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II. Climate, Basic Hydrologic Concepts, and Wetland Classification

II. Climate, Basic Hydrologic Concepts, and Wetland Classification

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WETLAND SOILS OF PRAIRIE POTHOLES



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varies from semiarid in the west to humid in the east. As an example of the

variation in average yearly precipitation in the PPR, Richardson et al. (1991)

had three sites representative of semiarid, subhumid, and humid regions. Mean

yearly precipitation (20-year norms) was 34 cm in semiarid regions, 50 cm in

subhumid regions, and 85 cm in humid regions. Yearly variations are also extreme. Droughts and pluvial cycles are the norm. The westerly winds that typically prevail in central North America provide little precipitation to the PPR,

because these air masses, which originate in the Pacific Ocean, lose most of their

moisture on the west side of the Rocky Mountains. Most of the precipitation in

the PPR occurs in the spring and summer, the result of weather systems occasionally bringing in moist air from the Gulf of Mexico. The frequency of weather

patterns that bring moist Gulf air decreases as one moves west in the PPR, explaining the west to east gradient in precipitation (personal communication, Dr.

John Enz, State Climatologist for North Dakota).

The interactions between precipitation, temperature, and evapotranspiration

(ET) are important factors in the water budget of wetlands, and can influence

wetland frequency on the landscape. Given the same landscape and landforms,

high precipitation coupled with low ET favors the development of wetlands because water inputs are maximized and ET losses minimized. Conversely, low

precipitation coupled with high ET inhibits the development of wetlands because

ET losses are maximized (Zoltai, 1988). In the PPR, potential yearly evapotranspiration (PET) generally exceeds mean yearly precipitation, with the ratio between PET and average precipitation being highest in the southern and western

portions of the region, and decreasing northward and eastward. The impacts of

high PET coupled with low precipitation on the water budget of prairie wetlands

are great. Shjeflo (1968) noted that on average usually more than 35% of the

water lost from wetlands in North Dakota is evapotranspired, but that the ET

loss as a percentage of the total water loss was greatly influenced by the dominance of seepage inflow over seepage outflow of groundwater. ET losses are a

smaller percentage of the total water loss in wetlands dominated by seepage

outflow, whereas ET losses can go up to 100% of total water losses in wetlands

dominated by groundwater seepage inflow. Millar (197 1) observed a positive

correlation between shoreline :wetland area ratios and wetland evapotranspiration, and noted that smaller and shallower ponds with a large shore1ine:pond

area ratio tended to have higher evapotranspiration rates and were only seasonally persistent. In summary, the high PET: precipitation ratio tends to mitigate

against high wetland density and permanence in the PPR. Permanent lakes are

few compared to the more humid glaciated regions north and east. Many wetlands of the PPR are only seasonally to semipermanently ponded, and wetland

density decreases from east to west.

A climatic factor that does favor the formation of wetlands in the PPR is the

timing and distribution of surface runoff. Prairie wetlands receive a significant



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J. L. RICHARDSON ET AL.



portion of their water volume as surface runoff during spring snowmelt (Hubbard

and Linder, 1986), when frozen ground minimizes infiltration, and low temperatures and dormant plant communities minimize ET losses (Shjeflo, 1968; Lissey,

1971; Sloan, 1972). Shjeflo (1968) determined that snow accounts for at most

25% of total yearly precipitation, yet it accounts for at least 50% of the water

that reaches the wetland. Generally, a wet winter means a wet year for PPR

wetlands. Whereas intense summer thunderstorms may result in runoff, infiltration in the upland areas of the catchment after thaw, coupled with high evapotranspiration rates, minimizes runoff during the growing season.



B. HYDROLOGIC

ASPECTSOF WETLAND-GROUNDWATER

INTERACTIONS

INTHE PPR

Typical PPR wetlands lack integrated drainage networks. However, the presence of overflow or high-water outlets in some PPR wetlands indicates that partially closed systems are more common during unusually wet climatic cycles or

more pluvial periods, such as that which existed immediately after deglaciation.

Under present climatic conditions, the lack of effective water removal by channels accentuates the importance of groundwater as a component of the PPR wetland water balance (Winter, 1988, 1992; Richardson et al., 1992). Surface runoff in closed systems or partly closed systems is depression focused-i.e., flow

converges on the depression occupying the lowest portion of the closed catchment. Convergent flow thus brings water (and the material it transports) to a

point on the landscape from the surrounding catchment. Once the water is collected in depressions, some equilibrium level in the wetland water balance is

reached where sediment saturation persists long enough to develop anaerobic

conditions in the soil zone favorable for the growth of hydrophytes (Cowardin

et al., 1979). This equilibrium level strikes a tenuous balance between the input

components of precipitation, overland flow, and seepage inflow, and the output

components of evapotranspiration and seepage outflow. Because precipitation

and temperature are so variable in the PPR, the exact elevation where saturation

persists long enough for hydrophytes to grow is extremely variable, and can

change several feet in horizontal distance from year to year. Thus, the presence

of hydrophytes represents a short-term indicator of wetness at best (Stewart and

Kantrud, I97 1). Several soil characteristics better indicate long-term saturation

status (the hydrologic regime) because they integrate the effects of anaerobic

conditions and water movement over time. Important morphological characteristics of the soil that are often-used indicators of wetness include ( 1 ) soil mottling, which reflects the distribution pattern of redox-sensitive soil constituents,

(2) high levels of organic matter, which build up due to slow rates of decomposition in anaerobic environments, and (3) the distribution of evaporites in the



WETLAND SOILS OF PRAIRIE POTHOLES



127



soil, which can indicate the intensity and direction of saturated and unsaturated

water flow. The spatial relationship of soil horizons within and between pedons

is also an often-used indicator of water movement in soils. We feel the hydrology

of PPR wetlands is reflected most clearly in the associated hydric soils. Because

groundwater fluxes in PPR wetlands are so important in explaining the development and morphology of hydric soils in the PPR, we will examine the characteristic groundwater hydrology in some detail.



1. Darcy’s Law

Groundwater movement can be analyzed and simulated by Darcy’s law and its

extensions (Freeze and Witherspoon, 1967). Darcy’s law (4 = Kdh/dl) indicates

that groundwater flow velocity ( 4 ) is the result of the presence of a hydraulic

gradient ( d h / d l ) in sediments with a characteristic hydraulic conductivity ( K ) ,

Darcy’s law applied to saturated conditions predicts that flow will increase if the

hydraulic gradient or hydraulic conductivity increases. We will first consider the

influence of topography and climate on the spatial distribution of hydraulic gradients in the PPR.



2. Distribution of Hydraulic Gradients:

Recharge and Discharge in the PPR

Seeps or hillslope wetlands often occur below steep slopes because the water

table gradient is also steep, favoring groundwater discharge (Fig. 2) (Winter,

1988). Many hydrology texts and mathematical simulations of groundwater

movement assume that groundwater recharge generally occurs in upland areas

adjacent to depressions and streams (e.g., Toth, 1963; Freeze and Witherspoon,

1967; Winter, 1983). Under these conditions it is universally assumed that the

water table will become a subdued replica of the surface topography: groundwater mounds will form under topographic highs and groundwater depressions

will be associated with topographic lows (wetlands, lakes, or streams). Using

this model all wetlands will be foci of groundwater discharge (Lissey, 1971).

The assumption of upland-focused recharge does explain many of the hydrologic

characteristics of wetlands in the glaciated humid regions near the PPR. Because

of an excess of precipitation relative to evapotranspiration, upland recharge often

occurs. Most streams in the humid areas are effluent (receive groundwater), and

it has been suggested that wetlands in humid regions typically receive water from

the groundwater flow system as well (Richardson et al., 1991, 1992). Many

wetlands in this situation fill and overflow, and form the “deranged” surface

drainages characteristic of glaciated humid areas north and east of the PPR.

However, low precipitation and high evapotranspiration limit the development of

surface drainages in much of the PPR. Instead, PPR wetlands typically form



128



J. L. RICHARDSON ET AL.

Recharge



A



Flow thrwah

Pond Sediments



e

0



2



DISTANCE (km)

Recharge



6



0



DISTANCE (km)



2



Figure 2 Recharge, flowthrough, and discharge wetlands with mineralogical controls and soil

types fresh to saline wetlands in Nelson County, North Dakota. After Arndt and Richardson (1988,

1989b) and Richardson et al. (1992).



relatively large complex wetland systems connected to each other by groundwater flow. Whereas upland groundwater recharge may be characteristic of humid areas where precipitation exceeds potential evapotranspiration, several researchers have indicated that groundwater recharge is much more complex in

subhumid to semiarid areas.

Wetlands in the PPR have been described as surficial expressions of the water

table (Sloan, 1972), the free water surface at atmospheric pressure that can be

located at, below, or above the hydric soil surface. In the absence of integrated

surface drainages, it is apparent that groundwater recharge, groundwater movement, and groundwater discharge are intimately associated with PPR wetlands.

Lissey ( I 97 1) noted that groundwater recharge and discharge are focused on

depressions in hummocky areas of the PPR. He also observed that interdepressional uplands are relatively inactive regarding water transfers to and from the

water table. This depression-focused nature of groundwater and surface water

movement is a direct result of hummocky topography and a subhumid to semiarid climate. In the PPR, groundwater recharge occurs first where the vadose

zone is thinnest (Winter, 1983). Because the vadose zone in uplands is thick in



WETLAND SOILS OF PRAIRIE POTHOLES



129



the PPR and precipitation is meager, much of the summer infiltration occurring

on upland slopes never reaches the water table. Two additional factors act to

reduce recharge in the uplands for PPR wetlands: (1) hydraulic conductivity of

surface sediments is often ansiotropic, favoring lateral water movement over

vertical movement, and (2) spring snowmelt and runoff occur when the soil is

impermeable due to the presence of frost layers.

Groundwater movement in the PPR has necessitated the development of several hydrologic concepts and terms suited to the region. The groundwater system

interaction with the surface water in PPR wetlands is expressed as recharge, discharge, orjowthrough, depending on the dominant process at each site (Fig. 2).

Recharge wetlands recharge groundwater within the wetland basin. When rapid

overland flow is discharged to a recharge-type depression, infiltration into and

percolation through the pond bed eventually recharges the groundwater, producing a water table mound that slowly dissipates through lateral and downward

groundwater movement (Miller et al., 1985; Knuteson et al., 1989). In the PPR,

wetlands that are usually dry by midsummer receive the majority of their water as

spring snowmelt and recharge shallow groundwater aquifers (Richardson et al.,

1991). Discharge wetlands receive groundwater that is discharged into the wetland basin. Evaporative discharge is the upward capillary flow of water from a

near-surface water table in response to hydraulic gradients set up by higher evapotranspiration rates at the soil surface (Fig. 3). Flowthrough wetlands both recharge the groundwater system and receive groundwater as discharge. On a landscape scale, water can be thought of as moving laterally through flowthrough

wetlands (Fig. 2).

The groundwater recharge and discharge characteristics of PPR wetlands are

strongly influenced by climate, and reflect a climatic gradient from the humid east

to the semiarid west (Richardson et al., 1991). As examined above, semipermanently ponded wetlands in humid areas are typically discharge type (Fig. 4A).

However, in subhumid areas, semipermanent wetlands typically receive and yield

water to the water table (Fig. 4B). In semiarid regions, recharge to groundwater

is common in nearly any wetland basin (Fig. 4C) (Miller et al., 1985; Mills and

Zwarich, 1986; Richardson et al., 1991).

Flow reversals may occur with changes in hydraulic gradient. Recharge wetlands may briefly become discharge wetlands if the water table in adjacent uplands rises above usual levels, and discharge wetlands can become recharge wetlands if the water table is drawn down and a subsequent precipitation event floods

the basin. Flow reversals affecting entire wetlands, or parts of wetlands, are

frequent occurrences in the PPR. When considered on a small scale, such as that

involving the wetland edge, several flow reversals may occur in one season. In

Fig. 4A, the presence of the wetland on the downslope side of the water table

indicates a discharge condition. After a rain, edge-focused recharge as discussed

by Winter (1983) and Arndt and Richardson (1993) results in groundwater



130



J. L. RICHARDSON E T A .



A

Bearden



0



B



-1



R



*



Rainfall



Distance (m)



*



Lindaas



30



Bearden_

I_ Lindaas



4

-



Figure 3 (A) Wetland recharge and edge soils (after Knuteson et al., 1989). The solid arrows

are saturated flow and the open arrows represent unsaturated flow from the water table to the drier

soil surface. The Bk-horizon here is from a short flow distance and lacks gypsum and salinity.

(B) Flownet illustrating a recharge wetland based on (A) and Knuteson et al. (1989) and Richardson

et al. (1992). Note that the equipotential lines are high in the surface and decrease with depth.



mounding at the edge (Fig. 4B) that produces a shallow, localized reversal of

flow landward of the groundwater mound. Because of the increase in hydraulic

gradient pondward of the mound, groundwater discharge to the pond is also



131



WETLAND SOILS OF PRAIRIE POTHOLES

A

Humid region



-..



Postprecipitationwet meadow mound

Direct discharge



.-..-../..



-..-..-..-



-Wetland

..-..-..



,



C

Phreatophyte wet meadow drawdown

vaporative discharge



/-.x

.._,._.._..-..

Wetland

~



'..



I



.1.-



..-.__..- .'/ '



Figure 4 (A) Expected flow. (B) A water table mound forms on a wetland edge shortly

after a precipitation event, creating a recharge point. (C) The wet meadow drawdown from evapotranspiration.



enhanced. Such reversals may be unimportant when considering large, deep flow

systems; however, reversals have been shown to be very important when examining soil morphology and salinity dynamics in wetland soils (Arndt and Richardson, 1993). Flow reversals induced by phreatophyte transpiration at the wetland edge were examined in detail by Meyboom (1966) in wetlands surrounded

by willow (Salix spp.). After the spring recharge, groundwater mounds typically

developed under these wetlands. However, transpiration by the phreatophytic

willow resulted in the formation of a water table depression around the wetland

periphery, which becomes a groundwater discharge site (Fig. 4C). Because high

rates of evapotranspiration are typical of the wetland periphery, the wetland edge

often becomes the location of focused discharge during significant drawdown



I32



J. L. RICHARDSON ETAL.



periods (Fig. 4C). Soils formed under such complex hydrologic conditions are

discussed by Wilding et al. (1963), Mills and Zwarich (1986), Seelig el al.

(1990, 1991), and Richardson et a1.(1992).



3. The Influence of Sediment Hydraulic Conductivity

Changes in recharge and/or discharge produce flow reversals as discussed

above. The influences of sediment characteristics affecting hydraulic conductivity (K) are less obvious but have a strong influence on groundwater movement.

The important components of K that influence groundwater movement in the

PPR are the texture and the anisotropic nature of unconsolidated surface sediments. Anisotropy is a term that indicates that hydraulic conductivity within the

sediment is not the same in all directions. Preferential lateral flow (also called

interflow, throughflow, and stormflow) results from higher hydraulic conductivity in the horizontal when compared to the vertical dimension. Preferential lateral

flow is the result of several factors. Compaction resulting from the weight of

overlying materials progressively increases sediment bulk density and reduces

porosity with depth. Other factors include plant root macroporosity, textural

changes, and the presence of bedding planes, frozen or partially frozen soil, and

restrictive layers such as an argillic horizon (Kirkby and Chorley, 1967; Zaslavsky and Sinai, 1981). Preferential lateral flow in surface sediments reduces

groundwater recharge in sloping uplands and influences the distribution of evaporites and soil morphology of hydric soils (Steinwand and Richardson, 1989).

Texture and stratigraphy are features of surface sediments that directly influence K , PPR wetland groundwater topography, and the magnitude of groundwater flow. In lacustrine sediments and fine-textured tills, high hydraulic gradients are often observed because the very low hydraulic conductivity associated

with fine textures limits the quantity of flow. As an example, a groundwater

mound with an unusually steep gradient was found to be associated with a recharge wetland in a very low-relief landscape with fine-textured lacustrine sediments (Knuteson et al., 1989). The relief of the groundwater mound was several

times greater than the relief of the land surface. The persistence and magnitude

of the mounding under the recharge wetland were associated with depressionfocused recharge and fine-textured sediments. The mounding explained the major differences noted in soil type and leaching regime over short distances

(Fig. 3).

Conversely, coarse-textured sediments have higher hydraulic conductivity than

do fine-textured sediments. The rapid movement of groundwater in these sediments prevents the development of steep hydraulic gradients (Winter, 1986).

Significant groundwater flows often occur in sand-dominated landscapes, although very steep hydraulic gradients will be absent. We speculate, based on our

field observations, that the presence of coarse textures enhances upland-focused

recharge, and discharge-type wetlands are common.



WETLAND SOILS OF PRAIRIE POTHOLES



133



4. Flownet Examples of Recharge, Flowthrough,



and Discharge Wetlands

A flownet is a mesh of equipotential lines and associated streamlines that indicates the direction and magnitude of groundwater flow. By convention, equipotential lines indicate constant values in head, and adjacent lines indicate equal

head drops. Streamlines indicate the linear path of water flow. Streamlines always meet equipotential lines at right angles, and groundwater flow is always

from higher to lower magnitude equipotential lines. All streamtubes formed by

adjacent streamlines have equal amounts of flow per unit time. Thus fast groundwater flow is indicated where streamlines converge. Conversely, where streamlines diverge, groundwater flow slows. Closely spaced equipotential lines indicate the presence of large hydraulic gradients. Conversely, where equipotential

lines are spread far apart, low gradients are found.

Figure 2 illustrates the groundwater-surface water interactions of recharge,

flowthrough, and discharge wetlands using flownet analysis. Recharge occurs as

water collects in a depression and infiltrates into the soil. The magnitude of

equipotential lines associated with recharge wetlands decreases from the surface

maximum (Fig. 2a), indicating the downward flow (Fig. 2b). Because the water

is snowmelt and runoff derived, intermittent ponding with fresh water leaches

the soil and promotes the development of an argillic horizon (Amdt and Richardson, 1988). In Fig. 3b we also illustrate a recharge wetland condition. The equipotential lines in the flowthrough wetland are perpendicular to the surface and

decrease in magnitude from left to right (Fig. 2a). Lateral water flow from left

to right is indicated (Fig. 2b). Leaching of minerals will not play an important

role in soil development in flowthrough wetlands because the presence of continuously saturated conditions and brackish water limits the development of an

argillic horizon. The magnitude of the equipotential lines steadily decreases as

the discharge wetland (20) is approached (Fig. 2a), indicating groundwater seepage (discharge) to the wetland (Fig. 2b). These discharge wetlands concentrate

salts in many cases.

The evaporites tend to accumulate in soils at the pond margin. Evapotranspiration on the edge of wetlands or any other landscape position with a shallow

water table creates a condition of abundant soil water loss. This evaporative

discharge allows evaporite minerals such as calcite and gypsum to accumulate in

the periphery of flowthrough and discharge wetlands (Arndt and Richardson,

1988; Steinwand and Richardson, 1989).



C. WETLAND

CLASSIFICATION

Several systems are currently used to classify wetlands in the PPR. The most

important classification systems include the Canadian system (Zoltai, 1988), the



134



J. L. RICHARDSON ETAL.



United States Fish and Wildlife system (Cowardin et al., 1979), the Stewart and

Kantrud (1971, 1972) system, and an extension of the Stewart and Kantrud

(1971, 1972) system that incorporates hydrology and landscape position (Arndt

and Richardson, 1988). We will examine the last three systems in some detail

below.



1. United States Fish and Wildlife Service System

The Cowardin et al. (1979) wetland classification was developed for use nationwide by the United States Fish and Wildlife Service. It has been adopted as

the standard system of wetland classification in the United States. The Cowardin

et al. (1979) classification is hierarchical in nature. The highest level of classification consists of five systems based on major ecological type: they are marine,

estuarine, lacustrine, riverine, and palustrine. Within each system, wetlands are

further differentiated into subsystems, classes, and dominance types based on

criteria that include ponding permanence, dominant vegetation, water chemistry

and pH, and soil type. In essence, the Cowardin et al. (1979) system is designed

to discriminate ecologically important zones within a wetland; however, basins

can often be classified at the highest levels. In the PPR virtually all “pothole”type wetlands would be classified into the Palustrine System. Basins would be

classified into the Palustrine Emergent class, based on the presence of emergent

vegetation. Zones within the wetland are classified at the more detailed subclass

and dominance-type levels. Specifically, Cowardin et al. (1979) define wetlands

and palustrine systems as follows:

Wetlands are lands transitional between terrestrial and aquatic systems where

the water table is usually at or near the surface or the land is covered by

shallow water. For purposes of this classification wetlands must have one or

more of the following three attributes: (1) at least periodically, the land supports predominantly hydrophytes; (2) the substrate is predominantly undrained

hydric soil; and (3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of each year.

The Palustrine System includes all nontidal wetlands dominated by trees,

shrubs, persistent emergents, and emergent mosses or lichens, and all such wetlands that occur in tidal areas where salinity due to ocean-derived salts is low. It

also includes wetlands lacking such vegetation, but with all of the following four

characteristics: (1) area less than 8 ha (20 acres); (2) active wave-formed or

bedrock shoreline features lacking; (3) water depth in the deepest part of basin

less than 2 m at low water; and (4) salinity due to ocean-derived salts is low.

The Palustrine System was developed to include vegetated wetlands traditionally called by names such as marsh, swamp, bog, fen, and prairie (Cowardin

et al., 1979). It also includes the small, shallow, permanent, or intermittent



WETLAND SOILS OF PRAIRIE POTHOLES



135



water bodies often called ponds. Palustrine wetlands may be situated shoreward

of lakes, river channels, or estuaries; on river floodplains; in isolated catchments;

or on slopes. They may also occur as islands in lakes or rivers. The erosive forces

of wind and water are of minor importance except during severe floods.



2. The Stewart and Kantrud System

The Stewart and Kantrud (1971, 1972) system was developed specifically as a

research tool for the PPR. It uses plant community composition criteria to describe and define distinctive wetland basins. The Stewart and Kantrud system is

based on zonal plant communities that, in addition to hydrologic variables, reflect salinity and pond permanence. Wetlands are assigned class numbers from I

to V depending on the plant species and zonation present. In classes I through

V, higher numbers indicate the presence of central vegetation zones that reflect

increasingly wet conditions: from central low prairie communities through central wet-meadow, shallow-marsh, and deep-marsh communities, to central open

water (Figs. 5 and 6 ) . Subgroupings based on species composition reflect salinity

tolerances: from fresh, through brackish, to saline subgroups (Table I).



3. Hydrologic Classification of Wetlands

We believe that a wetland basin classification based on pond permanence and

salinity is the best method to make ecologically meaningful delineations for most

wetlands in the PPR. The separations are easier, more accurate, and require less

time than other systems. Arndt and Richardson (1988) outlined a field-oriented

basin-delineation system that inferred recharge-flowthrough-discharge hydrology by relating soil morphology to the observations of Stewart and Kantrud

(1971, 1972), Sloan (1972), and Lissey (1971). Stewart and Kantrud (1971)

identified pond permanence and salinity as defined by wetland plant communities, and Sloan (1972) and Lissey (1971) related salinity, topographic position,

and pond in duration to groundwater recharge-discharge relationships. They

found that, although wetlands can be defined or classified by hydrologic function

as recharge, flowthrough, and discharge wetlands, these wetlands actually form

a continuum on the landscape. The emphasis is on dominant flow conditions,

because seasonal and climatic variation creates intermittent reversals. Recharge

and discharge wetlands are relatively easy to distinguish. Recharge wetlands are

temporarily to seasonally ponded with fresh water and have leached soil profiles

consistent with their hydrologic function. Discharge wetlands are semipermanent

to permanently ponded, and often have saline soils. Efflorescent salt crusts are

often evident around the pond edge. Even though they are permanently ponded,

the catchment area for the pond is small relative to the pond itself, indicating a

large amount of groundwater discharge. Flowthrough wetlands “bridge the gap”



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