WETLAND HYDROLOGY

6.4 WETLAND HYDROLOGY

Wetlands are part of a landscape mosaic that provides several watershed functions, and removal or alteration of wetlands through alteration of hydrology signi ficantly affects the health and function of the broader landscape. The hydrology of a wetland creates the unique physicochemical conditions that make such an ecosystem di fferent from both well- drained terrestrial systems and deepwater aquatic systems. Hydrologic pathways such as rainfall, surface runo ff, ground-water flow, tides, and flooding rivers transport energy and nutrients to and from wetlands. Water depth, flow patterns, and duration and frequency of flooding are the result of hydrologic inputs and outputs, and these factors have a direct in fluence on the biochemistry of soils and the biota of wetlands. Except in nutrient-poor wetlands such as bogs, water inputs are the major source of nutrients to wetlands. When hydrologic conditions in wetlands change, even slightly, the biota may respond with mas- sive changes in species composition, richness, and ecosystem productivity. Several ani- mals are particularly noted for their contributions to hydrologic modi fications and subsequent changes in wetlands. Beavers are noted for building dams, muskrats for bur- rowing, and geese (particularly Canada geese, shown in Figure 6.8) for consuming exces- sive amounts of wetland vegetation.

FIGURE 6.8 Canada geese. (From Jackson Bottom Wetlands Preserve, 2005.)

WETLANDS

The hydroperiod is the seasonal pattern of water level in a wetland and can be consid- ered a hydrologic signature of each wetland type. Factors that a ffect the hydroperiod include (1) the balance between in flows and outflows, (2) the surface contours of the landscape, and (3) the subsurface soil, geology, and ground-water conditions. The balance between in flows and out flows defines the water budget of the wetland, whereas the surface contours and sub- surface conditions de fine the capacity of the wetland to store water. For wetlands that are not subtidal or permanently flooded, the amount of time that a wetland contains standing water is called the flood duration, and the average number of times that a wetland is flooded in a given period is known as the flood frequency. Each depth of inundation has a range of possible flood durations, and each duration has a corresponding flood frequency.

The hydrologic budget of a wetland is given by the relation

ᎏ ∆ V ᎏ⫽P n ⫹S n ⫹G n ⫺ ET

where ∆V is the volume of water added to the wetland in time increment ∆t, P n is the incre- mental net precipitation, S n is the incremental net surface-water in flow, including flooding streams, G n is the incremental net ground-water in flow, and ET is the incremental evapo- transpiration. The incremental net precipitation is equal to the incremental precipitation minus the amount of this precipitation intercepted by the vegetation. The components of the wetland water budget are discussed below.

6.4.1 Net Surface-Water Inflow

Wetlands receive surface in flows in many forms. Overland flow is a nonchannelized sheet flow that usually occurs during and immediately following rainfall or a spring thaw, or as

tides rise in coastal wetlands. Channelization of flow, which is usually associated with urbanization, has a signi ficant impact on the functioning wetlands with significant over- land in flow. In addition, roads can block or severely alter the outflow dynamics of the sys- tem, and increased flow rates through culverts can be a major impediment to wetland functioning as well as fish migration. A special case of surface flow occurs in riparian wet- lands that are in floodplains adjacent to rivers or streams and are occasionally flooded by those rivers or streams. Examples of riparian wetlands are the delta marshes of the Mississippi River (Gosselink, 1984) and the hardwood swamps of the southeastern United States (Wharton et al., 1982). Some wetlands can be more isolated, receiving only inter- mittent surface-water input. These systems include vernal pools in California (Zedler, 1987), prairie pothole wetlands in the midwest (Kantrud et al., 1989), and playa lakes in the southern high plains.

6.4.2 Net Ground-Water Inflow

Wetlands can recharge ground water or can be located in areas where ground water is dis- charged to the wetland. Movement of ground water into or out of a wetland is a function of the permeability of the soils, which is partially a ffected by vegetation and soil type. The drawdown of the water table caused by urbanization of nearby areas can have a deleteri- ous e ffect on wetland function, and such an effect has been documented in the forested wetlands of Florida (Mortellaro et al., 1995). The e ffects of water-table drawdown on

CASE STUDY: THE EVERGLADES AND BIG CYPRESS SWAMP

wetlands include reduced hydroperiod, lower water levels, a shift to drier, nonwetland plant species, death of animal species, loss of fish and amphipods, reduced use by birds and wildlife, and increased fire damage.

6.4.3 Evapotranspiration

Evapotranspiration (ET) typically accounts for loss of water ranging from 20 to 80% of the annual water budget in most wetland systems (Mitsch and Gosselink, 2000). Evapotranspiration plays a role in the attenuation of floodwaters as well as maintenance of the soil-water-redox conditions in a wetland. Changing the water input, loss of plants, and changes in soil conditions will a ffect ET and in turn change the functioning of the wetland. It is noteworthy that transpiration can exceed open-water evaporation losses in prairie- pothole systems (LaBaugh et al., 1998).

As a general rule, natural wetlands are more prevalent in cool or wet climates than in hot or dry climates, and steep terrain tends to have fewer wetlands than gently sloping landscapes. Wetlands occur most extensively in regions where precipitation is in excess of losses such as evapotranspiration and surface runo ff. The dividing line between precipita- tion excess in the eastern United States and precipitation de ficit in the western United States is the Mississippi River.