Seasonal patterns in the soil water balance of a Spartina marsh site at North Inlet, South Carolina, USA

Time series of ground-water head at a mid-marsh site near North Inlet, South Carolina, USA can be classified into five types of forcing signatures based on the dominant water flux governing water-level dynamics during a given time interval. The fluxes that can be recognized are recharge by tides and rain, evapotranspiration (ET), seepage into the near surface soil from below, and seepage across the soil surface to balance either ET losses or seepage influxes from below. Minimal estimates for each flux can be made by multiplying the head change induced by it by the measured specific yield of the soil. These flux estimates are provide minimal values because ET fluxes resulting from this method are about half as large as those estimated from calculated potential evapotranspiration (PET), which place an upper limit on the actual ET. As evapotranspiration is not moisture-limited at this regularly submerged site, the actual ET is probably nearly equal to PET. Thus, all of the other fluxes are probably twice as large as those given by this method. Application of this method shows that recharge by tides and rain only occurs during spring and summer when ET exceeds upward seepage from below and is thereby able to draw down the water table below the marsh surface occasionally. During fall and winter, seepage of fresh water from below is largely balanced by seepage out of the soil into overlying tidal water or into sheet flow during tidal exposure. The resulting reduction in soil water salinity may thereby enhance the growth of Spartina in the following spring.