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).
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).