Dynamics of water-table fluctuations in an upland between two prairie-pothole wetlands in North Dakota

Journal

ELSEVIER

Journal of Hydrology

191 (1997) 266-289

Dynamics of water-table fluctuations in an upland between two prairie-pothole wetlands in North Dakota Donald 0. Rosenberry”, Thomas C. Winter USGeological

Survey, Box 25046, Mail Stop 413, Denver Federal Center, Denver, CO 80225, USA Received 24 October 1995; accepted 15 February 1996

_ Abstract Data from a string of instrumented wells located on an upland of 55 m width between two wetlands in central North Dakota, USA, indicated frequent changes in water-table configuration following wet and dry periods during 5 years of investigation. A seasonal wetland is situated about 1.5 m higher than a nearby semipermanent wetland, suggesting an average ground water-table gradient of 0.02. However, water had the potential to flow as ground water from the upper to the lower wetland during only a few instances. A water-table trough adjacent to the lower semipermanent wetland was the most common water-table configuration during the first 4 years of the study, but it is likely that severe drought during those years contributed to the longevity and extent of the water-table trough. Water-table mounds that formed in response to rainfall events caused reversals of direction of flow that frequently modified the more dominant water-table trough during the severe drought. Rapid and large water-table rise to near land surface in response’to intense rainfall was aided by the thick capillary fringe. One of the wettest summers on record en&d the severe drought during the last year of the study, and caused a larger-scale water-table mound to form between the two wetlands. The mound was short in duration because it was overwhelmed by rising stage of the higher seasonal wetland which spilled into the lower wetland. Evapotranspiration was responsible for generating the water-table trough that formed between the two wetlands. Estimation of evapotranspiration based on diurnal fluctuations in wells yielded rates that averaged 3-5 mm day-‘. On many occasions water levels in wells closer to the semipermanent wetland indicated a direction of flow that was different from the direction indicated by water levels in wells farther from the wetland. Misinterpretation of direction and magnitude of gradients between ground water and wetlands could result from poorly placed or too few observation wells, and also from infrequent measurement of water levels in wells.

* Corresponding author. 0022-1694/97/$17.00 Q 1997- Elsevier Science B.V. All rights reserved PII SOO22- 1694(96)03050-S

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1. Introduction Hydrologic setting is a significant and sometimes paramount control of the chemical md biological conditions of a wetland (Sloan, 1972; LaBaugh et al., 1987; Winter and Jamas, 1993). Many wetlands in North Dakota are in a semi-arid climate where the valance between precipitation and evaporation is highly variable, indicating that the rydrological, chemical, and biological stability of these wetlands may be susceptible to :hanges in climate (LaBaugh et al., 19%). Some wetlands respond greatly to seasonal as veil as longer-term changes in the balance between precipitation and evaporation. Many )f these wetlands dry up in summer or autumn. Other wetlands are fairly stable in response o changing climatic conditions, and dry up only after prolonged, severe drought. The lydrologic stability of some wetlands suggests that a relatively stable flux of ground water :ontributes to these wetlands and probably also significantly affects the water quality of hese wetlands. The role of ground water in determining the hydrologic setting and chemical charactersties of wetlands in the prairie-pothole region of North America is complex (LaBaugh :t al., 1987). Recent studies have indicated that ground-water fluxes to and from prairiemthole wetlands are highly variable temporally and spatially, and the direction of Ilux :hanges frequently (Mills and Zwarich, 1986; Woo and Rowsell, 1993; Winter and Rosennrry, 1995). Winter and Rosenberry (1995) indicated that transpiration can cause water.able troughs to form adjacent to some wetlands. Water moves toward these troughs from he wetlands, but these troughs also intercept ground water that otherwise would recharge he wetland. However, transpirationally induced troughs were not present at all wetlands, )r along the entire perimeter of most wetlands. Gerla (1992) indicated that in eastern North Dakota a relatively thick capillary fringe associated with clay-rich glacial drift contributed .o rapid and large water-table fluctuations in response to recharge events. He reported that he water table could rise as much as lo-20 times greater than the depth of precipitation hat caused the infiltration, which was important to maintain surface-water levels in amporary wetlands. Sloan (1972) and Winter and Carr (1980) reported that seepage from topographically righer wetlands can flow via ground water to discharge into wetlands at lower elevation. However, LaBaugh et al. (1987) indicated that transient reversals in direction of groundwater flow could interrupt this flow path for various periods of time. They also stated that dl processes associated with the interaction of wetlands and ground water were not :ommon to all wetlands in the wetland complex that they studied, and that some processes were dependent to a large extent on the physical setting of the wetland relative to the local .opography. Using numerical simulation of hypothetical settings, Winter (1983) found that transient water-table mounds resulting from near-shore focused recharge could block the flow of Iround water from an upgradient water body to a downgradient water body. Since then, bis process has been observed at numerous lake settings (e.g. Anderson and Munter, 1981; iosenberry, 1985; Sacks et al., 1992; Anderson and Cheng, 1993) and at wetland settings n Indiana (Doss, 1993) and in Delaware (Phillips and Shedlock, 1993). Recharge and lischarge of ground water focused adjacent to the edges of wetlands has been shown to E common in the prairie-pothole region (Amdt and Richardson, 1993). Amdt and

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Richardson also stated that complex flow patterns cause zones of evaporite salts to form in soils adjacent to wetlandedges. However, few studies have used a group of wells instrumented with continuous recorders to evaluate the complexity of flow processes adjacent to wetlands. The purpose of this paper is to use frequently collected data from a line of wells located between two wetlands in east-central North Dakota to: (1) determine the location, frequency and duration of water-table mounds that form in response to ground-water recharge between the higher and lower wetland, (2) determine the extent, frequency and duration of water-table troughs caused by evapotranspiration that form between the two wetlands; (3) determine the effect of variable ground-water configurations on the stage of the lower wetland. The occurrence and significance of these processes need to be addressed to further the understanding of the interaction between wetlands and ground water, especially with regard to proper management of other wetlands in the prairie-pothole region. Ground-water and wetland stage data discussed herein were collected from June 1989 to December 1993. 2. Site description The Cottonwood Lake study area is located in east-central North Dakota near the eastern flank of the northwest-southeast trending Missouri Coteau (Fig. 1). The site is hummocky, contains many wetlands of small to medium size, and local relief is about 26 m. The site has been described in detail by Winter and Cat-r (1980). The wetland of primary interest, Wetland Pl, is classified as semipermanent (Cowardin et al., 1979) and is situated near the center of the Cottonwood Lake study area. The area of closely spaced instrumentation is located on an isthmus of land of 55 m width between a seasonally flooded wetland (T3) and Wetland Pl (Fig. 1). Glacial till underlies the Cottonwood Lake study area and contains discrete deposits of sand and gravel (Winter, 1996). The soils between Wetlands Pl and T3 are primarily Typic Calciaquolls, relatively sandy near the surface and becoming finer below about 15 cm depth (Arndt and Richardson, 1993). Soils are considerably less dense and more fractured in the uppermost meter. Hydraulic conductivity, as determined by single-well slug tests, ranges from 2 x lo4 to 6 x lo4 cm s-’ for the shallow water-table wells, and from 1 x lo-’ to 3 x 10” cm s-’ for the deeper-screened wells. 3. Methods Shaiiow wells were installed between Wetlands T3 and Pl to provide data on the fluctuation of the water table, the upper surface of the ground-water system (Fig. 1). These wells were constructed by hand-augering a hole using a bucket auger of 8.9 cm diameter, installing PVC screen and pipe of 5.1 cm diameter, pouring a silica sand pack around the screen, and then backfilling the rest of the hole with drill cuttings. Screens were ten-slot and of continuous wire-wound construction. Depths of shallow wells were selected so the screened interval would intersect the water table most of the time. Deeper wells, designed to provide vertical hydraulic-head gradients when compared with shallow wells installed in the same location, were installed using the same technique, except a

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Fig. 1. Study site located between Wetlands T3 and PI at the Cottonwood Lake area, North Dakota. Wells 47-5 1 and 64 are instrumentedwith water-level sensors.

Giddings power auger was used to drill the holes. Head, as defined in this paper, is the hydraulic head at the midpoint of the screened interval of a well. For the shallow wells, head is assumed to be analogous to the water-table head. Six of the wells were instrumented with potentiometer-float systems described by Rosenberry (199oa). Potentiometers were queried each minute by a data logger that was programmed to provide 2 h and daily averages of heads in the wells. Manual check measurements were made weekly for wells equipped with stage recorders; all other wells were measured at 2 week intervals during the open-water period and monthly during winter. Water-level accuracy of automated wells was -CO.007m; manual water-level measurements had an accuracy of +0.004 m. The stage of Wetland Pl was measured using a potentiometer-float system installed in a stilling well in the center of the wetland. Stage values were averaged hourly and daily by a

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D.O. Rosenbeny, T.C. WiniedJoumd of Hydrology 191 (1997) 264-289

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data logger also located in the center of the wetland. The stage of Wetland T3 was indicated by a steel staff gage located in the center of the wetland. This staff gage was read weekly when the wetland contained water. When the stage of Wetland T3 was sufficiently high to flood the ground where Well 47 is located, the head at Well 47 was used as a surrogate for the stage of Wetland T3.

4. Results During the first 4 years of this study, and during the year before the beginning of this study, the climate in the region was significantly drier and warmer than normal (Rosenberry, 1996). Precipitation was below normal during 40 of the 60 months from 1989 to 1993 (Fig. 2). Wetland T3 contained water during only 1.5 months of the first 4 years of this period, and the stage of Wetland Pl was much below normal (Fig. 2). During each year from 1989 to 1992, Wetland Pl went dry. Before 1988, this wetland had not gone dry since 1978. Many of the wells between the wetlands also went dry during parts of the first 4 years of the study. Whereas precipitation was below normal during 7 of 12 months during 1993, the summer of 1993 was one of the wettest summers on record, and both wetlands and all of the wells contained water from early summer to the end of the year. Wetland Pl was dry in March, but by the end of August the wetland contained water about 1.3 m deep (Fig. 2). Wetland T3 filled to its maximum stage in the summer of 1993, at which point it began to spill into Wetland Pl . The extreme climatic variability during this 5 year period provided an opportunity to evaluate ground-water fluxes adjacent to wetlands in response to widely varying conditions. Three possible water-table configurations between Wetlands T3 and Pl are presented in Fig. 3. Only Configuration 1, termed continuous gradient herein, allows water to flow from Wetland T3 to Wetland Pl. In Configuration 2 a hydraulic-head dam, in response to a water-table mound, prevents water from flowing from Wetland T3 to Wetland Pl. In Configuration 3 a water-table trough, in response to evapotranspiration, intercepts any water that would travel from Wetland T3 to Wetland Pl . Each of these configurations was present at least once during the 5 year study. In addition, a number of other water-table configurations were present when either or both wetlands did not contain water. 4.1. Continuous gradient (Configuration 1) Periods during which water had the potential to flow from Wetland T3 to Wetland Pl were rare during the 5 year study because Wetland T3 contained water so infrequently during the first 4 years of the study. However, even when Wetland T3 contained water, a flow-through condition was not common. For example, during June-July 1990, weekly discrete measurements indicated that Wetland T3 contained water from about 17 June to about 9 July (Fig. 4). During the approximately 22 day period when Wetland T3 contained water there was a continuous gradient from Wetland T3 to Wetland Pl for only about 5 days, as indicated by the shaded portions of Fig. 4. The rest of the time there was either a water-table mound near Well 48 or a water-table trough near Well 51. During the wet summer of 1993, a flow-through configuration was common from June to early September,

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but from mid-September to mid-November a water-table trough was present near Wetland Pl. The water-table trough was small but it probably prevented the shallowest ground water from discharging into Wetland Pl. A continuous ground-water gradient toward Wetland Pl during times when Wetland T3 was dry was infrequent at the site. Disregarding times when wells or Wetland Pl were dry, a continuous gradient toward Wetland Pl was present only 18% of the remaining time during the study. 4.2. Water-table mounds (Configuration 2) Nearly all water-table mounds that formed between Wetlands T3 and Pl formed when

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ntinuous gradient \

559.5 E

Q E 559.0 .c 24% 5 12

Fig. 4. Daily average stage of Wetland Pl and beads of wells located between Wetlands T3 and PI, and daily total rainfall during 14 June to 14 July 1990. Values for Wetland T3 stage are from five discrete observations.

Wetland T3 was dry. As only those mounds that formed when Wetland T3 contained water had the potential to block flow from Wetland T3 to Wetland Pl, results associated with those mounds will be.presented first, followed by results associated with mound formation when Wetland T3 was dry. 4.3. Mound formution when Wetland T3 contained water A water-table mound formed only three times during the 5 year study when both wetlands contained water. During each occurrence, a mound was present for only a few days, but it dissipated for distinctly different reasons each time. Water-table configurations before, during and after mound formation for two of the three events are shown in Fig. 5. In 1990, 132 mm of rain fell during the first 3 weeks of June, creating the watertable configuration shown in Fig. 5(A). This configuration (similar to Configuration 1 in Fig. 3) provided the potential for water to flow through the ground-water system from Wetland T3 to Wetland PI. However, an additional 36 mm of rain that fell on 27-28 June (Fig. 4) led to the water-table mound configuration shown in Fig. 5(B) (similar to Configuration 2 in Fig. 3). This mound had an apex in the vicinity of Well 48, it was about

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0.06 m higher than the stage of Wetland T3, and it lasted for about 5 days, after which a continuous water-table gradient towards Wetland Pl resumed (Fig. 5(C)). This water-table mound had a short duration because not enough rain fell to maintain it. In contrast, a water-table mound which formed in early June 1993 (not shown in Fig. 5), after 59 mm of rain fell over a 5 day period, ceased to exist in part because of a preexisting steep water-table gradient between the wetlands. This mound was very short in duration (2 days), it had a much higher apex above the stage of Wetland T3 (0.17 m), and the apex was nearest to Well 49. The short duration of this mound was caused in part by the higher stage of Wetland T3 and the relatively steep gradient between Wetlands T3 and Pl. The steep water-table gradient between Well 49 and Well 50 allowed a rapid redistribution of ground water that caused the water table at Well 49 to quickly drop below the stage of Wetland T3. Additional rainfall following mound formation also resulted in the dissipation of a water-table mound. A water-table mound formed in July 1993 (Fig. 5(E)) that had an apex between Wells 48 and 49 and was 0.05 m higher than the stage of Wetland T3. It formed in response to 31 mm of rain that fell between 7 July and 14 July. However, additional rainfall caused the cessation of this mound. On 15 July 1993, the largest single-day accumulation of rainfall during the 5 year study (105 mm) caused Wetland T3 to reach its maximum stage and spill overland into Wetland Pl (Fig. 5(F)). At this time, the well and wetland stages were identical and both were coincident with the minimum elevation of the land surface between the two wetlands, effectively eliminating the water-table mound. 4.4. Mound formation when Wetland T3 was dry Many other water-table mounds of various magnitude and duration were present during times when Wetland T3 was dry. Although these mounds were not responsible for an interruption of ground-water flow from the higher to the lower wetland, they may have had an effect on the flux of ground water into Wetland Pl . These mounds were usually highly transient, both temporally and spatially, and they would have been difficult to document without the high density of continuously monitored water levels at the site. Water-table mounds were centered most frequently in the vicinity of Wells 48,49 and 51. Considering only the times when the wells and Wetland Pl contained water, the percentage of time the apex of a water-table mound was nearest to a given well was 13% for Well 48, 15% for Well 49, 4% for Well 50, and 14% for Well 5 1. (Although Well 64 was not installed until 1991, a water-table mound was present in the vicinity of Well 64 during 8% of the time when adjacent wells and the wetland contained water.) Therefore, a water-table mound was in existence somewhere between the wetlands during 46% of the time when Wetland Pl and adjacent wells both contained water. During the rest of the time, one of the following occurred: Wetland Pl or the wells were dry, a water-table trough was present, a combination of a mound and a trough was present, or a continual gradient toward Wetland Pl was present. The frequency of the formation of water-table mounds between Wetlands T3 and Pl, based on comparison of hydrographs of three wells and Wetland Pl, is shown in Fig. 6. Water-table mounds are indicated in Fig. 6 whenever the head at a well closer to Wetland

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Pl is higher than the head at a well more distant from Wetland Pl , and also higher than the stage of Wetland Pl. For example, in late August and early September 1989 a water-table mound is indicated near Well 5 1 because the head at Well 5 1 is higher than both the head at Well 49 and the stage of the Wetland. Well 47 is dry at the time, as indicated by the flat line representing the well hydrograph. Other water-table mounds are indicated during portions or all of April 1990, April, May, June and July 1991, April, May, June, July, September, and November 1992, and during April, May, June and July 1993. The configuration and duration of water-table mounds was dependent not only on the timing and distribution of rainfall, but also on the time of year that the mound formed. In general, water-table mounds that formed at the site occurred most often during spring and early summer (Fig. 6); they were highly transient and had many different configurations no matter what the season. Also, water-table mounds that formed in the spring or early summer usually had an apex that was farther away from Wetland Pl than water-table mounds that formed later in the summer. The response of the water table to early-summer vs. late-summer rainfall is shown in Fig. 7. Two series of cross-sections of water-table profiles over time illustrate early-season and late-season responses to two rainfall events separated by an intervening dry period of approximately 2 weeks duration. These periods were selected to show how the generally wetter environment during early summer leads to more frequent, longer-duration watertable mound formation. The early-summer series (Fig. 7(A-E)) show water-table configurations before and after rainfall of 51 mm on 14 June and 30 mm on 28-30 June. The late-summer series (Fig. 7(F-J)) show water-table configurations before and after rainfall of 52 mm on 18 August and 43 mm on 31 August-3 September. At the beginning of each of the series of panels in Fig. 7 the water-table gradient indicates that water had the potential to flow from Wetland Pl to ground water. After the first rainfall event, a water-table mound formed during the early-summer series but not during the late-summer series (Fig. 7(B), Fig. 7(G)). In the early-summer series the watertable mound was as much as 1.3 m higher than the stage of Wetland PI (Fig. 7(B)), which created a large gradient to drive potential flow of ground water into the wetland. However, following the late-summer rainfall only Well 51 contained water and a gradient away from the wetland still was present (Fig. 7(G)). The early-summer water-table mound lasted for approximately 12 days, after which a water-table trough formed near Well 51 to truncate the mound and provide the potential for flow from the wetland to ground water (Fig. 7(C)). There was no change during the week following the late-season rainfall (Fig. 7(H)). Following the second early-summer rainfall event, the water-table configuration indicated the presence of a mound with an apex near Well 49 that was 1.1 m higher than the stage of Wetland Pl (Fig. 7(D)). Following the second late-summer rainfall event, a water-table mound formed that had an apex near Well 51 and was 0.5 m higher than the stage of Wetland Pl (Fig. 7(I)). Both the early-summer and late-summer watertable mounds that formed in response to the latter rainfall events lasted for approximately 6 days, after which the water level at Well 51 dropped below the stage of Wetland Pl, forming a near-shore water-table trough. Immediately before the formation of a watertable trough, the apex of the early-summer mound was still 0.7 m higher than the wetland and centered near Well 49 (Fig. 7(E)), but the apex of the late-summer mound had

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nigrated from near Well 5 1 to near Well 49, and was only 0.25 m higher than the wetland Fig. 7(J)). U, Water-table troughs (Configuration

3)

A water-table trough located adjacent to Wetland Pl was the most common water-table :onfiguration between the two wetlands during the 5 year study period, and it was espe:ially common during the drier tirst 4 years. During extended dry periods water-table roughs formed first in the vicinity of Well 51 or Well 64 and then extended laterally oward Wetland T3 until, in extremely dry situations, the water table declined below all of he monitoring wells. Assuming that a water-table trough was present during times when Yell 51 was dry but Wetland Pl was not, a water-table trough was present 49% of the time luring the 5 year study period and 61% of the time during the first 4 years. Assuming that a vater-table trough was present during times when both Well 51 and Wetland Pl were dry, I water-table trough was present during 64% of the 5 year study and 71% of the time luring the first 4 years. Direct evaporation and transpiration from upland and near-shore vegetation caused a &line in the water table that was surprisingly consistent among wells. Median values of laily head decline during the 5 year study were 0.02 m for all wells except Well 5 1, where he median daily decline in head was 0.03 m. Maximum daily declines in head were much arger, reaching values as great as 0.3 m, but those large head changes were due to edistribution of ground water as larger pore volumes were dewatered following recharge :vents.

‘ig. 8. Daily avenge stage at Wetland Pl and head at selected wells. and daily total rainfall during an extended ry period, 1 July to 31 August 1990.

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The effect of evapotranspiration on water-table configuration was greatest during extended dry periods, an example of which is shown in Fig. 8. During July and August 1990,75 mm of rain fell on 11 of 62 days, totaling only 60% of normal rainfall for July and August. A water-tabletrough formed adjacent to Wetland Pl in response to this dry period, forming first at Well 51 on 4 July, reaching Well 50 on 19 July, reaching Well 49 on 1 August, and extending as far as Well 48 on about 13 August (Fig. 8). Median daily decline in head was a consistent 0.04 m for all four wells, although maximum daily declines in head ranged from 0.07 to 0.14 m. Maximum declines in all wells occurred on either 4 July or 5 July. Rainfall caused a brief rise in head for a day or two at some of the wells, but ensuing daily head declines were unchanged when compared with daily head declines before rainfall. At first glance, it appears that the break in slope of the head decline at Well 5 1 was being mitigated by lateral flow of water from nearby Wetland Pl after about 4 July, because the head of the well became lower than the wetland stage. However, hydrographs for the other wells show similar breaks in slope at the same time, which could indicate that a decrease in evapotranspirationbeginning on about 4 July affected all wells, and that the break in slope of the decline at Well 5 1 was not related only to lateral movement of water from Wetland Pl.

4.6. Effects of evapotranspiration

on configuration

of the water table

The unusually dry conditions at the site during the first 4 years of the study prevented extensive analysis of evapotranspirationbecause the wells were dry much of the time. The extremely wet conditions in July and August 1993 precluded the documentation of diurnal head fluctuations because most of the wells were under water during much of the summer. Furthermore, at the wells that were not under water, frequent rains interrupted periods when the effects of evapotranspiration could be quantified. Nevertheless, evidence of daily evapotranspiration, in the form of diurnal head fluctuations in wells, was documented during sufficient portions of the study to provide some estimates of evapotranspiration rates. Unlike the well-known studies that evaluated daily rises and falls of the water table in wells to determine evapotranspiration (White, 1932; Robinson, 1958; Meyboom, 1967). most water-table responses to evapotranspiration showed step-like changes with little or no rise of the water table at night. One reason for the lack of night-time recovery of the water table is that hydraulic conductivity decreases with depth at the site. This retards the ability for water from deeper in the aquifer to resupply water lost from the shallow portion of the aquifer to evapotranspiration. Large vertical hydraulic-head gradients (commonly 0.07-0.08) that developed during extended periods of evapotranspiration provide further evidence that the portions of the aquifer 5-6 m below the water table are not able to rapidly resupply the shallowest portions of the aquifer. Also, sensor limitations probably resulted in hydrographs having erroneously small amplitude. Potentiometer-float type water-level sensors in small-diameter wells (about 5 cm diameter) have been shown to truncate the apex and nadir of hydrographs owing to frictional resistance (Rosenberry, 1990b). The only time that overnight recovery in head was documented was at Well 51 during early August 1993, when the shoreline of Wetland Pl was within 1 m of Well 51. The close proximity of the wetland allowed water to resupply losses from

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Fig.9. Diurnalhead fluctuationsin wells in response to evapotranspiration,and rainfall, during 1 July to I5 July 1990. Water-level data are 2 h averages and rainfall data are 2 h totals.

ground water owing to evapotranspiration. However, overnight recovery was not documented at Wells 50 or 49, even though Well 50 was only 5 m from the shoreline of the wetland. An example of diurnal fluctuations of the water table during a time when the water table was close to land surface is shown in Fig. 9. Diurnal fluctuations are clearly shown in hydrographs of Wells 48 and 51, they are less obvious but still present in the Well 49 hydrograph, and they are absent in the Well 50 hydrograph. Spatial variability in diurnal fluctuations probably is due partly to spatial variability in permeability and specific yield of the glacial till in which the wells are installed, and partly to spatial variability in transpiration. Diurnal fluctuations of the water table at Well 48 continued until 8 August, at which time the well went dry. The depth to the water table at the time the well went dry was 1.7 m. At Well 49, diurnal fluctuations ceased after 14 July, at a depth to water of 1.2 m. Diurnal fluctuations at Well 51 ceased after 20 July at a depth to water of 1.4 m. After cessation of diurnal fluctuations, the heads at Wells 49 and 5 1 continued to decline at the same average daily rate, and at a nearly constant rate throughout each day. The following equation was used to estimate rates of evapotranspiration (White, 1932): q=Sy(24r

2 s)

(1)

where q is discharge of water by transpiration, Sy is specific yield, r is average hourly rise m stage from 0O:OOh to 04:oO h, and s is net daily rise or fall of the water table. The White method has been criticized for overestimating evapotranspiration because of he uncertainty in determining Sy. Specific yield commonly is determined by applying itress over long periods of time. However, the stress caused by evapotranspiration is on an sourly to daily time scale, which is relatively short. Meyboom (1967) suggested using half he normally determined value for Sy for estimating rates of evapotranspiration. Gillham 11984) stated that Sy is variable, especially when the capillary fringe extends to land anface. Gerla (1992), at a wetland study in eastern North Dakota, suggested using the .atio of precipitation to rise in head during times when the capillary fringe extends near or

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Fig. 10. Estimates of daily total evapotranspiration using 2 h water-level data from Wells 48.49 and 51, 1 July to 25 July 1990.

to land surface, as a means of obtaining a more representative value for Sy. This method, termed ‘effective specific yield’ by Gerla, also was used at a swampy site near Chalk River, Ontario (Novakowski and Gillham, 1988). The ratio of precipitation to rise in head was used to determine Sy for Wells 48,49, and 51. After excluding data for times when either the depth to water was excessive, or the unsaturated zone was relatively dry before rainfall, median values of Sy were 0.09 for Well 48,O. 11 for Well 49, and 0.06 for Well 5 1. Daily evapotranspiration during July 1990 ranged from 0 to 17 mm (Fig. 10). Daily rates averaged 4 mm at Well 48,7 mm at Well 49, and 3 mm at Well 5 1. Daily rates of 17 and 12 mm probably are erroneously large, and may have resulted from a decline in the water table at Well 49 that was due to redistribution of ground water and not due to evapotranspiration. Discounting these values, the average daily evapotranspiration rate at Well 49 was 5 mm. Evapotranspiration rates declined with time, indicating that as depth to water increased, evapotranspiration decreased. Missing data in Fig. 10 indicate days when no evapotranspiration signal was evident from the well hydrographs. Rates of evapotranspiration at Well 51 also were determined for the first few weeks of August 1993. Night-time rise in stage at Well 5 1 was documented only when the shoreline of Wetland Pl was within a meter of Well 51. Night-time rise in stage was subtle, it was never more than 0.01 m, and it averaged 0.004 m. Evapotranspiration rates during the first 10 days of August ranged from 1 mm day-’ to 3 mm day-‘, averaging 1.7 mm day-‘.

5. Discussion

The configuration of the water table between Wetlands T3 and Pl was highly variable, both spatially and temporally, during the 5 year study. Water-table fluctuations in response to precipitation and evapotranspiration were relatively large; head rise in wells following rainfall commonly exceeded 1 m. On repeated occasions, the head in a well close to

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Vetland Pl, when compared with the stage of Wetland Pl, indicated the gradient was way from the wetland, whereas at the same time the head in a well farther away indicated lat the gradient was toward the wetland. Occasionally, the opposite situation was bserved, when data from a well close to Wetland Pl indicated the gradient was toward le wetland, and at the same time data from a well farther away indicated the gradient was way from the wetland. Without the high density of wells at the site, these undulating rater-table conditions would have gone undocumented. Misinterpretation of the interaction between ground water and an adjacent wetland ould easily result using improperly positioned wells or too few wells. In the simplest ase where no wells are present, the difference in elevation between the higher and lower retland would indicate a gradient in the water table between the two wetlands of about .02, which would indicate ground-water flow from the higher to the lower wetland. The requent water-table mounds and water-table troughs that form and interrupt this continous gradient would not be documented. Similarly, a high degree of temporal variability rould go undocumented without frequent measurement of water levels in wells. Phillips nd Shedlock (1993) observed similar near-shore phenomena adjacent to some wetlands in Delaware,and they also speculated that they may have missed the development of some ansient water-table mounds by not having a sufficient number of wells in the proper Eations. .I. Water-table mounds Water-table mounds formed frequently between Wetlands T3 and Pl, even though the rst 4 years of the study were much drier than normal. However, many of the water-table iounds formed during times when the upgradient wetland was dry. One of the goals of iis study was to determine the frequency at which water-table mounds formed that would t-event water from flowing from Wetland T3 to Wetland Pl, mounds that would create a ydraulic-head dam between the two wetlands. During the few times when Wetland T3 ontained water, water-table mounds seldom were present between the two wetlands. Only uee times during the 5 year study were water-table mounds present, and in each case the iounds were present for a week or less, making them insignificant as a barrier to flow of rater between the two wetlands. It is likely that water-table mounds would have been larger and would have formed lore frequently’if the land-surface elevation between the two wetlands had been higher. &en the water table was less than about 0.5 m below land surface before rainfall, watertble rise at most wells located between the two wetlands was greatly diminished relative ) water-table rise when the water table was farther below land surface. Only when the :age of Wetland T3 was high would the water table rise above about 0.5 m below land &ace. Desiccation cracks, plant root channels, and other void spaces were more common ear land surface, which probably truncated the capillary fringe in many locations and revented the water table from rising beyond about 0.5 m from land surface. Arndt and ichardson (1993) noted that shallow soils at the site were less dense and more sandy than eeper soils, which would suggest that the air-entry value is lower and the capillary fringe oes not extend as readily into the shallowest soils. Gerla (1992) noted a similar result at a ady of wetlands in eastern North Dakota, where the water-table rise fell short of reaching

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land surface by 0.2 m. He also attributed the smaller than expected rise to unsaturated macropores in the shallow soil. Water-table rise at nearby wells where the unsaturated zone was thicker was not diminished when the water table was high. At Well 13-18, located 22 m southeast of the transect of instrumented wells (Fig. l), land surface is about 1.1 m higher than at Well 49. During times when small, short-lived water-table mounds were present at Well 49, water-table elevations at Well 13-18 were as much as 0.3 m higher. Data from Well 13-18, when compared with Wetland T3 water levels, indicated the presence of much larger water-table mounds that lasted for weeks, rather than days. Data from this study indicate that, even during extended dry periods, smaller scale water-table mounds can form adjacent to wetlands in response to small to moderate rainfall events. These mounds can interrupt the temporal continuity of longer-duration watertable troughs. Larger scale water-table mounds form in response to repeated rainfall, heavy rainfall, or rainfall that occurs in the spring or early summer, when the depth to the water table is less and the unsaturated zone is not as dry. Continuous water-table gradients from a higher wetland to a lower wetland can have a significant effect on the stage of the downgradient wetland by allowing flow of water from the higher to the lower wetland. However, the more transient, smaller-scale water-table mounds that form adjacent to the lower wetland also are significant in maintaining water in the lower wetland during dry conditions when the higher wetland does not contain water. Water-table mounds, if they extended along a significant portion of the shoreline of Wetland Pl, probably delayed the drying up of Wetland Pl during dry periods. These small mounds not only halt losses of water from the lower wetland, but they also aid in shunting overland flow to the lower wetland. On a number of occasions, when a small water-table mound was present adjacent to Wetland Pl , the rise in stage of the wetland following rainfall was greater than the depth of rainfall. Phillips and Shedlock (1993) also noted that the largest rises in stage of wetlands in Delaware occurted when a transient water-table mound was present. Winter (1983) indicated that recharge adjacent to a lake or wetland would occur first near the shoreline because that was where the unsaturated zone is thinnest. This process was documented between Wetlands T3 and Pl following significant recharge events, when the water table between the wetlands was relatively low. The water-table mound that formed in September 1989 formed first next to Wetland Pl and then the apex of the mound migrated away from the wetland with time (Fig. 7(I), Fig. 7(J)). However, when the water table between the wetlands was relatively high, such as during June 1990 or June 1993, water-table rises near Wetland Pl were quickly overshadowed by rises of equal or larger magnitude at wells farther away from the wetland. Commonly, within 2-6 h of a water-table rise at Well 51, the water table would rise at the other wells to elevations higher than at Well 51, making the process of focused recharge adjacent to the wetland short lived and insignificant. One explanation of these results is that during times when the water table is low the unsaturated zone must first be rewetted before recharge water can reach the capillary fringe and the water table. This results in a lag in response at wells where the unsaturated zone is thicker compared with wells where the unsaturated zone is thinner. However, when the water table between the two wetlands is relatively high and nearer land surface, recharge quickly reaches the capillary fringe, and the ensuing watertable rise is higher and more rapid.

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Several other studies have noted rapid water-table rises in response to compressed air trapped beneath a wetting front (Lisse effect) (Meyboom, 1967), or in response to the capillary fringe extending near or to land surface (reversed Wieringermeer effect) (Meyboom, 1967; O’Brien, 1982; Novakowski and Gillham, 1988; Gerla, 1992). These rapid water-level rises were reported to subside to pm-rise levels within minutes to days. At this study site many rapid water-table rises were documented during times when the capillary fringe extended to near land surface, but most of these rapid water-table rises did not result in the creation of a water-table mound between the two wetlands. The watertable rises commonly were lo-25 times larger than the depth of precipitation that caused them. Water-table rises commonly occurred within 4 h of rainfall. However, unlike the rapid water-table declines following rainfall that would indicate a Lisseeffect response, water-table declines following the end of rainfall at the site took from 4 days to 2 weeks to return to pre-rainfall stages. Gerla (1992) reported similar water-table responses following rainfall at a site in eastern North Dakota. He attributed the extension of the capillary fringe to near land surface as a mechanism for allowing even very small rainfall events to create standing water in wetlands that had been dry before rainfall. Bouma (1990) stated that infiltration via macropore Row, termed ‘bypass flow’, is common and results in rapid movement of recharge water to the water table. Snowmelt commonly is among the largest fluxes of water to a prairie-pothole wetland (Eisenlohr, 1972; Hubbard and Linder, 1986; Winter and Rosenberry, 1995), and snowmelt also can cause large rises in stage in adjacent wells. Data from the site indicate that ground-water recharge from snowmelt between the two wetlands caused large rises in the water table that occasionally resulted in the formation of a water-table mound. The effect of snowmelt on Wetland Pl stage is indicated in Fig. 6 during 1992 and 1993, when winter and early spring data were available. In late March 1993 Wetland Pl fluctuated from being dry to containing water 0.3 m deep during a period of very little precipitation, but there was little change in heads in adjacent wells. From late February to early March 1992 the stage of Wetland Pl rose 0.2 m in response to snowmelt during late January and late February to early March. During the 1992 snowmelt period the head in some of the wells between Wetlands T3 and Pl rose much more than the rise in wetland stage. A thawfreeze-thaw cycle probably caused the water-level fluctuations recorded during the late winter thaw. Water levels at Wells 47 and 49 (Fig. 6(E)) (and also at Well 48, not shown) rose significantly in response to infiltration from snowmelt and from 10 mm of precipitation that fell on 4-9 March. However, warm weather lasted only 2 days, and the shallow ground water at Well 49 probably froze inside the well when the head was within 0.3 m of land surface, freezing the float in place in the well, as evidenced by the flat line in the Well 49 hydrograph. It appears that this ice lens thawed after a few days when the head in the well began to decline. However, at Well 47, where the head rose later in response to snowmelt, an ice lens evidently remained in place throughout most of the rest of March, and did not melt until late in the month when the head in the well declined suddenly to helow the well screen. The intermittent freezing and thawing of water in the wells makes it difficult to determine whether the timing of stage rise and decline in the wells is representative of actual conditions in the ground water, which makes documentation of watertable mound formation uncertain. Another snowmelt period during 1992 occurred in early April, when the heads at Wells

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49 and 51 rose during a period of no rainfall. This time a water-table mound formed, centered near Well 49, and it remained in place for more than a month. 5.2. Water-table troughs Water-table troughs were the predominant water-table feature at the site during the first 4 years of this study, indicating that during most of the time water was flowing out of Wetland Pl. This condition was anomalous when compared with longer-term data reported by Winter and Rosenberry (1995), who used discrete well measurements collected from 1979 to 1990 to indicate that the predominant direction of flow between Wetlands T3 and Pl was toward Wetland Pl. However, they reported periods of flow from Wetland Pl to ground water during parts of 9 of the 12 years of their study, and they reported these periods of flow out of the wetland lasted up to 7 months. They also showed that reversals of flow and water-table troughs extended around nearly the entire perimeter of the wetland during the driest years. Sloan (1972) reported that it was common for drawdown zones to fringe North Dakota prairie potholes in the summertime. Steinwand and Richardson (1989) showed how drawdown zones fringing marshes caused areas of high salinity, commonly resulting in deposition of gypsum and/or calcite. Evapotranspiration was a significant process in determining the water-table configuration adjacent to Wetland Pl. With the possible exception of Canada thistle (Cirsium arvense), which flourished on the wet slope when Wetland T3 spilled into Wetland Pl in 1993, none of the plants located between the two wetlands is a known phreatophyte. However, based on diurnal water-table fluctuations that were recorded at all of the instrumented wells between the two wetlands, it may be possible that upland plants become opportunistic when phreatic or capillary-fringe water is within reach of their roots. The resupply of water to the tension-saturated zone by ground water, in response to removal of tension-saturated water by transpiring plants, probably is the dominant process resulting in diurnal water-table declines. Direct evaporation from the soil was responsible for part of the diurnal stage changes, but only during times when the water table was near land surface. Evapotranspiration declined as the water table dropped, a condition that has been documented at a number of other studies of evapotranspiration of ground water (e.g. Robinson, 1958; Nichols, 1994). Because diurnal fluctuations ended once the depth to the water table reached about 1.2- 1.7 m, either the root zones of the plants did not extend deeper than that depth, or if the roots were shallow, the capillary fringe was only about 1 m thick. Nightly recovery of the water table during extended periods of evapotranspiration was infrequent at the site, as mentioned above. Although the inability for rapid resupply of shallow ground water by deeper ground water is suspected for the lack of a night-time water-table recovery at this site, similar water-table responses to evapotranspiration at wetland sites in Indiana (Doss, 1993) and Delaware (Phillips and Shedlock, 1993) also have been reported. Both of these wetland sites were in relatively sandy settings where the vertical resupply of shallow ground water would not normally be retarded. However, at the Delaware site and probably also at the Indiana site, a water-table mound adjacent to the wetland was present during step-wise water-table fluctuations in response to evapotranspiration, so perhaps evapotranspiration was simply hastening the dissipation of the water-table mounds when the water table was already elevated above a steady-state head.

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latter process was the suggested cause of a lack of nightly recovery at a wetland study in North Dakota (Comeskey, 1993).

This

5.3. Impact ofwater-table configuration on the stage of Wetland PI The significance of reversals of flow direction between ground water and Wetland Pl to the water budget of Wetland Pl is beyond the scope of this paper; however, flow direction between Wetlands T3 and Pl correlates well with daily decline in stage of Wetland Pl. Average daily decline of stage of Wetland Pl during periods when the ground-water gradient was toward Wetland Pl was compared with average daily decline in stage of Wetland Pl during periods when the ground-water gradient was away from the wetland. Days when precipitation fell were excluded from the analyses. During July-August of each summer of 1990-1992, daily stage decline at Wetland Pl was at least twice as great when there was a ground-water gradient away from the wetland as when there was a gradient toward the wetland. During the wet summer of 1993 the gradient was never away from the wetland so a comparison could not be made. However, the daily stage decline of Wetland Pl during 1993 was within the range of other summers when the gradient was toward the wetland. Although other variables affect the stage of Wetland Pl, such as variations in rates of evapotranspiration and variations in ground-water gradients along other reaches of the wetland shoreline, this evidence indicates that temporal changes in water-table configurations adjacent to the wetland could be significant to the water budget of the wetland. It also is possible that the rate of change of wetland stage could be used as a simple tool to indicate the relative interconnectivity with ground water. In addition, it has been suggested by LaRaugh et al. (1987) and Amdt and Richardson (1993) that temporal changes in the direction of flow between ground water and an adjacent prairie-pothole wetland have a significant effect on the chemistry of the wetland. Phillips and Shedlock (1993) showed that transient water-table mounds adjacent to wetlands in Delaware caused intermittent pulses of low pH, sulfate-rich water that played a large role in maintaining acidic conditions in the wetlands. Steinwand and Richardson (1989) stated that deposition and dissolution of gypsum deposits along wetland margins could be affected seasonally by changes in hydrology. 6. Summary and conclusions Water-table configuration beneath an upland separating two wetlands was highly variable, both temporally and spatially. During 4 years of severe drought, the most common water-table configuration was a water-table trough that formed first adjacent to the lower wetland, and extended beneath the entire upland between the wetlands as dry conditions persisted. The last summer of the study was extremely wet, and a water-table gradient from the higher to the lower wetland was present much of the time. During late summer the stage of the upper wetland increased until it spilled into the lower wetland. Rainfall events during the dry first 4 years of the study commonly resulted in the formation of transient water-table mounds, the magnitude and duration of which depended on the timing and amount of rainfall. Rainfall that occurred in spring or early summer usually created larger water-table mounds that were centered near the center of the upland

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between the two wetlands. Rainfall in late summer or early autumn more often caused the formation of smaller water-table mounds that formed adjacent to the lower wetland. On two occasions these late-season water-table mounds formed first adjacent to the lower wetland, and then migrated away from the wetland as the height of the mounds dissipated. Only three times during the 5 year study was a water-table mound documented that blocked the flow of water from the higher wetland through the ground-water system to the lower wetland. All three mounds dissipated within days of formation and they were not significant barriers to the movement of ground water from the higher wetland to the lower wetland. Water-table mounds appeared to be truncated by land surface. At other sites, where the upland area is higher relative to the upgradient wetland, water-table mounds could be more common and persistent. During much of the study the capillary fringe probably extended to near land surface, aiding in rapid and large water-table rises following precipitation. However, at most wells the capillary fringe appeared not to extend beyond about 0.5 m from land surface, perhaps because of a greater pore volume owing to a high density of desiccation cracks and plant root channels that prevented capillary rise. Evidence of evapotranspiration was seen in diurnal water-table fluctuations at all instrumented wells. Diurnal water-table fluctuations were not documented once the depth to water exceeded about 1.7 m. Evapotranspiration by upland plants around the perimeter of the wetlands was estimated using the White (1932) method and using a modified specific yield determined from the ratio of precipitation to water-table rise following precipitation. Evapotranspiration rates ranged from 0 to 17 mm day-‘; averages at individual wells varied from 3 to 5 mm day-‘. Reversals in gradient probably are significant to the hydrology of wetlands. Daily stage decline of the semipermanent wetland at this study site was greater during times when the water-table gradient was away from the wetland than during times when a water-table gradient was toward the wetland. References Anderson,M.P. and Cheng. X., 1993. Long and short term transience in a groundwater/lakesystem in Wisconsin, U.S.A. J. Hydrol., 145: 1-18. Anderson,M.P. and Munter,J.A., 1981. Seasonal reversals of groundwaterflow aroundlakes and the relevanceto stagnation points and lake budgets. Water Resour. Res., 17(4): 1139- 1150. Arndt, J.L. and Richardson, J.L.. 1993. Temporal variations in the salinity of shallow groundwaterfrom the peripheryof some North Dakota wetlands (USA). J. Hydrol., 141: 75-105. Bouma, J.. 1990. Using morphometricexpressions for macroporesto improve soil physical analyses of field soils. c&de46: 3-11. Comeskey, A& 1993. Estimation of a water budget for Agnes Marsh, Grand Forks County, North Dakota. Masters Thesis, University of North Dakota, Grand Forks, 133 pp. Cowardin,L.M., Carter,V., Golet, F.C. and LaRoe, E.T., 1979. Classification of wetlands and deepwaterhabitats of the United States. US Fish and Wildlife Service Office of Biological Service, Washington,DC, FWS/OBS79/31,131 pp. Doss, P.K., 1993. The nature of a dynamic water table in a system of non-tidal, freshwater coastal wetlands. J. Hydrol., 141: 107-126. Eisenlohr, W.S., 1972. Hydrologic investigations of prairie potholes in North Dakota, 1959-68. US Geol. Surv. Prof. Pap., 585-A. 102 pp.

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of Hydrology

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Gerla P.J., 1992. The relationship of water-table changes to the capillary fringe, evapotranspiration. and pre-

cipitation in intermittent wedands. Wetlands, 12(2): 91-98. Gillham, R.W.. 1984. The capillary fringe and its effect on water-table response. J. Hydrol., 67: 307-324. Hubbard, D.E. and Linder. R.L., 1986. Spring runoff retention in prairie pothole wethmds. J. Soil Water Conserv., 41(2): 122-125. LaBaugh. J.W., Winter, T.C., Adomaitis. V.A. and Swanson, G.A., 1987. Hydrology and chemistry of selected prairie wetlands in the Cottonwood Lake area, Stutsman County, North Dakota, 1979-1982. US Geol. Surv. Prof. Pap., 1431,26 pp. LaBaugh, J.W., Winter, T.C., Swanson, G.A., Rosenberry. D.O.. Nelson, R.D. and Euliss, Jr., N.H., 1996. Changes in atmospheric circulation patterns affect mid-continent wetlands sensitive to climate. Limnol. Oceanogr., 41(5). Meyboom, P., 1967. Groundwater studies in the Assinihoine River drainage basin, Part II, hydrologic chamcteristics of phreatophytic vegetation in south-central Saskatchewan. Geol. Surv. Can. Bull., 139. 64 pp. Mills, J.G. and Zwarich, MA.. 1986. Transient groundwater flow surrounding a recharge slough in a till plain. Can. J. Soil Sci., 66: 121-134. Nichols, W.D., 1994. Groundwater discharge by phmatophyte shrubs in the Great Basin as related to depth to groundwater. Water Resour. Res., 30(12): 3265-3274. Novakowski, KS. and Gillham. R.W., 1988. Field investigations of the nature of water-table response to ptecipitation in shallow water-table environments. J. Hydrol.. 97: 23-32. O’Brien, A.L.. 1982. Rapid water table rise. Water Resour. Bull., 18(4): 713-715. Phillips. P.J. and Shedlock, R.J., 1993. Hydrology and chemistry of groundwater and seasonal ponds in the Atlantic Coastal Plain in Delaware, USA. J. Hydrol., 141: 157-178. Robinson, T.W., 1958. Phreatophytes. US Geol. Surv. Water-Supply Pap. 1423.84 pp. Rosenberry, D.O., 1985.Factors contributing to the formation of transient water-table mounds on the oudlow side of a seepage lake, Williams Lake, Central Minnesota. M.S. Thesis, University of Minnesota, Minneapohs. 127 pp. Rosenberry, D.O.. 199Oa.Inexpensive groundwater monitoring methods for determining hydrologic budgets of lakes and wetlands. In: Proc. National Conference on Enhancing the States’ Lake and Wetland Management Programs, 1989. North American Lake Management Society, Chicago, IL, pp. 123-131. Rosenberry, D.O., 1990b. Effect of sensor error on interpretation of long-term water-level data. Ground Water, 28(6): 927-936. Rosenbetry, D.O., 1996. Climate of the Cottonwood Lake area, Stutsman County, North Dakota. US Fish Wildl. Serv. Tech. Rep., in ptess. Sachs, L.A.. Herman, J.S.. Konikow, L.F. and Vela, A.L.. 1992. Seasonal dynamics of groundwater-lake interactions at DoIiana National Park, Spain. J. Hydrol., 136: 123-154. Sloan, C.E.. 1972. Ground-water hydrology of prairie potholes in North Dakota. US Geol. Surv. Prof. Pap., 585C, 28 pp. Steinwand. A.L. and Richardson, J.L.. 1989. Gypsum occurrence in soils on the margin of semipermanent prairie pothole wetlands. J. Soil Sci. Sot. Am., 53(3): 836-842. White, W.N., 1932. A method of estimating ground-water supplies based on discharge by plants and evaporation from soil. US Geol. Surv. Water-Supply Pap., 659: l-105. Winter, T.C., 1983. The interaction of h&es with variably saturated porous media. Water Resour. Res., 19(5): 1203-1218. Winter. T.C., 1996. Geohydrdlogic setting of the Cottonwood Lake area Stutsman County, North Dakota. US Fish Wildl. Serv. Tech. Rep., in press. Winter, T.C. and Carr, M.R.. 1980. Hydrologic setting of wetlands in the Cottonwood Lake arta, Stutsman County, North Dakota. US Geol. SW. Water-Resour. Invest., 8099.42 pp. Winter, T.C. and Llamas, M.R., 1993. Introduction to the 28th International Geological Congress Symposium on the Hydmge-oiogy of Wetlands. J. Hydrol., 141: 1-3. Winter. T.C. and Rosenberty, D.O., 1995. The interaction of ground water with prairie pothole wetlands in the Cottonwood Lake area, east-central North Dakota, 1979-1990. Wetlands, 15(3): 193-211. WOO,M.-K. and Rowsell, R.D., 1993. Hydrology of a prairie slough. J. Hydrol., 146: 175-207.