Environmental Engineering Reference
In-Depth Information
wetlands on hillslopes or wetlands that drain rapidly following rain or flooding
events. In those settings, the accounting unit may need to be set based on topogra-
phy or areal extent of specific types of wetland vegetation. If the wetland surface is
considered to be saturated virtually all the time, then one could reasonably assume
that
V
is zero. In this case, any additions of water to the wetland must instantly be
balanced by an equal volume of loss terms. The accounting unit also could be the
quantification of water stored in the vadose zone or the sum of water contained in
the vadose zone and in groundwater beneath the vadose zone. Assumptions may
need to be made regarding the level of saturation of the wetland soils to quantify a
change in stored volume. If the water table decreases to below the wetland bed,
water-volume change could be estimated based on water-level measurements in
monitoring wells and assumptions about volumetric storage capacity of the wetland
soil. A further complication is associated with the typically small distance of the
water table below the land surface or wetland bed. The capillary fringe is a zone of
tension saturation that exists above the water table in all settings; the thickness is
inversely proportional to the grain size of the soil. In the generally fine-grained
sediments found in most wetland settings, the soils may be essentially saturated
beneath much to all of the wetland bed. If this is the case, even small rainfall or
recharge events can bring the water table directly to land surface and result in
surprisingly large amounts of overland flow to the wetland (Gerla
1992
).
Δ
3.2.2 Determining the Accounting Period
The proper time interval (
t
in
3.1
) over which a water budget is determined
depends on the questions asked, the duration of the study, or the reasons for
quantifying a water budget (Healy et al.
2007
). If the question is related to wetland
response to climate change, an annual water budget may be all that is necessary.
If determining the relative significance of a particular hydrologic component is
important, the study may need to extend over several years and quarterly or monthly
time steps would be appropriate. If the concern is related to the response of a
wetland to individual recharge events or to specific physical, chemical, or
biological processes, daily time steps may be the most appropriate. In general,
because of technological advances in data collection, scientists are tending to use
shorter time steps. Whereas monthly measurements may have been the norm during
previous decades, it is more likely that data are collected every minute to every hour
and hourly or daily values are then calculated based on those data.
Δ
3.3 Water-Budget Hydrology
The volume of water contained in a wetland,
V
, is an integrated response to all of
the hydrological processes that add or remove water. Therefore, if all of the
components of a wetland water budget were measured perfectly, the sum of those
