SHORT-TERM EVAPORATION AND WATER BUDGET COMPARISONS IN SHALLOW ARCTIC LAKES USING NON-STEADY ISOTOPE MASS BALANCE

Abstract

Shallow lakes in a continental, low Arctic setting are found to undergo substantial fluctuations in heavy-isotope content during the annual cycle due to extreme seasonality in water balance processes. Progressive isotope enrichment during the ice-free period occurs as a consequence of isotope exchange during evaporation under seasonally arid conditions, while enhanced input and flushing by heavy-isotope depleted precipitation and snowmelt during late fall and spring tend to deplete the lakes in heavy isotopes once again. Time-series sampling surveys in a group of nearby lakes, ranging in volume from 41,000 to 2,250,000m3, was carried out for two consecutive ice-free periods, in conjunction with a comprehensive hydrological measurement program in a detailed study lake, to assess models that describe isotopic enrichment in lakes and to test their suitability for comparing evaporation rates and water balance. From a non-steady isotope balance analysis, it is found that isotope enrichment rates in lakes during the ice-free period are determined primarily by evaporation rates and volume of the lakes, and are less sensitive to water balance variations for short time intervals when evaporation is less than 50mm or so. A basic assessment of best-fit and step-wise models is presented which suggests that the former are useful only for predicting evaporation and water balance during month-long periods with relatively stable atmospheric and hydrologic conditions. A step-wise isotope balance approach is presented which demonstrates how isotope-based estimates of evaporation rates can be applied to effectively compare short-term (weekly) water balance in nearby lakes. Practical applications in the region include water balance assessments to assist in design and maintenance of tailings ponds for gold and diamond mining operations.