A study was undertaken to examine the potential effects of surface water withdrawals on the floodplain wetland vegetation communities of the St. Johns River in Florida. Among the foci of the project were potential changes in the extent or placement of wetlands in the landscape and alterations in seasonal hydrologic patterns, which are critical to wetland species. A geographic information system application (hydroperiod tool or HT) was used to estimate change in area-wide temporal and spatial patterns of inundation and water depth. Input to the HT model is elevation data in the form of a digital elevation model and water levels (time series) derived from a suite of hydrologic models combining various water withdrawal rates, land use conditions, sea level and status of water management projects in the upper reaches of the St Johns River. Intermediate steps within the tool involve interpolation of the point based hydrologic data to represent the water surface. Output from the HT were used to create metrics that relate to the depth, areal coverage and seasonality of wetland inundation – useful for understanding historical patterns of inundation and for comparing the various management (water withdrawal) scenarios.
Results/Conclusions
Two study areas in the upper river basin were evaluated, totaling 25,678 ha of herbaceous and woody wetland communities. Withdrawal scenarios were compared to an empirical baseline data set based on 10 years of water surface elevation data. Areas receiving inundation significantly less than historic levels were considered to be negatively impacted and therefore at risk of transition. Potential effects were greatest under the full withdrawal scenario based on 1995 land use, without the regional water projects (27.5% of the total area negatively impacted). Effects largely disappeared with scenarios based on future land use (2030) that incorporated regional water projects (3.82% of the total area impacted). Implications of the change in habitat availability for wood stork nesting and foraging were explored. Application of this relatively simple geographic information system method (the HT) is limited by the availability of accurate elevation data representing the wetland area, sufficient hydrologic data and current interpolation methods in the geographic information system. On the other hand, the simplicity of the HT model approach may provide faster results compared to more highly parameterized models, which can be critical to water resource management agencies operating under limited time frames.