DETERMINATION OF PRAIRIE POTHOLE WATER STORAGE DYNAMICS AND AREA-VOLUME RELATIONSHIPS

Millions of small depressions occur in the glaciated plains of North America from Alberta to Iowa. They host critical habitat for migratory birds and provide an array of ecological and hydrological functions that vary both temporally and spatially. The water budget of these wetlands is perhaps their single most important feature, with storage regulated by basin geometry, and input and output controlled by precipitation and evapotranspiration. Many studies have shown a large change of pothole area with climate variation, but the depths and shapes of the basins are required to estimate water storage.

We surveyed about 100 potholes in the upper Turtle River watershed of northeastern ND using a GPS receiver and an auto-level to determine basin geometry. Most potholes are roughly elliptical, and measurements were made along only the long and short axes. Data were computer contoured to estimate volume and to provide calibration of parametric area-depth-volume relationships. For each wetland, cross-sections were generated to characterize the significance of asymmetry in the volume analysis. Statistical analysis of the results were then used to estimate volumes for wetlands where only surface area is known, such as data from remote sensing.

Most of the potholes surveyed are ephemeral or seasonal, and although similar in size and shape, each exhibits different geometrical characteristics. A few are asymmetrical. Wind erosion may be an important mechanism that causes asymmetry, but we do not yet know why only a small subset of the potholes shows this characteristic. Currently we lack data on larger, more permanent potholes; as size increases they become more difficult to measure. Preliminary results suggest that shape of these small lakes differs significantly from most potholes. We plan to continue pothole measurement, so that general relationships between water persistence, area, depth, and shape can be determined. These data will provide us with knowledge on the distribution of pothole shape, shoreline characteristics, and volume-area relationships, and information on the physical factors that lead to these features. Based on data extracted from the National Wetlands Inventory, we believe it will be possible to characterize the asymmetry and area-volume relationship for wetlands throughout the prairie pothole region.