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United StatesEnvironmental ProtectionAgencyOffice of WaterOffice of Science and TechnologyWashington, DC 20460EPA-822-R-08-024December 2008www.epa.govMethods for Evaluating Wetland Condition#20 Wetland Hydrology
United StatesEnvironmental ProtectionAgencyOffice of WaterOffice of Science and TechnologWashington, DC 20460EPA-822-R-08-024December 2008www.epa.govMethods for Evaluating Wetland Condition#20 Wetland HydrologyPrincipal ContributorUniversity of GeorgiaTodd RasmussenPrepared jointly by:The U.S. Environmental Protection AgencyHealth and Ecological Criteria Division (Office of Science and Technology)andWetlands Division (Office of Wetlands, Oceans, and Watersheds)
NoticeThe material in this document has been subjected to U.S. Environmental ProtectionAgency (EPA) technical review and has been approved for publication as an EPA document.The information contained herein is offered to the reader as a review of the “state of thescience” concerning wetland bioassessment and nutrient enrichment and is not intended tobe prescriptive guidance or firm advice. Mention of trade names, products or services doesnot convey, and should not be interpreted as conveying official EPA approval, endorsement,or recommendation.Appropriate CitationU.S. EPA. 2008. Methods for Evaluating Wetland Condition: Wetland Hydrology. Officeof Water, U.S. Environmental Protection Agency, Washington, DC. EPA-822-R-08-024.AcknowledgementsEPA acknowledges the contribution of Todd Rasmussen of the University of Georgiafor writing of this module.This entire document can be downloaded from the following U.S. EPA blicat.htmlii
ContentsForeword viList of “Methods for EvaluatingWetland Condition” Modules viiWetland Hydrology 1Introduction 1Hydrologic Measures 2Wetland Water Budgets 11Water Budget Components 14Open Channel Measurements 23Control Structures 24Evolution and Alteration of Wetland Hydrology 29References 35List of TablesTable 1a:Tidal wetland types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Table 1b:Non -tidal wetland types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Table 2:I llustrative water budget components(mm/year) for selected wetland types. . . . . . . . . . . . . . . . . . . 16Table 3:Types of Channel Control Structures . . . . . . . . . . . . . . . . . 25Table 4:Two General Types of Weirs . . . . . . . . . . . . . . . . . . . . . . . . . . . 26iii
List of FiguresFigure 1: Multiple staff gages can be used to determine wetlandwater levels for a wetland with highly variable stages.The higher gage is used during wet periods and is easierto read than the more distant one. Once water levelsfall below the higher gage, then the lower gage is used . . . 5Figure 2: Piezometers are used to monitor water level changesin the subsurface. Multiple piezometers (called a nest )can be installed next to each other, one in each soilhorizon, if vertical flow between soil horizons is expected.Additional piezometers can be installed in the bedrock ifsuch materials affect the hydrology of the wetland. . . . . . 6Figure 3: Time Domain Reflectometer probes can be used tomonitor soil moisture saturation over time, providingan estimate of the water change in the subsurface. . . . . . . . 6Figure 4: Hydrograph for a short period of time showing thewater level variation. Note that the hydroperiod ismarked for a few stages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 5: Left: Wetland hydroperiod plot showing durationof flooding versus stage. Right: Wetland stage- frequencyplot showing number of exceedances per year. Note thatthe longest duration of flooding occurs at the lowerstages, and vice versa.Lower stages have a higherfrequency of being flooded than higher stages. . . . . . . . . . . 8Figure 6: Wetland stage- frequency- duration plot showing durationof flooding versus stage for a range of frequencies. . . . . . 9Figure 7: Stage-Area (top) and Stage-Volume (bottom) curvesshowing changes in wetland area and volume as afunction of stage.Flooded wetland area is zero oncewater stage drops below the ground surface in thedeepest section of the wetland.Volume of waterstorage is not zero because the bed sediments may beable to store and release water.Wetland area alsoincreases rapidly if a levee is overtopped. . . . . . . . . . . . . . . 13iv
List of Figures (continued)Figure 8: Relationship between hydrologic exchangesand nontidal wetland types. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Figure 9: ffect of wetlands on surficial aquifer movement.EInflow to wetland is on side where water table levelsare higher than wetland. Outflow is on side wherewater tables are lower. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 10: Network of piezometers required to map water levelsin the vicinity of a wetland. Note that the water levelcontours can be made based upon interpolation betweenmeasurements within individual piezometers. . . . . . . . . . . . . . 21Figure 11: Water level behavior showing slow decline in baseflowalong with a discrete storm event with its associatedstormflow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22Figure 12: Rating curve showing relationship between water levelstage and stream discharge. Note change in slope in relationship as different parts of the channel arewetted as the stage changes. . . . . . . . . . . . . . . . . . . . . . . . . . .23Figure 13: Three types of weirs (rectangular, triangular, trapezoidal,flooded orifice) used as control structures for measuringstream discharge. Water level (stage) is measured in weirbasin upstream of weir blade. . . . . . . . . . . . . . . . . . . . . . . . . . 26v
ForewordIn 1999, the U.S. Environmental Protection Agency (EPA) began work on this series of reportsentitled Methods for Evaluating Wetland Condition. The purpose of these reports is to helpStates and Tribes develop methods to evaluate (1) the overall ecological condition of wetlandsusing biological assessments and (2) nutrient enrichment of wetlands, which is one of the primary stressors damaging wetlands in many parts of the country. This information is intendedto serve as a starting point for States and Tribes to eventually establish biological and nutrientwater quality criteria specifically refined for wetland waterbodies.This purpose was to be accomplished by providing a series of “state of the science” modulesconcerning wetland bioassessment as well as the nutrient enrichment of wetlands. The individualmodule format was used instead of one large publication to facilitate the addition of otherreports as wetland science progresses and wetlands are further incorporated into water qualityprograms. Also, this modular approach allows EPA to revise reports without having to reprintthem all. A list of the inaugural set of 20 modules can be found at the end of this section.This last set of reports is the product of a collaborative effort between EPA’s Health andEcological Criteria Division of the Office of Science and Technology (OST) and the WetlandsDivision of the Office of Wetlands, Oceans and Watersheds (OWOW). The reports wereinitiated with the support and oversight of Thomas J. Danielson then of OWOW, Amanda K.Parker and Susan K. Jackson (OST), and seen to completion by Ifeyinwa F. Davis (OST). EPArelied on the input and expertise of the contributing authors to publish the remaining modules.More information about biological and nutrient criteria is available at the followingEPA website:http://www.epa.gov/ost/standardsMore information about wetland biological assessments is available at the followingEPA website:http://www.epa.gov/owow/wetlands/bawwgvi
List of “Methods for Evaluating WetlandCondition” ModulesModule #Module Title1 . Introduction to Wetland Biological Assessment2 . Introduction to Wetland Nutrient Assessment3 . The State of Wetland Science4 . Study Design for Monitoring Wetlands5 . Administrative Framework for the Implementation ofa Wetland Bioassessment Program6 . Developing Metrics and Indexes of Biological Integrity7 . Wetlands Classification8 . Volunteers and Wetland Biomonitoring9 . Developing an Invertebrate Index of BiologicalIntegrity for Wetlands10 . Using Vegetation to Assess Environmental Conditionsin Wetlands11 . Using Algae to Assess Environmental Conditions in Wetlands12 . Using amphibians in Bioassessments of Wetlands13 . Biological Assessment Methods for Birds14 . Wetland Bioassessment Casestudies15 . Bioassessment Methods for Fish16 . Vegetation -Based Indicators of Wetland Nutrient enrichment17 . L and -Use Characterization for Nutrient and SedimentRisk Assessment18 . Biogeochemical Indicators19 . Nutrient Loading20 . Wetland Hydrologyvii
Wetland HydrologyThe purpose of this module is to describeand discuss the general hydrologic properties that make wetlands unique, and to provide an overview of the processes that controlwetland hydrologic behavior. The intent is toprovide a general discussion of wetland hydrologic processes and methods in the hopeof fostering an understanding of the important attributes of wetland hydrology relevantto the monitoring and assessment of thesesystems. As such, it is not intended to addressthe narrower definition of wetland hydrologyfor jurisdictional or classification purposes.Also, this module should not replace moreadvanced wetland texts. If the need arises toobtain more specific information, the readeris advised to refer to wetland books or articles, including those referenced within thisdocument.IntroductionWetlands are a unique hydrologic featureof the landscape. One particularly important attribute is their position as the transition zone between aquatic and terrestrialecosystems. Wetlands share aspects of bothaquatic and terrestrial environments becauseof this position. On one hand, most freshwater and marine aquatic environments, suchas lakes, rivers, estuaries, and oceans, arecharacterized as having permanent water. Onthe other hand, terrestrial environments aregenerally characterized as having drier conditions, with an unsaturated (vadose) zonepresent for most of the annual cycle. Wetlandsthus occupy the transition zone between predominantly wet and dry environments.A diagnostic feature of wetlands is the proximity of the water surface (or water table below the surface) relative to the ground surface.In freshwater and marine aquatic habitats, thewater surface lies well above the land surface,while in terrestrial environments it lies somedistance below the root zone as a water tableor zone of saturation. The shallow hydrologicenvironment of wetlands creates unique biogeochemical conditions that distinguish itfrom aquatic and terrestrial environments.Geomorphic PositionWetlands are a fundamental hydrologiclandscape unit (Winter 2001) that generallyform on flat areas or shallow slopes, whereperennial water lies at or near the land surface,either above or below. Wetlands tend to formwhere surface and ground water accumulatewithin topographic depressions, such as alongflood plains, within kettles, potholes, bogs,fens, lime sinks, pocosins, Carolina Bays, vernal pools, ciénegas, pantanos, tenajas, and playas, and behind dunes, levees, and glacial moraines. Seepage wetlands form where groundwater discharges on slopes, as well as near theshores of streams, lakes, and oceans. Fringewetlands also form along shorelines, with periodic inundation not caused by ground water discharges but, rather, by water exchangeswith adjacent waterbodies, such as by periodicfloods and tidal action. And, finally, perchedwetlands form above low-permeability substrates where infiltration is restricted, suchas above permafrost, clay, or rock (Novitzki1989).Brinson (1993) provides a methodology forusing hydrogeomorphic indicators to classifywetlands based on their unique hydrologic,geomorphic, and hydrodynamic characteris-1
tively flat in these areas (Orme 1990). Becausewetlands lie in relatively flat landscapes, theirsurface area expands and contracts as the water stage changes, allowing for the storage oflarge volumes of water. Wetlands thereforeserve as a moderator of hydrologic variability—storing flood flows and reducing flowvelocities during wet weather in particular. Inaddition, shallow depths and low slopes, consistent with low energy environments, are important for trapping nutrients and sediments.tics. In this way, the dominant landscape andhydrologic factors can be synthesized to better develop an understanding of wetland formsand functions.Energy as the Driving ForceThe direction and rate of water movementinto and out of wetlands is controlled by thespatial and temporal variability of energy. Achange in energy with distance generates aforce that causes water to move from zones ofhigh energy to zones of lower energy. Gravitational forces account for most water movement, in that water tends to flow from higherto lower elevations. Resisting the gravitationalforce are viscous (friction) forces that retardthe fluid velocity. Inertial (momentum) forcesresist a change in velocity, causing water tomove at a constant velocity and in a straightline, unless additional energy is expended toeither accelerate, decelerate, or deflect the water.Hydrologic Measureshree hydrologic variables can be definedthat are useful for characterizing wetland hydrologic behavior; the water level, hydropattern, and residence time. Each of thesewetland descriptors are described in greaterdetail in subsequent sections. What followshere is a brief introduction of these concepts.TOne hydrologic descriptor is the generalelevation of wetland water levels relative tothe soil surface. Open water usually occursin deeper areas with few, if any, emergentmacrophytes. Any vegetation present in theseareas is usually not attached to the wetlandbottom, but vegetation may be floating on thewater surface. An emergent zone may alsobe present in areas shallower than the openwater zone, containing substantial quantitiesof emergent macrophytic vegetation, eitherliving or dead. Yet, other wetlands may havelarge areas of exposed, saturated soil that isgenerally covered with macrophytic vegetation. The water level can, therefore, be used asan indicator of the vegetation types likely tooccur in each of these zones.Water can also move due to a change inpressure, from zones of high pressure to zonesof low pressure. This is common in groundwater systems, where confined aquifers flowto the surface because of the greater pressureat depth. Artesian flow from a confined aquifer to the surface occurs when the rechargearea to the aquifer lies at a higher elevationthan the ground surface where the dischargeoccurs. Classical artesian springs exist in lowlying areas that are supplied with flows fromhigher elevation areas.Wetlands are normally found in low-energyenvironments—that is, in areas where waternormally flows with a slow velocity. This results, in part, because the land surface is rela-2
A second descriptor of wetland hydrologyis the temporal variability of water levels. Thetiming, duration, and distribution of wetlandwater levels are, together, commonly referredto as the wetland hydropattern, which incorporates the duration and frequency of waterlevel perturbations. The hydropattern of somesystems, such as tidal marshes, fluctuate dramatically over short periods of time; othersystems, such as seasonally flooded bottomland hardwood communities, fluctuate moreslowly over time. Yet, other wetland systemsare more static and may not display substantialshort- or long-term variability. The wetlandhydropattern is a function of the net difference between inflows and outflows from theatmosphere, ground water, and surface water.A third descriptor of wetland hydrology isthe residence, or travel time, of water movement through the wetland. Some wetland systems exchange water quickly, with water remaining within the wetland for only a shortduration of time, while water may travel veryslowly through other wetland systems. Theresidence time is the ratio of the volume ofwater within the wetland to the rate of flowthrough the wetland. Short residence times occur when the flow through the wetland is largecompared to its volume—longer residencetimes occur when the flow is small comparedto its volume. The residence time of a wetlandis often related to its hydropattern, in that wetlands with large water level fluctuations mayhave shorter residence times, such as in tidalmarshes. On the other hand, some wetlandsmay fluctuate rapidly due to large changes ininflow, yet have very long residence times dueto slow loss rates.Wetland Water LevelAn important feature of wetlands is the condition of oxygen deficiency in wetland soils.Anaerobic conditions develop more quickly insaturated soils than in unsaturated soils dueto low oxygen solubility in water, slow ratesof water advection, and slow diffusion ratesof oxygen through water. Anaerobic conditions in wetland soils affect vegetation bycreating adverse conditions for root survivaland growth. Thus, the presence of water substantially affects soil oxygen concentrations,which affects plant growth and survival.Yet, despite these low oxygen concentrations, wetlands are among the most biologically productive ecosystems on the landscape.They support a diverse assemblage of vegetative species having special physiological adaptations that enable them to survive and prosperTable 1a: Tidal wetland typesTIDA L WETLANDS :Su btida l - Tidal w ater p erm ane ntly covers the land s urface .Irregularly Exp osed - Tidal w ater us ually covers t he land sur face, but is not exposed daily.Regularly F lood ed - Tidal w ater a lternately covers and daily exposes the land surface.Irregularly Flooded - Tidal w ater c overs the land s urface less often than daily.3
Table 1b: Non-tidal wetland typesNO N-TID AL WE TLANDS :Per m anently Flooded - Water covers the land su rface throughout the year in all years.Vegetation is comp osed of obligate hydrophytes.Intermi ttently Expos ed - Water covers the land su rface throughout the year except in years ofextreme drought.Sem iperma nently Flooded - Water covers the land su rface throughout the growing s eason inmost years. The water table is at or v ery n ear the surface when the land su rface is e xposed.Season ally Flooded - Water covers the land su rface for extended periods, especially early inthe grow ing s eason, but is absent by the end of the season in m ost years. The water table is ator near the surface when t he land surface is exposed. Saturated water never covers the landsurfac
e t h o d s f o r. e. va l u at ni g. W. e t l a n d. C. o n d ti oi n #20 . Wetland Hydrology. Principal Contributor . University of Georgia Todd Rasmussen . Prepared jointly by: The U.S. Environmental Protection Agency . Health and Ecological Criteria Division (Office of Science and Technology)
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