A model for wetland surface water dynamics

Abstract

Hammer and Kadlec [1986] present a model and data describing the dynamics of water flowing horizontally through a wetland. Their model is based on a pair of mass balance equations, one used in the case in which the free surface is above the sediment surface and the other used in the case in which the free surface (water table) is below the sediment surface. This pair of the mass balance equations can be derived from a single mass balance equation. The model of Hammer and Kadlec [1986] is successful in describing the distribution of head in the wetland because their mass balance equations contain the dominant flux terms in the more general equation for the cases of the free surface above and below the sediment surface. The use of two mass balance equations implies that surface water and pore water are decoupled and independent , when in fact there exists a gradual transition between the two regimes in wetlands. The purpose of this commentary is to suggest a form for the general mass balance equation for one-dimensional flow in a wetland that retains the coupling between surface water and pore water. The resulting equation is operationally equivalent to the pair of equations used by Hammer and Kadlec. There is no conflict between discussion presented here and the results and discussion of Hammer and Kadlec regarding the dynamics of surface water flow in wetlands. The advantage of the proposed formulation is that the coupling between surface water and pore water is represented more accurately. This becomes important when one begins to consider the role of the hydrologic cycle in biogeochemical cycles in wetlands. DERIVATION OF THE MASS BALANCE EQUATION The mass balance on water on a vertical slice taken through the full width w of a wetland, perpendicular to the direction of flow (Figure 1), is described by wS dt-dz w vz dy + w[P-E + A-I] (1) b The rate of change of water content in the slice is equal to the total inflow minus outflow due to horizontal velocities perpendicular to the slice, v z, plus the net contribution due to the vertical fluxes of precipitation, P; evapotranspiration, E; lateral inflows and additions, A; and infiltration to an underlying aquifer, I. The rate of change of water content is related to the rate of change of hydraulic head, h, by a storage coefficient S, defined here as the change in water content of the sediment and overlying water column per unit change in head, per unit surface area. The storage coefficient is dimensionless and operationally equivalent to the porosi-ties, •s and (I)g, defined by Hammer and Kadlec, but it differs in concept as will be discussed below. A one-dimensional mass balance is thus developed following the notation of Hammer and Kadlec as much as possible. The integral of horizontal fluxes over the depth interval [hb, h] can be replaced by expressions that give the net horizontal flux above and below the sediment surface: wS •-[ = •zz w(h-hg)a(h-hg) tt + •zz w(hg-hb)[(•zz + W[P-E + A-I] (2) It is assumed that pressure is hydrostatically distributed in the ve...

Modeling the Flood Modification Effects of a Mid-Sized Unregulated Wetland

“…As a result ofthe difficulties inherent in the application of Manning' s equation to wetlands, several researchers have adopted a more general power law: Kadlec and Knight (1996) report a range ofl.0 < P <2.4 for the hydraulic depth exponent. Past studies (Kadlec etaL, 198 I;Hammer and Kadlec, 1986) have indicated that the computed water surface profiles are not sensitive to the selection ofP within normal operating ranges. For application to wetland systems, Kadlec and Knight (1996) recommend a laminar flow formulation (X ::::: 1.0), unless additional research data becomes available.…”

“…Finally, for many types of wetland vegetation, there is a pronounced vertical distribution in vegetation density. As a result ofthe difficulties inherent in the application of Manning' s equation to wetlands, several researchers have adopted a more general power law: The wetland model utilizes a laminar friction law model as proposed by Hammel ' and Kadlec (1986). Overland flow velocity (v x ) can be expressed:…”

“…The wetland model utilizes a laminar friction law model as proposed by Hammel ' and Kadlec (1986). Overland flow velocity (v x ) can be expressed:…”

Hydrogeomorphic (HGM) Approach to Assessing Wetland Functions: Guidelines for Developing Guidebooks (Version 2)

“…There are several good sources of information that can be used to identify potential variables, including several reports that summarize information on the physical, chemical, and biological variables used in a wide variety of assessment methods (Canter and Hill 1979, Adamus and Brandt 1990, Adamus 1992, Simenstad et al 1991, Solomon and Sexton 1994. Potential variables can also be identified using the literature dealing with more quantitative approaches to assessing hydrologic and biogeochemical functions (Brunner 1988;Chescheir et al 1987Chescheir et al , 1984Faulkner et al 1989;Guertin et al 1987;Gunderson 1989;Hammer and Kadlec 1986;Heliotus and DeWitt 1987;Kadlec 1988;LaBaugh 1986;Rosenberry 1990;Tiedje et al 1981;Tiedje 1982;Welcomme 1979;Winter 1981).…”

Numerical Modeling of the Effects of Hydrologic Conditions and Sediment Transport on Geomorphic Patterns in Wetlands

“…Hydrological models of wetland surface flow fall into two categories: they are either used as water inventory (budgeting) tools, such as in the case of ecological and regional ecosystem models, or used to simulate runoff and stream flow as input to hydrodynamic transport models for water quality applications (Hammer and Kadlec, 1986 Each macrocosm consists of one ridge, one deep slough, two tree islands located inside the deep slough, and one shallow slough build to resembles the ridge and slough and tree island landscape features of the Everglades ( Fig. 2.2).…”