Wetland HydrologyR W Tiner, University of Massachusetts, Amherst, MA, USA

ã 2009 Elsevier Inc. All rights reserved.

Introduction

Wetland hydrology – frequently occurring prolongedinundation and/or soil saturation (waterlogging) –is the driving function that creates and maintainswetlands and provides wetlands with unique qualitiesand significant ecological functions that are highlyvalued by society. Wetlands may be generally definedas shallow water areas or lands that are periodicallyflooded or saturated long enough to support hydro-phytic vegetation and/or other forms of aquatic life.Conceptually, wetlands lie between dry land and deepwater and as a result have often been referred toas ecotones (a transitional habitat; part land, partwater). Depending on the wetland type, the wetlandmay be subjected to flooding or soil saturation or acombination of both. From a hydrologic standpoint,wetlands encompass a wide range of wetness con-ditions from permanent to periodic inundation orwaterlogging. Differences in climate, geologic setting,and other factors have created a diversity of wetlandtypes globally with varied hydrologies that affectplant and soil development, their use by wildlife,their functions and values. This article is a generalintroduction to wetland hydrology for a nontechnicalaudience; for more advanced coverage, consult‘Further Reading.’ Since the focus of this encyclopediais on inland waters, the discussion of wetland hydrol-ogy emphasizes that of inland wetlands (nontidalwetlands and tidal freshwater wetlands) and doesnot address marine and estuarine wetlands.

Water Sources

Nontidalwetlands receivewater frommeteoric sources(precipitation, snow, sleet, hail, fog, and mist) or tellu-ric sources (groundwater), while tidal wetlands receivea significant inflow of water from tides in addition tothe other sources.Meteoric sources affect all lands, butfor wetlands to form, water must persist either on thesurface or in the soil for sufficient time to promotethe colonization, growth and survival of hydrophyticvegetation and the development of hydric soils andsubstrates, and to create environmental conditionsthat support other aquatic life.

778

Factors Contributing to WetlandHydrology

Topography (landform), landscape position (proxim-ity to a water source), soil properties, geology, andclimatic conditions are important factors in wetlandformation. Depressions and broad flats with poordrainage are places where water can accumulate insufficient quantities to create wetlands. Mountainousareas tend to have less wetland than coastal andglaciolacustrine plains largely due to drainage proper-ties (e.g., rainwater drains readily from slopes andcollects on flats). Wetlands inmountainous areas likelyreceive considerable groundwater inflow (groundwaterwetlands), while wetlands on broad flats in areas ofhigh rainfall may be supported by rainwater (surfacewater wetlands; Figure 1) or groundwater. Inlandwetlands occur (1) along the shores of lakes andponds where high water levels and the presence of apermanentwaterbody lead to permanent inundation ofshallow water zones and periodic inundation of low-lying neighboring areas, (2) on floodplains where theyare subject to seasonal inundation, (3) in depressionsthat receive runoff from adjacent areas and ground-water discharge, (4) on broad flats of coastal plains orglaciolacustrine plains where drainage is poor, (5) attoes of slopes where subsurface water reaches the sur-face, (6) on slopes associated with springs and seepswhere groundwater discharges to the surface, (7) inpaludified landscapes where low evapotranspirationand an excess of water allow peat mosses to growover once dry land covering the landscape with peat(e.g., blanket bogs), (8) in permafrost areas where fro-zen soils serve as an impermeable layer that percheswater at or near the surface, and (9) in areas belowglaciers and snowfields where the seasonal flow ofmeltwater creates wet conditions. Soils with lowhydraulic conductivity such as clayey soils and soilswith restrictive layers (e.g., hardpans) near the surfacefavor wetland development over sandy soils that tendto have good internal drainage due to large effectiveporosity (high hydraulic conductivity). Sandy soilsbecome wet if external drainage is poor or if periodi-cally flooded for long duration or saturated by waterfrom external sources (e.g., regional water tables).Geological features, such as contacts between different

Groundwaterdepressional wetland

Surface waterdepressional wetland

Groundwaterslope wetland

Surface waterslope wetland

Overlandflow

Groundwaterinflow

Seasonal highwater table

Eva

potr

ansp

iratio

n

Pre

cipi

tatio

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Overlandflow

Water table (may temporarily rise to wetlandlevel, but ground water inflow is minorcompared to surface water inflow)

Eva

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Pre

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nOverlandflow

Watertable

Groundwater inflow

Streamflow

Eva

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ansp

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flow

Water table (may temporarily rise to wetlandlevel, but ground water inflow is minorcompared to surface water inflow)

Lake or riverflood waterlevel

Eva

potr

ansp

iratio

n

Pre

cipi

tatio

n

Figure 1 Four types of wetlands defined by wetland hydrology and topography: groundwater depressional wetland, groundwater

slope wetland, surface water depressional wetland, and surface water slope wetland. Following this approach, wetlands on broad flats

may be considered surface water flat wetlands; they are not inundated (except in microdepressions) but have seasonal high water tables.Source: Tiner RW (1988) Field guide to nontidal wetland identification. Newton Corner, Massachusetts, USA: U.S. Fish andWildlife Service

and Maryland Dept. of Natural Resources. Reprinted by Institute for Wetlands and Environmental Research, Inc., Leverett, MA; redrawn

from Novitski RP (1982) Hydrology of Wisconsin wetlands, Information Circular 40. Reston, VA: U.S. Geological Survey.

Hydrology _ Wetland Hydrology 779

rock types that outcrop along hillsides, create condi-tions favoring the formation of seeps and associatedwetlands where groundwater or near-surface interflowintersects the land surface. Desert spring-fed wetlandsare the result of groundwater discharge from region-al aquifers. Dissolution of limestone formations allowfor the establishment of wetlands in karst terrain.Morainal deposits in glaciated regions typically

create deranged (nonintegrated) drainage patternsthat produce isolated depressions on low-permeabilitysubstrates where wetlands develop, whereas kettlebasins in outwash deposits will also produce wetlandswhere such basins are in contact with aquifers. Precipi-tation patterns significantly influence wetland forma-tion. Wetlands are naturally more abundant in regionswithwarmhumid climates than in hot arid climates for

780 Hydrology _ Wetland Hydrology

obvious regions. The latter regions may support‘ephemeral wetlands’ during extremely wet years thatcreate conditions in landscape positions that typicallysupport wetlands in humid regions.

How Wet is a Wetland?

One would think it would be relatively easy to definea wetland from a hydrologic standpoint; after all if anarea is wetland, it must be wetter than dry land. Yet tofully describe and understand hydrology requireslong-term measurements and for wetlands this in-volves monitoring water tables (soil saturation) aswell as surface water. Given the diversity of wetlandtypes, that wetlands are transitional habitats betweendry land and open water, and the need to conductstudies over long time periods, it is little wonder thatsuch information is lacking for most wetlands andthat it is not a simple matter to define the minimumwetness of wetland.Both surface and groundwater sources need to be

considered in determining minimum wetness fordefining wetland hydrology. Scientists in the UnitedStates probably have spent more time contemplatingthis topic because wetlands on both public and pri-vate lands are regulated by the federal governmentand by many state and local governments. Such regu-lation requires identifying specific limits of wetlandson the ground. To be a wetland for jurisdictionalpurposes, an area typically must have a positive indi-cator of hydrophytic vegetation, hydric soils, andwetland hydrology. The former two features aremani-festations of wetland hydrology and serve as validindicators of wetlands in the absence of drainage.Yet, many situations are encountered where somedrainage has been performed, thereby raising thequestion – is the area still wet enough to be identifiedas wetland? In such areas, the plants and soilsmay better reflect past hydrologic conditions andmay no longer be valid indicators of current sitewetness. So to answer this question, wetlands hydrol-ogy must be defined and a lower threshold of wetnessestablished. (Note: The upper threshold of wetnessclearly is permanent inundation or saturation to thesurface.)

Wetland Hydrology Defined

A group of distinguished American wetland scientistsstudied the topic of wetland delineation for 2 yearsand came up with the following definition:

An area has wetland hydrology when it is saturated

within one foot of the soil surface for two weeks or

more during the growing season in most years (about

every other year on average).

Depth of Saturation

Roots supply plants with nutrients and water neededfor growth and reproduction. Heavy rains that satu-rate the soil for as little as a few hours can cause roothairs to die of oxygen starvation as evidenced by wilt-ing of vegetable crops (e.g., squash and cabbages) afterrainfall. Prolonged saturation and accompanyinganaerobic conditions adversely affect root function,causing changes in root morphology (aerenchyma de-velopment), root rot and dieback, and the formationof shallow roots (near the surface). Since they live inareas of frequent soil saturation, wetland plants havemost of their roots located in the upper, partly aeratedzone of the soil. This zone is typically within 30 cm(1 ft) of the soil surface.

Duration of Wetness

Although flooding for as little as a day can createanaerobic conditions under special circumstances insome soils, most plants need to be wet longer toadversely affect their growth and survival. Some wet-land plants begin to develop morphological properties(e.g., aerenchyma tissue, adventitious roots, and hyper-trophied lenticels) within 1–2weeks of flooding orwaterlogging. Soil scientists have identified long andvery long durations as important periods of floodingfor soils. Long duration is defined as inundation from7 to 30days, while very long duration is longer than30days. Flooding (from overbank flows or runoff) orponding (standing surface water in a closed depression)for long duration or more during the growing season inmost years canbe used to identify ahydric (wetland) soilregardless of its morphological properties.

Frequency of Prolonged Wetness

In order to be a wetland under most definitions, anarea has to be frequently wet, which has been definedas every other year on average. This definition worksfor wetlands in humid regions where average condi-tions may have some significance. Yet for arid andsemiarid regions, this definition is problematic, forsuch regions may experience long-term droughtsthat have a profound effect on the ‘average.’ In theseregions, a series of wet years can create conditionslong enough for wetlands to become established.While there are typical wetlands supported by springsand river overflows in these areas, some ‘wetlands’are ephemeral types showing up only during periodsof extreme wetness. The term ‘episodic’ has beenapplied to describe such wetness and these infre-quently wet areas are viewed as wetlands in somecountries (e.g., Australia). Soils in arid regions maydevelop hydromorphic properties when saturated forless than 5weeks at a frequency of once every 3 years.

Hydrology _ Wetland Hydrology 781

Growing Season

Although the wetland hydrology definition is explicitin stating the frequency and duration of wetness andthe depth of saturation, the seasonality of wetness –‘growing season’ – can be defined inmanyways whichcould lead to different interpretations of wetlandhydrology. For example, the growing season hasbeen traditionally used in agriculture to assist in deter-mining planting times for crops like corn, wheat, andrice. As such, the growing season could be defined asthe frost-free periodwhere there would be virtually norisk of crop failure due to frost or freeze. This conceptis not useful for natural plant communities that areadapted to local environmental conditions. For exam-ple, by the time the frost-free period arrives, manynative plants have already flowered and leafed-out.So, from an ecological standpoint, the growing seasonhas commenced well before there is no risk of frost.The growing season is actually a concept that is bestapplied to a particular plant. What is the growingseason for red maple (Acer rubrum) or corn (Zeamays)? Such a definition would be based on whenthey initiate new growth after a period of dormancy.Yet, this concept is also not useful for definingwetlandhydrology. Given that the focus of wetland hydrologyis often to define conditions that affect plant establish-ment, growth, and survival, and sometimes the forma-tion of hydric soils, the time of year should, at least, berelated to plant activity and possibly to soil formationprocesses. From the botanical standpoint, one couldsay the start of the growing season for wetlandsshould be predicated on the vegetation growing inwetlands. Observations of plant growth in wetlandswould therefore serve as valid indicators that thegrowing season has begun. The earliest of the plantsthat leaf out or flower in spring (e.g., willows – Salixspp., alders – Alnus spp., marsh marigold – Calthapalustris, leatherleaf – Chamaedaphne calyculata, andskunk cabbage – Symplocarpus foetidus) would be thebest indicators for determining this. Amore generalizedapproachwould be to consider the growing season foran area to have commenced when any plants (wet-land or upland) show signs of growth (e.g., budbreak,leaf emergence, or blooming). The end of the growingseason should be defined by the end of plant growthin wetlands or in the local area. Recognizing that thefall is an important time for root growth, the growingseason extends beyond the time when leaves fall offdeciduous-leaved plants. In all likelihood, it continuesuntil the ground freezes. Minimum temperatures forroot growth may be from slightly above 0–7 �C(32–44.6 �F), with optimum temperatures rangingfrom 10 to 25 �C (50–77 �F). Soil scientists haveused the term ‘biologic zero’ to define conditionsthat relate to microbial activity in the soil: 5 �C

(41 �F) degrees measured at a depth of 50 cm (20 in.)in the soil. This concept may have some utility intemperate and tropical regions, but it is not valid forsubarctic and arctic regions where permafrost or fro-zen soil occurs at shallow depths and therefore nogrowing season would exist.

Despite the significance of flooding and soil satura-tion on growth of nonevergreen plants, there areother factors to consider when defining wetlandhydrology (i.e., should the focus be on growing sea-son or year-round conditions?). For example, whatare the needs of wetland-dependent animals and aresignificant wetland functions being performed out-side the ‘growing season’. Consider the following:

1. Evergreen plants and persistent graminoids (grassand grasslike plants) continue to grow during the‘dormant period’ for nonevergreens and satura-tion during this time should have some effect onthese and competing species.

2. Water conditions during the dormant period havea profound influence on hydrologic conditionsduring the early part of the growing season andmay prevent winter dessication of some plants.

3. Hydric soil properties have developed underreducing conditions that extend beyond thegrowing season.

4. Critical activities of some animals require dormantseason flooding or soil saturation (e.g., woodlandvernal pool breeders and pond animals).

5. Aquatic animals like fish need water year roundand are active year round.

6. Wetland functions such as nutrient transformationand cycling, shoreline stabilization, surface waterdetention, and sediment retention are independentof the growing season.

7. Wetness limitations during the dormant periodalso affect the potential uses of the land.

Defining wetland hydrology based on year-roundconditions appears to be justified from ecological andfunctional perspectives. This approach also wouldbetter reflect how wet some of the drier-end wetlandsreally are. Many of these drier-end wetlands (e.g., wetflatwoods of the southeastern United States) are wetfor significant periods (months) during the year, butare saturated near the surface for relatively shortperiods (weeks) during the growing season.

Does Prolonged Saturation GuaranteeAnaerobic and Reducing Conditions?

Although the definition of hydric soil emphasizesanaerobic reducing conditions and most wetlandsare exposed to periodic anaerobiosis, there may besome situations where wetlands exist in aerobic

782 Hydrology _ Wetland Hydrology

environments. Also, soils saturated for long periodsare not always reduced; there must be a source oforganic matter, sufficiently high temperatures to sup-port microbial activity responsible for reduction, anda population of reducing microbes present. (Note:The latter is typically present if the first two condi-tions are satisfied.) Possible ‘aerobic’ wetlands occurin seepage areas where oxygenated water is continu-ously flowing downhill, especially in colder climatesand high-elevation sites, and along coldwater moun-tain streams on cobble-gravel or sandy substrates.Saturated soils may not be reduced under the follow-ing circumstances: (1) in cold climates with averagetemperatures of less than 1 �C (33.8 �F), (2) in verysaline waterlogged desert soils where salinity restrictsgrowth of reducing microbes, (3) in areas with littleor no organic matter and moderate to high levels ofcalcium carbonate (e.g., irrigated rice basins in north-west India lack a low chroma matrix soil), and (4) inareas subject to groundwater discharge where dis-solved oxygen is present in water (e.g., in areas ofmoderate relief and soils on the edges of valleys).

Wetland Water Regimes

The hydrology of wetlands can be described innumerous ways. The duration of flooding and soilsaturation can be defined by various water regimes(Table 1). The flow of water can be classified asinflow (water coming into a wetland with no outlet;a sink), outflow (water flows out of wetland; source),

Table 1 Inland wetland hydrology descriptors based on the U.S. F

Water regime modifiers General definition

Permanently flooded* Inundated continuously, year-round in

Intermittently exposed Inundated year-round in most years, bSemipermanently flooded* Inundated throughout the growing sea

Seasonally flooded* Inundated for extended periods during

season; water table may be near the s

saturated) or may be well below the sTemporarily flooded* Inundated for brief periods during the g

growing season), with the water table

Intermittently flooded Inundated for variable periods with no

Saturated Water table is at or near the surface fowhen soil is saturated only seasonally

as seasonally saturated

Artificially flooded The frequency and duration of inundatpurposeful by means of pumps or siph

qualify, including irrigation

An asterisk (*) denotes water regimes that can also be modified to desc

(i.e., permanently flooded-tidal, semipermanently flooded-tidal, seasonally floo

Adapted from Cowardin LM, Carter V, Golet FC, and LaRoe ET (1979)Classifica

DC: U.S. Fish and Wildlife Service, FWS/OBS-79/31.

throughflow (water comes into and exits wetland),and bidirectional flow (water levels rise and fall inwetlands due to tides, lake or pond levels). If thewetland is surrounded by dry land and there is noknown flow into or out of the wetland (besides runoffand near-surface flow from adjacent upland), thewetland is hydrologically isolated from a surface-water perspective, but it may not be hydrologicallyisolated from a groundwater perspective. The topo-graphic position of ‘isolated wetlands’ may determinewhether they are sources, throughflows, or sinksbased on their location in groundwater flow systems.In semiarid regions like the North American PrairiePothole Region, topographic position and local geol-ogy influence water salinity and vegetation patterns,with freshwater wetlands at the highest elevations orlevels of the groundwater flow system (sources), themost saline wetlands at the lowest levels (sinks orsumps), and flow-through wetlands with intermedi-ate salinities in between (Figure 2). Because salt toler-ance is an adaptation possessed by certain plants,vegetation can be used to infer the hydrologic func-tion in these regions.

Hydrographs for Different Wetland Types

Due to variations in climate, soils, vegetation, geologicsetting, and other factors, the presence of water and itslocation in wetlands varies around the globe. Evenwithin local areas, the hydrology of wetlands differs

ish and Wildlife Service’s wetland classification system

all years

ut exposed during extreme droughtsson in most years

the growing season, but usually not flooded later in the growing

urface for much of the time when not flooded (seasonally flooded/

urfacerowing season (usually a couple of weeks or less early in the

typically well below the surface for extended periods thereafter

detectable seasonality; area is usually exposed

r most of the growing season and surface water usually absent;, usually early in the growing season, the hydrology is referred to

ion is controlled by humans; in the strictest sense, the control isons, but more generally, wetlands flooded by any artificial means

ribe freshwater tidal wetland hydrology by adding ‘-tidal’ to the term

ded-tidal, and temporarily flooded-tidal).

tion of Wetlands and Deepwater Habitats of the United States. Washington,

Rechargewetland

RechargewetlandFlow-through

wetland Dischargewetland

Explanation

Regional flow

Local flow Local flow

Intermediate flow Intermediate flow

Direction of groundwater flow

Average water table

Flow-throughwetland Discharge

wetland

Figure 2 Groundwater flow patterns in North American prairie pothole wetlands. Characteristics of the three types shown differ:

recharge wetlands (that recharge groundwater) are more fresh and have standing water for only a few months, whereas discharge

wetlands (that receive groundwater) are the most saline and are permanently flooded and flow-through wetlands are intermediate in

salinity and duration of surface water. Source: Berkas WR (1996) North Dakota wetland resources. In: Fretwell JD, Williams JS, andRedman PJ (eds.) National Water Summary on Wetland Resources, Water-Supply paper 2425, pp. 303–307. Reston, VA:

U.S. Geological Survey.

Hydrology _ Wetland Hydrology 783

by wetland type. Some wetlands are permanentlyflooded or nearly so, others are never flooded butpermanently saturated, and the rest are either periodi-cally inundated or seasonally saturated. The fluctua-tions of the water table and water levels in wetlandsmay be depicted graphically by hydrographs (Figure 3).

Changing Water Levels

Site wetness varies seasonally, annually, and long-term. Seasonal changes are reflected in the hydro-graphs that clearly show the wet season and dryseason for certain wetland types. Some types are per-manently flooded or saturated near the surface withthe latter showing slight changes in water table dur-ing the dry season (e.g., summer in the northeasternUnited States). Other types show marked fluctuationsin the water table during the year. Wetlands alsoexperience changes from year to year. During wetyears, water tables are higher than normal andwater may persist on the surface for longer periods,while in dry years they are lower (Figure 4).The long-term hydrologic cycle encompasses years

of normal precipitation, above normal precipita-tion and below normal precipitation. If wetlandhydrology were monitored for decades, the effect of

these precipitation patterns on wetland water levelsand water tables could be readily seen. Unfortunate-ly such data are lacking for most wetland types.Water levels in wetlands along the shores of NorthAmerica’s Great Lakes fluctuate with changes in lakelevels which vary from about 1–2m (3.5–6.5 ft) fromextremely wet years to extremely dry years. Thesechanges have a significant impact on plant commu-nities with aquatic beds predominating in high wateryears and wet meadows in low water years (Figure 5).Many woody plants that colonize these wetlands dur-ing dry years are killed by high water during wetyears. These wetlands are among the most dynamicof inland wetlands in North America from a plantcomposition standpoint. Wetlands in arid and semi-arid regions (e.g., prairie potholes and playas) alsoexperience somewhat similar vegetation changes dueto variability in regional precipitation patterns.

Water Budget

The water budget of an area, wetland or nonwetland,is an accounting of water inflows (gains or inputs) andoutflows (losses or outputs) to determine the changein storage (Figure 6). Inputs include water sources(precipitation, surface water, and groundwater),

−6

−4

−2

0

Jan Feb Mar Apr May June July Aug Sep

Floodplain forest

Ground level

Ground level

Ground level

Ground level

Ground level

Ground level

Fen or bog

Wet meadow

Marsh

Flatwood

Hardwood swamp

Oct Nov Dec

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Pos

ition

of w

ater

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eed

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e or

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vel)

+2

+2

−1

0

+1

−1

−2

−3

0

+1

0

+1

+2

+3

+4

Figure 3 Hydrographs of some common wetland types in the northeastern United States. Note that there is considerable variability

within types that are not reflected in these hydrographs and that these hydrographs are intended to represent general tendencies inwater levels for illustration purposes. Source: Tiner RW (2005) In Search of Swampland: A Wetland Sourcebook and Field Guide. New

Brunswick, NJ: Rutgers University Press.

784 Hydrology _ Wetland Hydrology

while outputs or water losses are attributed to evapo-ration, transpiration by plants, surface and subsurfacewater runoff, and groundwater recharge. Becausewet-lands have an excess of water, at least seasonally, theinputs are greater than the outputs at such time. Thewater budget equation is used by hydrologists and otherscientists to evaluate the net change of the volume ofwater in a defined area over time. Change in volume is

the sum of the inputs minus the sum of the outputs asexpressed by the following equation:

�V ¼ ½Pþ SiþGiþ Ti� � ½ETþ SoþGoþ To�

Inputs

The four sources of water are precipitation (P), sur-face water inflow (Si), groundwater inflow (Gi), and

Soil surface

Depthbelow

surface(inches)

0

20

40

60

80

A M

Surface waterpresent for

variable periods

Active plant growthand increasing

evaporation

Plants dormantlower evaporation

Water table

Surface waterpresent for

variable periods

J J AYear 1 Year 2

S O N D J F M A M J J A S O N D

Figure 4 Seasonal and annual differences in water levels in a forested wetland in the northeastern United States. Note that Year 1 is a

year of normal precipitation, while Year 2 represents a wet year, with corresponding changes in water table levels. Sources: Tiner RW,and Burke DG (1995) Wetlands of Maryland. National wetlands inventory cooperative publication. Hadley, MA: U.S. Fish and Wildlife

Service, Northeast Region and Maryland Dept. of Natural Resources; based on data from Lyford WH (1964) Water table fluctuations in

periodically wet soils of central New England, Harvard forest paper No. 8. Petersham, MA: Harvard University Forest.

Feet

Southwest

Drowned sedges

1973 (High water)

Water level

Fine grained deposits

1977 (Falling stage)

1965 (low water)

Extensive meadow

Peat accumulation

?

Old channel Emergent marsh

Betsieriver

Clay Sand

Marl

177.5

177.0

176.5

176.0

175.5

177.5

177.0

176.5

176.0

175.5

Water levelSand

Sand

Peat

Die back

Northeast

Meters

177.5

177.0

176.5

176.0

582

580

578

582

580

578

576

582Shrub zone

Water level

580

578

576

0 250 500 750 1000

0 100 200 300

1250

400

1500

500

1750 2000

600

2250 ft

700 M

?

Figure 5 Vegetation dynamics in a Great Lakes coastal wetland in response to changing lake levels. Source: Herdendorf CE,

Hartley SM, and Barnes MD (eds.) (1981) Fish and wildlife resources of the Great Lakes coastal wetlands within the United States, vol. 1:overview. Washington, DC: U.S. Fish and Wildlife Service, Biological Services Program, FWS/OBS-81/02-v1.

Hydrology _ Wetland Hydrology 785

GWI

GWO

SWO

SWI P

ET

High watertable

Low watertable

Δ S

Figure 6 Components of the wetland water budget: P, precipitation; SWI, surface water inflow; GWI, groundwater inflow; ET,

evapotranspiration; SWO, surface water outflow; GWO, groundwater outflow; and DS, change in storage. Sources: Carter V (1996)

Wetland hydrology, water quality, and associated functions. In Fretwell JD, Williams JS, and Redman PJ (eds.)National Water Summary

on Wetland Resources, Water-Supply Paper 2425, pp. 35–48. Reston, VA: U.S. Geological Survey.

786 Hydrology _ Wetland Hydrology

incoming or flood tides (Ti). The contributions ofthese sources vary daily, seasonally, and yearly.Inflows are typically natural, but can be human inducedby releases of water from dams and similar inputs.

Outputs

Water is lost through evapotranspiration (ET), sur-face water outflow or runoff (So), groundwater out-flow or recharge (Go), and outgoing or ebb tides (To).Evapotranspiration is a combination of evaporationand plant transpiration. Evaporation increases withrising air temperature and exposure of the land orwater surface to the sun, while plant transpiration isthe natural uptake of water from the soil by plantsand eventual loss to the atmosphere in the form ofwater vapor. Groundwater outflow could rechargegroundwater supplies or result from water with-drawals by humans.

Annual Water Budgets

Water budgets may be calculated for a wetland or awatershed and for different time periods. As men-tioned, wetlands form in areas inundated or water-logged for long periods of time; the wet season could

be from winter to early spring (as it is in the easternUnited States), summer to fall (as in Florida and thesouthwest United States), or in winter (in Mediterra-nean climates like that of California). During the wetseason, the inputs would exceed the outputs, so thatwater is stored in the wetland. When evaluated over alonger period, however, outputs may exceed inputs,especially for wetlands that are nearly permanentlywet. Some examples of annual water budgets areshown in Table 2.

Wetland Hydrology Indicators

In the absence of site-specific hydrological data (e.g.,hydrographs or recorded data from water-level gagesor observation wells), various features can be used toverify recent flooding or waterlogging. These featuresmay be direct observations of water on the surfaceor near the soil surface, indirect evidence such asfeatures left by recent flood or saturation events(Table 3), or inferred from soil properties (e.g., hydricsoil indicators, presence of a hard pan, dense claylayer, or permafrost layer near the surface) or vegeta-tion (e.g., presence of obligate hydrophytes or plantswith certain morphological adaptations to excessivewetness like hypertrophied lenticels and water roots).

Table 3 Potential wetland hydrology indicators for North American wetlands

Direct Evidence

Visual observation of surface water

Visual observation of a water table within 30 cm (12 in.) of the surface of nonsandy soils

Visual observation of a water table within 15 cm (6 in.) of the surface of sandy soilsVisual observation of soil glistening, or shaking or squeezing pore water out of the soil within 30 cm in nonsandy soil or within 15 cm in a

sandy soil

Positive reaction to a ferrous iron test with 30 cm of the surface in a nonsandy soil or within 15 cm of a sandy soil

Observed soil color change when exposed to air due to oxidation of ferrous ironSulfidic odor (rotten egg smell) from soil sample within 30 cm of the surface

Indirect Evidence of Flooding or Ponding

Water marks (e.g., blackish stains or silt lines)

Drift or wrack lines (piles of water-carried debris)Sediment deposits

Algal crusts on or near the ground

Drainage patterns (e.g., braided streams, network of minor streams, scoured areas, scouring around roots, and living plants bent over orlying in the direction of water flow)

Water-stained leaves

Mud cracks or surface polygons

Live or dead remains of aquatic invertebratesPresence of crayfish burrows

Presence of periphyton (aufwuchs) growing on plants

Aerial photo showing flooding or ponding

Indirect Evidence of Recent Soil SaturationOxidized rhizospheres within 30 cm of soil surface (e.g., iron oxide plaques on living roots or redox concentrations in soil surrounding

roots)

Presence of muck or mucky mineral soil on surfacePresence of deep soil cracks in clayey soils (e.g., Vertic soils)

Presence of salt deposits on soil surface

Presence of redox features in soil horizon despite bioturbation (i.e., mixing of soil by animals like earthworms)

Presence of deep impressions in soil left by heavy objects (e.g., vehicles or livestock)Observed water table between 30 and 60cm (12–24 in.) deep during dry season or dry year

Note that all these indicators are not of equal stature in verifying that the site still has wetland hydrology.

Source

Noble CV, Martel DJ, and Wakely JS (2005) A national survey of potential wetland hydrology regional indicators. Vicksburg, MS: U.S. Army Corps of

Engineers, Waterways Experiment Station, ERDC TN-WRAP-05–1.

Table 2 Examples of annual water budgets for selected wetland in North America

Wetland (location) Inputs Outputs Net storage

P Si Gi ET So Go

Bog (Massachusetts) 100 – – 70 8 8 14

Cypress River Swamp (Illinois) 23 70 7 22 71 6 1

Okefenokee Swamp (Georgia) 76 23 – 55 43 2 �1Prairie Pothole Marshes (North Dakota) 48 52 – 78 – 22 0

Hidden Valley Marsh (Ontario) 11 53 36 12 35 50 3

Arctic Fen (Northwest Territories) 22 52 26 27 40 <1 >32

Experimental Marsh (Ohio) 3 97 – 2 72 26 0

A positive storage value indicates a net gain in water.

Sources

Carter V (1996) Wetland hydrology, water quality, and associated functions. In Fretwell JD, Williams JS, and Redman PJ (eds.) National Water Summary on

Wetland Resources, Water-Supply Paper 2425, pp. 35–48. Reston, VA: U.S. Geological Survey.

Winter TC (1989) Hydrologic studies of wetlands in the northern prairie. In Van der Valk A (ed.) Northern Prairie Wetlands, pp. 16–54. Ames, IA: Iowa State

University Press.

Zhang L and Mitsch WJ (2002) Water budgets of the two Olentangy River experimental wetlands in 2001. In Mitsch WJ and Zhang L (eds.) Annual Report –

Olentangy River Wetland Park, pp. 23–28. Columbus, OH: Ohio State University.

Note that numbers reflect percent of inputs or outputs.

Hydrology _ Wetland Hydrology 787

788 Hydrology _ Wetland Hydrology

Glossary

Adventitious roots – Roots formed above ground; inwetland and aquatic plants they are induced byprolonged inundation.

Aerenchyma – Air-filled tissue in plants (typically inwetland and aquatic plants) that facilitates airmovement from aboveground plant parts to roots.

Aufwuchs – Plants and animals attached to plants,rocks, pilings, or other erect materials in water.

Biologic zero – A term used by soil scientists to referto the temperature threshold that generally causessoil microbes to become relatively inactive, so thatreducing conditions do not readily develop insaturated soils; defined by the soil temperature of5 �C or 41 �F measured at a depth of 50 cm (20 in.)below the soil surface.

Flatwood – Low flat, forested landscape typical ofcoastal or glaciolacustrine plains; soils vary frompoorly drained to well drained with slight changesin topography.

Flooding – In general terms, a condition where anarea is inundated (covered by water); soil scientiststend to restrict the term to inundation resultingfrom overbank flooding of a river or stream.

Floodplain – Nearly level alluvial land subject to peri-odic inundation (overflow from river or stream).

Glaciolacustrine plain – Low flat landscape asso-ciated with a former glacial lake, the exposed bedof a former glacial lake.

Graminoids – Grasses (members of the FamilyPoaceae – true grasses) and grasslike plants (typicallymembers of the families – Cyperaceae: sedges, bul-rushes, spikerushes, beakrushes, and cotton-grassesand Juncaceae: rushes, plus other herbaceous plantswith long narrow grasslike leaves).

Hydraulic conductivity – Measure of the ability ofwater to flow through the soil; low conductivityresists flow, while high conductivity favors flow.

Hydric soils – Soils formed under frequent and pro-longed reducing conditions due to excessive wet-ness; soils typical of wetlands.

Hydrology – The scientific study of water properties,distribution, and circulation; also the dynamics ofwater presence and movement in a particular habi-tat (e.g., wetland hydrology, lake hydrology, or for-est hydrology) or the study of these patterns.

Hydrophytic vegetation – Plants adapted for life inpermanently to periodically flooded or waterloggedsubstrates; plants growing in water and wetlands.

Hypertrophied lenticels – Expanded, enlarged corkypores on woody plants typically the result of pro-longed inundation.

Isolated wetlands – Better referred to as ‘geographi-cally isolated wetlands,’ wetlands that are sur-rounded by upland (nonhydric soils), with nosurface water connection to other wetlands orwaters; these wetlands may be connected hydrologi-cally to groundwater.

Karst terrain – A landscape formed in a limestoneregion where the topography is shaped by dissolu-tion of limestone (or dolomite, gypsum, or salt),characterized by caves, springs, seeps, sinkholes,and disappearing streams.

Lenticels – Corky roundish pores or lines on bark ofwoody plants that facilitate gas exchange betweeninner plant tissue and the atmosphere.

Meteoric water –Water precipitating from the atmos-phere as rain, snow, sleet, hail, fog, or mist.

Morainal deposit – Unsorted rocky and soil material(till) carried by and deposited by glaciers, typicallymarking the extent of glacier advance (terminalmoraine) or the sides of the glacier (lateral moraine).

Paludified landscape – Peat-dominated landscape inregions of high rainfall and low evapotranspiration(often cool, wet climates) formed by the processof paludification where peat mosses (Sphagnumspp.) grow over once dry land converting it to bog(peatland; blanket bogs).

Ponding – A term mostly used by soil scientists todescribe inundation resulting from surface water run-off into a closed depression or water accumulating ina depression from precipitation or high groundwater.

Porosity – The state of having pores or space filledwith gases or liquids; also a measure of the volumeof pores in a material relative to the total volumeof the material; sandy soils have higher porosity(a higher ratio of pore space to a given volume)than clayey or other fine-grained soils.

Sink (or sump) – From a hydrological standpoint, anarea lacking an outflow where water accumulatesor is absorbed, including terminal basins associatedwith watersheds in some arid regions (e.g., theGreat Basin of the southwestern United States).

Hydrology _ Wetland Hydrology 789

Telluric water – Water from the earth, groundwater.

Throughflow – A condition where water both entersand exits an area; water moves through the wetland,for example.

Vernal pool – A type of open-water wetland wherewater is present seasonally (spring in temperateregions); vernal poolsmaybe imbedded inwoodlandsas in the eastern United States or in grasslands as inthe western United States (e.g., California, Oregon,and Washington); in Mediterranean climates, thesewetlands are typically inundated in winter.

Waterlogging – A condition where the substrate issaturated at or near the surface for extended peri-ods; in wetlands, saturation is usually long enoughto create anaerobic (low oxygen) and reducing soilconditions that affect plant growth.

Wetland – In general terms, a shallow-water ecosystemor at least, periodically wet ecosystem subject to fre-quent inundation or prolonged soil saturation (water-logging) that is often characterized by hydrophyticvegetation, other aquatic organisms, and hydricsoils/substrates; a variety of specific definitions havebeen created for a host of purposes (legal and scientif-ic) including land use regulation, habitat protection,and natural resource inventories.

Wetland hydrology – The recurrent, sustained satura-tion of substrates at or near the surface by eithersurface or groundwater sufficient to create condi-tions that support aquatic life including the growthof hydrophytic vegetation, and the formation ofhydric soils or substrates; the dynamics of waterpresence and movement in wetlands.

See also: Atmospheric Water and Precipitation; Ecologyof Wetlands: Classification Systems; Ecology of Wet-lands; Evapotranspiration; Flood Plains; Ground Water;Hydrological Cycle and Water Budgets; Marshes – Non-wooded Wetlands; Peat and Peatlands; Swamps –Wooded Wetlands; Tidal Freshwater Wetlands; WetlandEcology and Management for Birds and Mammals;Wetland Ecology and Management for Fish, Amphibiansand Reptiles; Wetland Plants; Wetlands of Large Lakes;Wetlands of Large Rivers: Floodplains.

Further Reading

Carter V (1996) Wetland hydrology, water quality, and associated

functions. In: Fretwell JD, Williams JS, and Redman PJ (eds.)

National Water Summary on Wetland Resources, Water-supply

paper 2425, pp. 35–48. Reston, Virginia, USA: U.S. GeologicalSurvey. http://water.usgs.gov/nwsum/WSP2425/hydrology.html.

Fretwell JD, Williams JS, and Redman PJ (eds.) (1996) NationalWater Summary on Wetland Resources, water-supply paper

2425. Reston, VA: U.S. Geological Survey.GilmanK (1994)Hydrology andWetland Conservation.Chichester,

England: John Wiley & Sons.

Ingram HAP (1983) Hydrology. In: Gore AJP (ed.) Mires: Swamp,Bog, Fen, andMoor. Ecosystems of theWorld 4A,General Studies,ch. 3, pp. 67–158. Amsterdam, The Netherlands: Elsevier Sci-

ence.

Jackson CR (2007) Wetland hydrology. In: Batzer DP and

Sharitz RR (eds.) Ecology of Freshwater and Estuarine Wet-lands, ch. 3, pp. 43–81. Berkeley, CA: University of California

Press.

Mitsch WJ and Gosselink JG (2000) Wetlands. New York, NY:John Wiley & Sons.

National Research Council, Committee on Characterization of

Wetlands. (1995) Wetlands: Characteristics and Boundaries.Washington, DC: National Academy Press.

Price JS, Branfireun BA, Waddington JM, and Devito KJ (2005)

Advances in Canadian wetland hydrology, 1999–2003. Hydro-logical Processes 19: 201–214.

Richardson JL, Arndt JL, and Montgomery JA (2001) Hydrologyof wetland and related soils. In: Richardson JL and VepraskasMJ

(eds.)Wetland Soils: Genesis, Hydrology, Landscapes, and Clas-sification, ch. 3, pp. 35–84. Boca Raton, FL: Lewis Publishers.

Stone AWand Lindley Stone AJ (1994)Wetlands and Groundwaterin the United States.Concord, NH: The American GroundWater

Trust.

Tiner RW (1999) Wetland Indicators: A Guide to Wetland Identi-fication, Delineation, Classification, and Mapping. Boca Raton,

FL: Lewis Publishers, CRC Press.

Tiner RW (2005) In Search of Swampland: AWetland Sourcebookand Field Guide. New Brunswick, NJ: Rutgers University Press.

Williams TM (1998) Hydrology. In: Messina MG and Conner WH

(eds.) Southern Forested Wetlands: Ecology and Management,pp. 103–122. Boca Raton, FL: Lewis Publishers.

Winter TC (1989) Hydrologic studies of wetlands in thenorthern prairie. In: Van der Valk A (ed.) Northern PrairieWetlands, pp. 16–54. Ames, IA: Iowa State University Press.

Winter TC and Woo MK (1990) Hydrology of lakes and wetlands.In: Wolman MG and Riggs HC (eds.) Surface Water Hydrologyof North America, vol. 1, pp. 159–187. Boulder, CO: Geological

Society of America.

Relevant Websites

http://www.wcc.nrcs.usda.gov/wetdrain.

http://www.srs.fs.usda.gov/pubs/2083.

http://www.nap.usace.army.mil/cenap-op/regulatory/

water_monitor_technote.pdf.http://el.erdc.usace.army.mil/elpubs/pdf/tnwrap06-2.pdf.

http://el.erdc.usace.army.mil/elpubs/pdf/tnwrap05-1.pdf.

http://www.gret-perg.ulaval.ca/Price_et_al_HP18_2005.pdf.http://www.lk.iwmi.org/ehdb/wetland/displayallreferences.asp.

http://www.info.usda.gov/CED/ftp/CED/EFH-Ch19.pdf.