4.6 Conceptual Models of Wetlands

Wetlands in some locations are dominated by one of the exchange conditions described above. However, variations in hydrologic conditions over seasons or multiple years may cause transitioning through one or more exchange mechanisms (e.g., effluent to influent, effluent to flow-through to effluent, and so on). Water budgets are extremely useful tools for determining the degree of groundwater exchange.

Generating a wetland water budget requires quantifying the same components found in budgets for rivers and lakes (Figure 49). Instrumentation is required to measure each component as described in Healy and others (2007). Often groundwater characterization includes creating a network of monitoring wells surrounding the wetland, installing mini-piezometers and/or seepage meters in the wetland, sampling for surface water and groundwater quality, and, in some studies, placing water temperature monitors in the wetland water and underlying groundwater (see Rosenberry and LaBaugh, 2008) as discussed in Section 5 of this book. The presence, absence and duration of water in a wetland is dependent on the changes in the magnitudes of inflows and outflows (Figure 49).

Figure showing etland water budget components
Figure 49 – Wetland water budget components (P+SWI+GWI=E+ET+SWO+GWOSP). Inputs are the volumes of precipitation (P), surface-water inflow (SWI), groundwater inflow (GWI). Outputs are direct evaporation from the water surface of the wetland pool (E), evapotranspiration (ET), surface-water outflow (SWO), and flow out to the groundwater system (GWO). The change in volume of the wetland pool (ΔSP) is positive if the volume increases and negative if the volume decreases, so the negative sign of the equation produces the appropriate balance (modified from Tiner, 1996).

The development of wetland vegetation and hydric soils is dependent on the length of time water is present in a wetland. The seasonal changes in the wetland surface and subsurface water levels define a wetland hydroperiod (Figure 50). Mitsch and Gosselink (2000) present numerous examples of wetland hydroperiods. Analyses of hydroperiods are used to compare wetland stability from year to year. With additional water budget information, including the degree and timing of wetland exchange conditions, a wetland hydroperiod can be used to examine changes in exchange conditions over time (Figure 50).

Figure showing examples of hydroperiods, wetland water level changes over time
Figure 50 – Examples of hydroperiods, wetland water level changes over time (blue lines), for four hypothetical temperate wetland settings. Relative water depths are plotted against months1 through 12 for one year in a semi-arid climate of the northern hemisphere. The wetland base (lowest ground surface elevation) is shown as the black horizontal line (0) at the top of the rectangle that represents the subsurface (gray). Monitoring wells/mini-piezometers, which are open only at the bottom and located in the wetland, show groundwater levels in light gray. Groundwater exchange direction is indicated by white arrows. a) A wetland hydroperiod dominated by effluent exchange and the influence of spring rains. Wetland base water levels are sustained above the ground surface for the entire year. b) A wetland hydroperiod where groundwater exchange (effluent) and rainfall raise the water level in the spring. In the summer months the wetland becomes influent and by month 9 (September) the wetland has dried, and the water level is below the wetland base. c) A wetland hydroperiod during a drought as compared to conditions in (b). Limited exchange is present in the winter and the wetland receives groundwater discharge (effluent) and some precipitation in the spring combined with some rainfall until summer. Then the wetland becomes influent and by September remains influent or becomes disconnected from the underlying groundwater system. d) A wetland hydroperiod for a disconnected wetland. Water in the wetland is from precipitation and runoff. Evaporation/evapotranspiration dominate the water balance and no groundwater exchange occurs (after Mitsch and Gosselink, 2000).

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