Environmental Engineering Reference
In-Depth Information
impossible to measure diffuse overland flow over a large area, this component is
frequently neglected or treated as a residual in the water-budget equation, especially
when there is evidence indicating that the magnitude of diffuse overland flow is much
smaller than stream inflow or groundwater inflow. However, diffuse overland flow is
a major water-budget component, at least temporarily, in some wetlands without
channelized stream inputs. Examples include many prairie wetlands in the Northern
Prairies region of North America (Winter 1989 ) and ephemeral forest pools in the
New England region of the United States (Brooks 2009 ).
Diffuse overland flow generally moves toward a wetland and is nearly always
considered as an input term in wetland water budgets, but this is not always the
case. In low-gradient settings where surface-water outflow is very slow and occurs
over a broad area with an ill-defined channel, loss of wetland water could be
considered either as slow surface-water flow or diffuse overland flow that is moving
away from the wetland. Although some have separated diffuse overland flow into
separate input and loss terms (LaBaugh 1986 ), here we will consider any slow-
moving flow occurring over a broad area that is leaving a wetland basin to be
surface-water outflow, as quantified with Eqs. 3.29 and 3.30 .
Since the amount of diffuse overland flow input entering a wetland is propor-
tional to the length of wetland perimeter, the effect of diffuse overland flow is
particularly pronounced in relatively small (e.g.,
10 4 m 2 ) wetlands that have large
perimeter-to-area ratios. A shallow water table in areas adjacent to a wetland can
rise to the surface with a relatively small amount of infiltration during storm events
(Gerla 1992 ), which precludes further infiltration and generates runoff (Dunne and
Leopold 1978 :268). In cold regions that have seasonally or permanently frozen soil,
reduced infiltrability of frozen soil causes a large amount of snowmelt runoff in the
surrounding uplands, which can be the dominant mode of water input to wetlands
(e.g., Winter and Rosenberry 1995 ; Hayashi et al. 1998 ). Using the same logic,
low-permeability soils also will retard infiltration, resulting in a greater percentage
of precipitation reaching the wetland as diffuse overland flow. Although difficult to
quantify over a large area, there are a number of methods for measuring flow
volume on a local scale or for individual storm events, or for obtaining order-of-
magnitude estimates of flow volume using simple models.
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3.7.1 Measurement of Diffuse Overland Flow
Flow traps are commonly used to measure diffuse overland flow over a small area.
Figure 3.19 shows a very simple flow trap consisting of an isolated area and a pit to
collect water. In this example, the pit is emptied after each storm event to determine
the overland flow volume for individual events. A pit also can be equipped with a
V-notch weir and water-level recorder for continuous monitoring of overland flow.
If the area contributing diffuse overland flow to a wetland is delineated with
reasonable accuracy, then the data obtained using small flow traps may be
extrapolated to a larger area to estimate the total water input to the wetland.
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