Site-specific estimates of water yield applied in regional acid sensitivity surveys across western Canada

Runoff or water yield is an important input to the Steady-State Water Chemistry (SSWC) model for estimating critical loads of acidity. Herein, we present site-specific water yield estimates for a large number of lakes (779) across three provinces of western Canada (Manitoba, Saskatchewan, and British Columbia) using an isotope mass balance (IMB) approach. We explore the impact of applying site-specific hydrology as compared to use of regional runoff estimates derived from gridded datasets in assessing critical loads of acidity to these lakes. In general, the average water yield derived from IMB is similar to the long-term average runoff; however, IMB results suggest a much larger range in hydrological settings of the lakes, attributed to spatial heterogeneity in watershed characteristics and landcover. The comparison of critical loads estimates from the two methods suggests that use of average regional runoff data in the SSWC model may overestimate critical loads for the majority of lakes due to systematic skewness in the actual runoff distributions. Implications for use of site-specific hydrology in regional critical loads assessments across western Canada are discussed.