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A Hydrological Budget (2002-2008) for a Large Subtropical Wetland Ecosystem Indicates Marine Groundwater Discharge Accompanies Diminished Freshwater Flow
Water availability and flow are the principal determinants of structure and function in wetland ecosystems (Mitsch and Gosselink 1993), where the juxtaposition of seasonally varying water level and topography creates a mosaic of flooded and emergent areas. Natural wetlands all over the world have been degraded by human activities from drainage to pollution and wetland biodiversity continues to decrease (Butchart et al. 2010). Restoration of a large, heterogeneous wetland ecosystem requires knowledge of how the specific hydrologic regime interacts with the structure and function of the inherent plant and animal communities. A first step in understanding the eco-hydrology of a large wetland would be to quantify the hydrological pathways leading into and out of the ecosystem, together with noting their seasonal and inter-annual variation (Carter 1986; Dadaser-Celik et al. 2006). Relating the inputs and outputs to changes in water level and water storage produces a water budget (Bedient et al. 2008). A water budget is necessary to predict how flow, residence time, and water level would respond to changes in the sources (LaBaugh 1986) that can arise from environmental change and water management. This information is critical in both natural and constructed hydrologically controlled ecosystems (German 2000; Lott and Hunt 2001) because water depth, frequency of flooding, and flow patterns govern groundwater recharge (Goes 1999), biogeochemical cycling, carbon sequestration and greenhouse gas emission (Mitsch et al. 2010), nutrient availability (Childers et al. 2006), species composition based on flood/drought tolerances, and the extent of aquatic habitat in the dry season to enumerate a few examples.
The calculation of water budgets for constructed wetlands and small natural wetlands (surface area <400 km2) has been well documented in the literature (e.g., Koerselman 1989; Becht and Harper 2002; Zhang and Mitsch 2005, Favero et al. 2007, Rodriguez-Rodriguez et al. 2007) while larger wetland ecosystem water budget studies are not as numerous; examples include the Nigerian Hadejia-Nguru wetlands (Goes 1999), Okavango delta floodplain wetlands (Ramberg et al. 2006) and the Ortuluakar Marsh in Turkey (Dadaser-Celik et al. 2006). Large wetland ecosystems such as the Everglades and the Pantanal possess complexity in hydrological pathways; for instance, evapotranspiration can vary spatially with vegetation type (Camacho et al. 1974; Rawson et al. 1977, Tardeau and Simonneau 1998) and seasonally with plant phenology. Groundwater-surface water interactions can vary across spatial scales and can be important in not only transporting water, but also chemicals and nutrients from one water reservoir to another (Sophocleous 2002). Landscape grid water budget models account for spatial heterogeneity in water inputs and outputs (e.g., Fitz et al. 2004) by estimating these for each grid cell and summing up to get a basin-wide estimate. However the possibilities of error propagation in finite element modeling approaches indicate that it would be beneficial to have an independent validation of the basin-level water inputs and outputs. Here, we describe the development of a basin-level water budget for Shark River Slough (SRS), at around 1,700 km2 the main drainage in the Everglades National Park, as the backbone in understanding its eco-hydrology as well as to provide a means of validation for landscape spatial models.
The Everglades is a slowly flowing subtropical wetland ecosystem with the bulk of its discharge entering the Gulf of Mexico and Florida Bay (Fig. 1). Flow velocities vary seasonally from upto 2 cm/s in the wet season to <0.1 cm/s in the dry season (e.g., Riscassi and Schaffranek 2004). Large scale hydrological alterations in the northern Everglades through construction of canals and levees for flood-control, water storage and drainage in since the early twentieth century have compartmentalized the previously free-flowing system (Duever et al. 1986; Light and Dineen 1994). Water management upstream of Everglades National Park (ENP) has greatly reduced inflows, altered flow regimes and water depth in ENP, thereby severely impacting the Everglades ecosystem as well as that of downstream Florida Bay (Smith et al. 1989; Fourqurean 1999). In addition, the reduction in freshwater inflows along with ongoing sea level rise is responsible for the advancing seawater intrusion into the Everglades (Fitterman et al. 1999; Price et al. 2006; Saha et al. 2011) that is accompanied by landward encroachment of mangroves into the previously freshwater portions of the Everglades (e.g., Ross et al. 2001). The central issue in Everglades restoration is to restore surface water towards pre-drainage levels and flow rates in order to achieve the hydrological requirements for different native plant and animal communities. To mention a few examples, hardwood hammock plants occupy a narrow ecological niche bounded by susceptibility to both flooding and drought (Saha et al. 2009) as well as salinity (Olmsted and Loope 1984). Everglades freshwater fish communities regulate their population size based on seasonal expansion and shrinking of pools of water; thus droughts or water diversions/sudden releases adversely affect these communities (DeAngelis et al. 2010) leading to cascading effects throughout trophic levels in the entire ecosystem (Rehage and Trexler 2006).
Everglades National Park has two watersheds (Fig. 1) with distinct ecosystems that are only connected in very wet years: SRS that has tides from the Gulf of Mexico reaching 30 km inland and Taylor Slough that experiences very small tidal influence. Water budgets have been created for portions of the northern Everglades (Nungesser and Chimney 2006), Taylor Slough (Sutula et al. 2001), Florida Bay (Nuttle 1995), and for the entire Everglades ecosystem (Fitz et al. 2004) that includes the entire ENP as a single unit. As the major drainage of ENP, Shark River Slough requires its own water budget that will aid the calculation of water residence times and link the hydrological cycle with ecosystem processes of Shark Slough and the estuary. Hence we calculate a water budget for the SRS based on daily data available from 2002 to 2008. Inter-annual variability in annual rainfall together with an equally large variation in water releases to the park and discharges to the Gulf of Mexico (e.g., Donders et al. 2008) results in water budgets acquiring this wide range of inter-annual variability. Thus, a multiyear calculation of the SRS water budget provides a better representation of the natural range of water budget components. We also report water budgets at a monthly scale, to demonstrate seasonal changes in the inputs and outputs. Given the range of woody and herbaceous plant communities with associated differences in transpiration, we estimate evapotranspiration using various models and modify a model to account for water limiting conditions. In addition, the budget estimates net groundwater discharge to the SRS that has been hypothesized to occur (Price et al. 2006) in the estuarine zone; hence we compare the groundwater discharge estimated by the budget with surface water salinity data at the mangrove-sawgrass ecotone that marks the dynamic boundary between halophytes and freshwater vegetation (Fig. 1).
U.S. Department of the Interior, U.S. Geological Survey
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