The dynamic nature of groundwater is not readily apparent, except where discharge is focused at springs or where recharge enters sinkholes. Yet groundwater flow and storage are continually changing in response to human and climatic stresses. Wise development of groundwater resources requires a more complete understanding of these changes in flow and storage and of their effects on the terrestrial environment and on numerous surface-water features and their biota.

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.

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Surface water and ground water watersheds commonly do not coincide. This condition is particularly relevant to understanding biogeochemical processes in small watersheds, where detailed accounting of water and solute fluxes commonly are done. Ground water watersheds are not as easily defined as surface watersheds because (1) they are not observable from land surface; (2) ground water flow systems of different magnitude can be superimposed on one another; and (3) ground water divides may move in response to dynamic recharge and discharge conditions. Field studies of relatively permeable terrain in Wisconsin, Minnesota, and Nebraska indicate that lakes and wetlands in small watersheds located near the lower end of extensive ground water flow systems receive ground water inflow from shallow flow systems that extend far beyond their surface watershed, and they may also receive ground water inflow from deeper regional flow systems that pass at depth beneath local flow systems. Field studies of mountainous terrain that have low‐permeability deposits in New Hampshire and Costa Rica also indicate that surface water bodies receive ground water inflow from sources beyond their local surface watersheds. Field studies of lakes and wetlands in North Dakota, Nebraska, and Germany indicate that ground water divides move in response to changing climate conditions, resulting in a variable source of ground water inflow to those surface water bodies.

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Lake-atmosphere CO 2 flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake-water CO 2 storage and with flux predictions from boundary-layer and surface-renewal models. Over a 3-yr period, lake-atmosphere exchanges of CO 2 were measured over 5 weeks in spring, summer, and fall. Observed springtime CO 2 efflux was large (2.3-2.7 mol m Ϫ2 s Ϫ1 ) immediately after lake-thaw. That efflux decreased exponentially with time to less than 0.2 mol m Ϫ2 s Ϫ1 within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO 2 concentrations, lake CO 2 storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (Ϫ0.17 mol m Ϫ2 s Ϫ1 ) to the lake's surface, while late fall flux was trendless and smaller (Ϫ0.0021 mol m Ϫ2 s Ϫ1 ). Large springtime efflux afforded an opportunity to make direct measurement of lake-atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air-water gas-transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake-water storage change in CO 2 , agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundarylayer and surface-renewal models. Comparison of measured and modeled transfer velocities for CO 2 , along with measured and modeled cumulative CO 2 flux, indicates that in most instances the surface-renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary-layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.

The hydraulic potentiomanometer described herein consists of a potentiometer connected to a manometer by a flexible tube. The device is used to directly measure the direction of seepage as well as the hydraulic‐head difference between groundwater and surface water. The device works most effectively in sandy materials. For accurate measurements the device must be free of air leaks.