalines. On the other hand, when the 5 and 10 psu isohalines enter the downstream distributaries, ≤5 and 10 psu salinity habitats are most sensitive to the SGD.