Abstract - GWET

 

Cohen, D. M., M. Person, R. Daannen, S. Locke, D. Dhalstromnb, V. Zabiel, D. O. Rosenberry, H. Wright, E. Ito, J. L. Nieber and W. J. Gutowski, 2005: Groundwater supported evapotranspiration within glaciated watersheds under conditions of cliamte change. J. Hydrology (submitted).

This paper analyzes the effects of geology and geomorphology on surface water/groundwater interactions, evapotranspiration, and runoff generation under conditions of long-term climate change. Our analysis uses hydrologic data from the glaciated Crow Wing watershed in central Minnesota, USA, as well as saturated/unsaturated mathematical modeling.

Analysis of historical water table (1970-1993) and lake level (1924-2002) records indicate that larger amplitude, longer period fluctuations occur within the upland portions of watersheds due to the response of the aquifer system to climatic fluctuations. Under dust-bowl type climatic conditions, lake and water table levels fell by as much as 2-4 meters in the uplands but by only a meter in the lowlands. The same pattern can be seen on millennial time scales. Analysis of Holocene lake core records indicate that Moody lake, located near the confluence of the Crow Wing and Mississippi rivers fell by as much as 4 meters between about 4400 and 7000 yr BP. During the same time period, water levels in Lake Mina, located near the watershed divide near Alexandria, MN, fell by about 15 m. These findings are consistent with analytical calculations that indicate that the response time and magnitude of water table and lake level fluctuations will be greatest near the water table divide of large watersheds.

A sensitivity analysis was carried out using a transient saturated-unsaturated hydrologic model (HYDRAT2D) to study how aquifer hydraulic conductivity, land surface topography and watershed size can influence water table fluctuations, wetlands formation, evapotranspiration, and runoff. The models were run by recycling relatively wet (1985, 87 cm annual precipitation) climatic records over a period of 10 years followed by 20 years of a dryer (1976, 38 cm precipitation) and warmer climate record. Model results indicated that aquifer-supported evapotranspiration accounted for as much as 12% (10 cm) of evapotranspiration. The highest hydraulic conductivity aquifers had the least amount of groundwater-supported evapotranspiration owing to deep water tables. Runoff generation due to high water tables was even more sensitive to aquifer conductivity, especially in the lowland regions. Increasing the length scale of the basin resulted in more aquifer-supported evapotranspiration due to the relatively higher water tables produced. These findings have important implications for paleoclimatic studies since the hydrologic response of a surface water body will vary across the watershed to a given climate signal.