Estimating the ground-water contribution in wetlands using modeling and digital terrain analysis

Wetland classification and management often requires information on the contribution of ground water to a wetland’s water budget. Direct estimation of this parameter, however, is time-consuming, expensive, and can typically only be accomplished for small areas. Thus, a method to characterize ground-water flow in wetland areas and regions may be useful in many applications. The estimation technique described combines the use of a digital elevation model (DEM) with transient numerical modeling and assumes that the water table reflects the general pattern of surface topography. The DEM grid elevations are used as initial heads in the model. Stepwise ground-water drainage from the flow domain is simulated until a reasonable match is obtained between the observed and model water tables. By knowing or assuming hydraulic conductivity and using the model water-table configuration, an estimate for ground-water flow to and from each discretized grid node can be estimated from Darcy’s Law and the Dupuit approximation. The net result, when mapped, shows the simulated distribution of recharge and discharge within and surrounding the wetland. Two examples from the Shingobee River headwaters in central Minnesota indicate how the method may be used. Geologically recent development of glacial landforms has led to numerous lakes, ponds, and wetlands in the region. Using a 30-m, 1∶24,000 scale DEM grid in combination with data from the U.S. Fish and Wildlife National Wetlands Inventory, the model predicts the most likely areas of ground-water interaction in and near wetlands and lakes. More quantitative results can be obtained by applying observed water budget and soil/aquifer parameter data.