Chapter 14Wetland HydrologyThe purp ose of this chapter is to describe and dis-cuss the general hydrologic properties that make wetlandsunique, and to provide an overview of the processes thatcontrol wetland hydrologic behavior. The intent is to pro-vide a general discussion o f wetland hydro logic processesand methods in the hope of fostering an understanding ofthe important attributes of wetland hydrology relevant tothe monitoring and assessment of these systems.As such, it is not intended to address the nar rowerdefinition of wetland hydrology fo r jurisdictional or clas-sification purposes. Also, this module should not replacemore advanced wetla nd texts. If the need arises to obtainmore s pecific information, the reader is advised to referto wetland books or articles, including those referencedwithin this document.14.1 IntroductionWetlands are a unique hydrologic feature of the landscape.One particularly important attribute is their position asthe transition zone between aquatic and terrestrial ecosys-tems. Wetlands share aspe c ts of both aquatic and terres-trial environments because of this position. On one hand,most freshwater and marine aquatic environments, suchas lakes, rivers, estuaries, and oceans, are characterized ashaving permanent water. On the other hand, terrestrialenvironments are genera lly characterized as having drierconditions, with an unsaturated (vadose) zone present formost of the annual cycle. Wetlands thus occupy the tran-sition zone between pre dominantly wet and dry environ-ments.A diagnostic feature of wetlands is the proximity of thewater surface (or water table below the surface) re lativeto the ground surfa c e . In freshwater and marine aquatichabitats, the water surface lies well above the land sur-face, while in terrestrial environments it lies some distancebelow the root zone as a water table or zone of satura-tion. The shallow hydrologic environment of wetlands cre-ates unique biogeochemical conditions that distinguishesit from aquatic and terrestrial environments.Geomorphic PositionWetlands are a fundamental hydrologic landscape unit(Winter, 2001) that generally form on flat areas, or onshallow slopes, where perennial water lies at or near theland surface, either above or below. Wetlands tend to formwhere surface water and ground wa ter accumulate withintopographic depressions, such as along flo od plains, withinkettles, potholes, bogs, fens, lime sinks, pocosins , Car-olina Bays, vernal pools, ci´enegas, pa ntanos, tenaja s, andplayas, and behind dunes, levees, and glacial moraines.Seepage wetlands form where ground-wa ter discharges onslopes, as well as near the shor e of streams , lakes, andoceans. Fringe wetlands a lso form along shorelines, withperiodic inundation not caused by ground-water discharges,but rather by water exchanges with adjacent waterbodies,such as by periodic floods and tidal action. And finally,perched wetlands form above low-permeability substrateswhere infiltration is restricted, such as above permafro st,clay, or rock (Novitzki, 1989).Brinson (1993) provides a methodology for using hy-drogeomorphic indicators to classify wetlands based ontheir unique hydrologic, geomorphic, and hydrodynamiccharacteristics. In this way, the dominant landscape andhydrologic factors can be synthesize d to better develop anunderstanding of wetland forms and functions.Energy as the Driving ForceThe direction and rate of water movement into and outof wetlands is controlled by the spatial and temp oral vari-ability of energy. A change in energy with distance gener-ates a force that causes water to move from zones of highenergy to z ones of lower energy. Gravitational forces ac-count for most water movement, in that water tends toflow from higher to lower elevations. Resisting the gravi-tational force ar e viscous (friction) forces that retard thefluid velocity. Inertial (momentum) forces resist a changein velocity, causing water to move at a consta nt velocity,and in a stra ight line, unless additional energy is expendedto either accelerate, decelerate, or deflect the water.Water can also move due to a change in pres sure, fr omzones of high pressure to zones of low pressure. This iscommon in gro und-water systems, wher e confined aquifers1CHAPTER 14. WETLAND HYDROLOGY 2flow to the surface because of the greater pres sure at depth.Artesian flow from a c onfined aquifer to the surface occur swhen the recharge area to the aquifer lies at a higher eleva-tion than the gro und surface where the discharge occurs.Classical artesian springs exist in low-lying areas that aresupplied with flows from higher elevation areas.Wetlands are normally found in low-energy environ-ments - that is, in areas where water normally flows witha slow velocity. This r e sults, in part, because the land sur-face is relatively flat in these ar eas (Orme, 1990). Becausewetlands lie in r elatively flat landscapes, their surface areaexpands and contracts as the water stage changes, allow-ing for the storage of large volumes of water. Wetlandstherefore serve as a moderator of hydrologic variability -storing flo od flows and reducing flow velocities during wetweather in particular. In addition, shallow depths andlow slopes, consistent with low energy environments, areimpo rtant for trapping nutrients and sediments.14.2 Hydrologic M easuresThree hydrologic variables can be defined that are usefulfor characterizing wetland hydrologic behavior: the wa-ter level; the hydropattern; and the residence time. Eachof these wetland descriptors are described in greater de-tail in subsequent sections. What follows here is a briefintroduction of these concepts.One hydrologic descriptor is the general elevation ofwetland water levels relative to the soil surface. Open wa-ter usually occurs in deeper areas with few, if any, emer-gent macrophytes. Any vegetation present in these areasis usually not attached to the wetland bottom, but vege-tation may be floating on the water surface. An emer-gent zo ne may also be present in areas shallower thanthe open water zo ne, containing substantial quantities ofemergent mac rophytic vegetation, either living or dead.Yet other wetlands may have large areas of exposed, satu-rated soil that is generally covered w ith macrophytic veg-etation. The water level can, therefore, be used as anindicator of the vegetation types likely to o ccur in each ofthese zones.A second descriptor of wetland hydrology is the tem-poral variability of water levels. The