A 6-YEAR ISOTOPIC RECORD OF LAKE EVAPORATION AT A MINE SITE IN THE CANADIAN SUB-ARCTIC - RESULTS AND VALIDATION

An isotopic method is applied in conjunction with a water balance meth od and the Penman combination method to estimate evaporation from a sm all, high closure (low outflow) lake near Yellowknife, Northwest Terri tories, Canada (62 degrees 03'N 111 degrees 24'W). The study provides baseline hydrological information for assessment of tailings pond desi gn and management at nearby mine sites, and, notably, enables intercom parison of several field-based evaporation methods and a standard clim ate approach in a subarctic setting. A non-steady isotope mass balance method is applied to estimate evaporation over time intervals ranging from five days to three weeks, based on isotopic surveys of lake wate r, groundwater, precipitation and atmospheric moisture during the open water periods of 1991 to 1996. Use of a relatively high precision non -steady technique, in contrast to the commonly employed approach assum ing steady state, is feasible in the present setting owing to pronounc ed seasonal evaporative enrichment in lake water (20-30 times analytic al uncertainty of delta(18)O), A comparative analysis reveals that the isotopic method is conservative relative to the Penman combination me thod, but less conservative than standard water balance, although esti mates for the open water period are in agreement to within 20% in both cases. Interannual variability in evaporation is revealed to be 30-50 % greater than predicted from standard pan-to-lake algorithms, and of the same order of magnitude as the annual snow water equivalent (appro ximate to 155-175 mm), which has important implications for the design and management of tailings ponds in the area. (C) 1998 John Wiley & S ons, Ltd.