MOVING TARGETS: HOW THE CHANGING AREA AND VOLUME OF WETLANDS AFFECT THE ERROR IN WETLAND WATER BUDGETS

The water budget approach is commonly used to seek answers to fundamental questions about the source and fate of water and solutes in lakes and wetlands. In lakes, because of their depth and permanence, seasonal changes in stage typically cause small relative changes in lake surface area over a budget period. For this reason, changes in lake surface area are rarely considered a significant source of error to the water budget. The surface area of wetlands, in contrast, can undergo comparatively large changes within a monthly or even weekly budget time frame because of their shallow depths (often <1 m) and the seasonal oscillation between flooded and dry conditions. Changes in the ponded surface area continuously reshape the boundaries of the system being budgeted. Errors in determining the ponded surface area affect the calculated volume entering and leaving the system through rainfall and evapotranspiration, as well as the estimated change in the stored water in the wetland. The rigor of wetland water budgets, and the meaningfulness of the residual term, therefore, can hinge on our ability to accurately describe wetland bathymetry.

Detailed bathymetric and hydrologic data were collected for 10 marsh and forested wetlands in the mantled karst terrain of west-central Florida during a 4-year period. Bathymetric data and water-budget calculations for these isolated wetlands are used to examine the potential effect of errors in volume and area on the calculated residual term – in this case, wetland leakage to the deeper aquifer. Bathymetric data were collected at high resolution in this study. Subsets of the bathymetric data points are analyzed to determine the optimal density needed to calculate accurate stage/area and stage/volume relationships. Hydrologic data, collected for 18-24 months at most sites, are used to explore whether water-budget calculations should exclude periods when the ponded surface area is changing substantially. Errors in the wetland water budgets are compared to the magnitude of residual terms to indicate how large residual terms should be to be physically meaningful and interpretable.