Donald O. Rosenberry

The wetland continuum: A conceptual framework for interpreting biological studies

We describe a conceptual model, the wetland continuum, which allows wetland managers, scientists, and ecologists to consider simultaneously the influence of climate and hydrologic setting on wetland biological communities. Although multidimensional, the wetland continuum is most easily represented as a two-dimensional gradient, with ground water and atmospheric water constituting the horizontal and vertical axes, respectively. By locating the position of a wetland on both axes of the continuum, the potential biological expression of the wetland can be predicted at any point in time. The model provides a framework useful in the organization and interpretation of biological data from wetlands by incorporating the dynamic changes these systems undergo as a result of normal climatic variation rather than placing them into static categories common to many wetland classification systems. While we developed this model from the literature available for depressional wetlands in the prairie pothole region of North America, we believe the concept has application to wetlands in many other geographic locations.

Hydrology of prairie pothole wetlands during drought and deluge: A 17-year study of the Cottonwood Lake wetland complex in North Dakota in the perspective of longer term measured and proxy hydrological records

From 1988 to 1992 the north-central plains of North America had a drought that was followed by a wet period that continues to the present (1997). Data on the hydrology of the Cottonwood Lake area (CWLA) collected for nearly 10 years before, and during, the recent dry and wet periods indicate that some prairie pothole wetlands served only a recharge function under all climate conditions. Transpiration from groundwater around the perimeter of groundwater discharge wetlands drew water from the wetlands by the end of summer, even during very wet years.Long-term records of a climate index (Palmer Drought Severity Index), stream discharge (Pembina River), and lake level (Devils Lake) were used to put the 17-year CWLA record into a longer term perspective. In addition, proxy records of climate determined from fossils in the sediments of Devils Lake were also used. These data indicate that the drought of 1988-92 may have been the second worst of the 20th century, but that droughts of that magnitude, and worse, were common during the past 500 years. In contrast, the present wet period may be the wettest it has been during the past 130 years, or possibly the past 500 years.

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

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 ended 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−1. 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.

The interaction of ground water with prairie pothole wetlands in the Cottonwood Lake area, east-central North Dakota, 1979–1990

The interaction of ground water with prairie wetlands in the Cottonwood Lake area has been the focus of research by the U.S. Geological Survey and the U.S. Fish and Wildlife Service since 1977. During this time, climatic conditions at the site ranged from near the driest to near the wettest of the century. Water levels in wetlands and in water-table wells throughout the study area responded to these changing climate conditions in a variety of ways. The topographically highest wetlands recharged ground water whenever they received water from precipitation. The wetland of principal interest, Wetland P1, which is at an intermediate altitude, received ground-water discharge much of the time, but it also had transpiration-induced seepage from it along parts of its perimeter during all but the wettest year. The large fluctuations of the water table in response to recharge and transpiration reflect the ease with which water moves vertically through the fractured till. Lateral movement of ground water is much slower; pore-water moves vertically through the fractured till. Lateral movement of ground water is much slower; pore-water velocities are generally less than 3 m yr−1. The water supply to the wetlands is largely from precipitation during fall, winter, and spring. During these periods, precipitation either falls directly on the wetland, or precipitation that falls on the upland runs over frozen soils or saturated soils into the wetland. The average ratio of stage rise to total overwinter precipitation was 2.59 for the 12-year study period. After plants leaf out, precipitation generally results in much lower rises of the wetland water level. The average ratio of stage rise to over-summer precipitation was less than 1.0.

Evaluation of 11 Equations for Determining Evaporation for a Small Lake in the North Central United States

Eleven equations for calculating evaporation were compared with evaporation determined by the energy budget method for Williams Lake, Minnesota. Data were obtained from instruments on a raft, on land near the lake, and at a weather station 60 km south of the lake. The comparisons were based on monthly values for the open-water periods of 5 years, a total of 22 months. A modified DeBruin-Keijman, Priestley-Taylor, and a modified Penman equation resulted in monthly evaporation values that agreed most closely with energy budget values. To use these equations, net radiation, air temperature, wind speed, and relative humidity need to be measured near the lake. In addition, thermal surveys need to be made to determine change in heat stored in the lake. If data from distant climate stations are the only data available, and they include solar radiation, the Jensen-Haise and Makkink equations resulted in monthly evaporation values that agreed reasonably well with energy budget values.

Energy budget evaporation from Williams Lake: A closed lake in north central Minnesota

Evaporation from Williams Lake, computed by the energy budget method for the five open-water seasons of 1982–1986, varied from a maximum seasonal rate of 0.282 cm/d in 1983 to a minimum seasonal rate of 0.219 cm/d in 1982. The pattern of monthly values of evaporation is not consistent from year to year. The normally expected pattern of low evaporation values in May, followed by increasing values in June to maximum values in July is true for only 3 of the 5 years. Comparison of annual evaporation calculated by the energy budget and mass transfer methods indicates that energy budget values varied from 13% greater to 11% less than mass transfer values. Furthermore, there is no seasonal bias in the pattern. Large differences exist in the magnitude of energy fluxes to and from Williams Lake. By far the greatest energy fluxes, having magnitudes of hundreds of watts per square meter, are incoming solar radiation, incoming atmospheric radiation, and outgoing long-wave radiation emitted by the lake water. The least energy fluxes are related to advection, which generally have magnitudes less than 5 W m−2.

Hydrological and chemical estimates of the water balance of a closed-basin lake in north central Minnesota

Chemical mass balances for sodium, magnesium, chloride, dissolved organic carbon, and oxygen 18 were used to estimate groundwater seepage to and from Williams Lake, Minnesota, over a 15-month period, from April 1991 through June 1992. Groundwater seepage to the lake and seepage from the lake to groundwater were determined independently using a flow net approach using data from water table wells installed as part of the study. Hydrogeological analysis indicated groundwater seepage to the lake accounted for 74% of annual water input to the lake; the remainder came from atmospheric precipitation, as determined from a gage in the watershed and from nearby National Weather Service gages. Seepage from the lake accounted for 69% of annual water losses from the lake; the remainder was removed by evaporation, as determined by the energy budget method. Calculated annual water loss exceeded calculated annual water gain, and this imbalance was double the value of the independently measured decrease in lake volume. Seepage to the lake determined from oxygen 18 was larger (79% of annual water input) than that determined from the flow net approach and made the difference between calculated annual water gain and loss consistent with the independently measured decrease in lake volume. Although the net difference between volume of seepage to the lake and volume of seepage from the lake was 1% of average lake volume, movement of water into and out of the lake by seepage represented an annual exchange of groundwater with the lake equal to 26–27% of lake volume. Estimates of seepage to the lake from sodium, magnesium, chloride, and dissolved organic carbon did not agree with the values determined from flow net approach or oxygen 18. These results indicated the importance of using a combination of hydrogeological and chemical approaches to define volume of seepage to and from Williams Lake and identify uncertainties in chemical fluxes.

COMPARISON OF 13 EQUATIONS FOR DETERMINING EVAPOTRANSPIRATION FROM A PRAIRIE WETLAND, COTTONWOOD LAKE AREA, NORTH DAKOTA, USA

Evapotranspiration determined using the energy-budget method at a semi-permanent prairie-pothole wetland in east-central North Dakota, USA was compared with 12 other commonly used methods. The Priestley-Taylor and deBruin-Keijman methods compared best with the energy-budget values; mean differences were less than 0.1 mm d−1, and standard deviations were less than 0.3 mm d−1. Both methods require measurement of air temperature, net radiation, and heat storage in the wetland water. The Penman, Jensen-Haise, and Brutsaert-Stricker methods provided the next-best values for evapotranspiration relative to the energy-budget method. The mass-transfer, deBruin, and Stephens-Stewart methods provided the worst comparisons; the mass-transfer and deBruin comparisons with energy-budget values indicated a large standard deviation, and the deBruin and Stephens-Stewart comparisons indicated a large bias. The Jensen-Haise method proved to be cost effective, providing relatively accurate comparisons with the energy-budget method (mean difference=0.44 mm d−1, standard deviation=0.42 mm d−1) and requiring only measurements of air temperature and solar radiation. The Mather (Thornthwaite) method is the simplest, requiring only measurement of air temperature, and it provided values that compared relatively well with energy-budget values (mean difference=0.47 mm d−1, standard deviation=0.56 mm d−1). Modifications were made to several of the methods to make them more suitable for use in prairie wetlands. The modified Makkink, Jensen-Haise, and Stephens-Stewart methods all provided results that were nearly as close to energy-budget values as were the Priestley-Taylor and deBruin-Keijman methods, and all three of these modified methods only require measurements of air temperature and solar radiation. The modified Hamon method provided values that were within 20 percent of energy-budget values during 95 percent of the comparison periods, and it only requires measurement of air temperature. The mass-transfer coefficient, associated with the commonly used mass-transfer method, varied seasonally, with the largest values occurring during summer.

Movement of Road Salt to a Small New Hampshire Lake

Runoff of road salt from an interstate highway in New Hampshire has led to contamination of a lake and a stream that flows into the lake, in spite of the construction of a diversion berm to divert road salt runoff out of the lake drainage basin. Chloride concentration in the stream has increased by over an order of magnitude during the 23 yr since the highway was opened, and chloride concentration in the lake has tripled. Road salt moves to the lake primarily via the contaminated stream, which provides 53% of all the chloride to the lake and only 3% of the total streamflow to the lake. The stream receives discharge of salty water from leakage through the diversion berm. Uncontaminated ground water dilutes the stream downstream of the berm. However, reversals of gradient during summer months, likely caused by transpiration from deciduous trees, result in flow of contaminated stream water into the adjacent ground water along the lowest 40-m reach of the stream. This contaminated ground water then discharges into the lake along a 70-m-wide segment of lake shore. Road salt is pervasive in the bedrock between the highway and the lake, but was not detected at all of the wells in the glacial overburden. Of the 500 m of shoreline that could receive discharge of saly ground water directly from the highway, only a 50-m-long segment appears to be contaminated.

Water source to four U.S. wetlands: Implications for wetland management

Results of long-term field studies of wetlands in four different hydrogeologic and climatic settings in the United States indicate that each has considerably different sources of water, which affects their response to climate variability and land-use practices. A fen wetland in New Hampshire is supplied almost entirely by ground water that originates as seepage from Mirror Lake; therefore, stream discharge from the fen closely follows the pattern of Mirror Lake stage fluctuations. A fen wetland in northern Minnesota is supplied largely by discharge from a regional ground-water flow system that has its recharge area 1 to 2 km to the east. Because of the size of this wetland’s ground-water watershed, stream discharge from the fen has little variability. A prairie-pothole wetland in North Dakota receives more than 90 percent of its water from precipitation and loses more than 90 percent of its water to evapotranspiration, resulting in highly variable seasonal and annual water levels. A wetland in the sandhills of Nebraska lies in a regional ground-water flow field that extends for tens of kilometers and that contains numerous lakes and wetlands. The wetland receives water that moves through the ground-water system from the upgradient lakes and from ground water in local flow systems that are recharged between the lakes. The difference in sources of water to these wetlands implies that they would require different techniques to protect their water supply and water quality.