Environmental Engineering Reference
In-Depth Information
the wetland surface. Although this method requires specific conditions and is not
applicable to all water-budget calculations or all wetland settings, data obtained
using Eq.
3.32
may be used to calibrate or validate models used for estimation of
diffuse overland flow (see below).
3.7.2 Estimation Using Simple Rainfall-Runoff Models
If the measurement of diffuse overland flow is impossible, it can be estimated from
other variables using a hydrological model. One of the simplest models is the
“rational method”, which estimates the volume of overland flow (
O
fc
,m
3
) generated
from a contributing area (
A
c
,m
2
) as a fixed ratio of precipitation (Mitsch and
Gosselink
2007
:128):
O
fc
¼
R
c
pA
c
(3.33)
where
R
c
is a dimensionless “rational coefficient” taking values between 0 and
1, and
p
(m) is the amount of precipitation. Equations similar to Eq.
3.33
are
commonly used by engineering hydrologists for estimating storm runoff generation
in urban areas. However, their applicability to wetlands is limited because
R
c
is
dependent on many factors including soil, vegetation, and the depth to the water
table. It is usually difficult to represent the variable conditions in a wetland
catchment, both in time and space, with a single parameter.
More sophisticated models consider soil type, vegetation, land use, and numerous
other factors, and treat soil moisture (and water-table) conditions as time-dependent
variables. A relatively simple example of such models is the “curve number”
(CN) method developed by the U.S. Soil Conservation Service in the 1950s and
1960s to estimate storm runoff from agricultural lands. Since then, the CN method
has become a standard tool for hydrologists and has been used in numerous
computer-based hydrological models such as the Soil and Water Assessment Tool
(SWAT) (
http://swatmodel.tamu.edu/
)
and Hydrologic Engineering Center (HEC)
as a non-linear function of precipitation. The non-linear dependence of runoff on
precipitation is represented by a CN coefficient, which is determined by soil texture
and drainage condition, land use, and the amount of rain during a period prior to the
storm event (i.e., antecedent moisture). Step-by-step instructions of the CN method
and examples of applications are found in introductory hydrology textbooks such as
Dunne and Leopold (
1978
:291-298), and can be easily implemented in a computer
algorithm.
Regardless of model sophistication, estimates made by these methods commonly
have a large degree of uncertainty resulting from violations of model assumptions
and uncertainty in model parameters and input variables. Therefore, estimated
overland flow should be verified with measured data whenever possible using the
methods described above.
