Geoscience Reference
In-Depth Information
Wetland hydrology
4
4.1 Introduction
whereas the latter is typical of semiarid and arid
environments.
Storage capacity of a wetland refers to the
volume of water held in pools, soil, and the
shallow subsurface. Storage capacity is deter-
mined by many factors
-
bedrock, sediment, soil,
landform, water level, and vegetation. For peat-
land, Charman (2002, p. 42) likened a mire to a
“giant bubble of water held together by a mass
of living and dead plant material.” The storage
capacity may be relatively large or small in rela-
tion to the area of the wetland. Likewise, storage
capacity may be relatively large or small com-
pared with the annual water gains and losses.
These factors, working together, inl uence the
hydroperiod, which is the seasonal pattern of
water level (Welsch et al. 1995).
Marine coastal and estuarine wetlands exhibit
daily and monthly l uctuations associated with
tides (Fig. 4-4). Given the widespread occur-
rence of wetlands along the continent-ocean
interface, tides play an important and inl u-
encing factor in determining coastal wetland
compositions and distributions. Hydrological
conditions shaped through tidal action along
with the coni guration of the coastline, slope of
the land, and inl ux of fresh water determine
the two principal types of coastal wetlands
-
saline or brackish
-
found across the world.
Tides caused by the gravitational pull of the
moon and sun l ood coastal regions diurnally,
submerge coastal soils and plants, and deposit,
erode and redistribute sediment in somewhat
The quantity and quality of water are crucial
factors for wetlands as well as all other environ-
ments (Niering 1985). Hydrology is the science
of water, particularly its l ow and storage, physi-
cal and chemical properties at the surface and
in the subsurface. Water is constantly in move-
ment at the Earth's surface via the water or
hydrologic cycle, which is powered by solar
energy (Fig. 4-1). Wetlands represent a storage
point within the hydrologic cycle, and the rela-
tionship of water gains, losses and storage is
known as the water budget. In simple terms,
these three components must balance for a
given wetland (Charman 2002), so that:
−
−
=
Gains
Losses
Storage
zero
Wetlands may gain water from direct precipita-
tion, surface inl ow, and ground-water discharge.
Water is lost via evaporation, transpiration, sur-
face outl ow, and ground-water recharge. The
amounts lost by evaporation from soil and water
bodies and transpiration by plants are usually
difi cult to measure separately, so the term eva-
potranspiration is often used for the combined
effects of both mechanisms. Wetlands represent
a delicate balance between these factors. The
balance may be relatively stable through time,
so that a wetland does not appear to change
much (Fig. 4-2), or may l uctuate seasonally or
erratically from year to year (Fig. 4-3). The
former is common in relatively humid climates,
