Prediction of groundwater characteristics in forested and harvested basins during spring snowmelt using a topographic index

Consistent relationships between groundwater conditions and topographic properties in small drainage basins would assist the study and modelling of basin hydrology and hydrochemistry. Hydrochemical simulation using topographically based models would benefit in particular from the linkage of groundwater residence times to topographic indices, such as the ln(a/tanbeta) index of Beven and Kirkby. Hypothesized associations between this index and groundwater characteristics were tested during the spring 2000 snowmelt in two sub-basins in the Turkey Lakes Watershed near Sault Ste Marie, ON. One sub-basin is in mature hardwood forest, and the other was clearcut in the fall of 1997. Piezometric surface elevations were monitored in each sub-basin throughout the melt, and the topographic index value for each piezometer was determined. The delta(18)O signatures of input water and groundwater were also measured. Mean groundwater residence times were obtained using the exponential system response function, which generally produced good fits to the observed groundwater delta(18)O time series in both sub-basins. There were significant contrasts in groundwater conditions between the sub-basins. The forested sub-basin exhibited higher and more temporally variable piezometric surface elevations and a greater contrast between 8180 signatures and water residence times in shallow versus deeper groundwater relative to the harvested sub-basin. Patterns of groundwater residence times in the sub-basins were supported by pre-harvest groundwater chemistry; however, data from the two sub-basins largely failed to support the hypothesized relationships between groundwater conditions and topographic index values. Methodological limitations that may have precluded a more rigorous test of the hypotheses are reviewed, and the potential for using groundwater residence times to evaluate the impacts of forest harvesting on basin hydrochemistry is briefly discussed. Copyright (C) 2001 John Wiley Sons, Ltd.