Water-yield estimates for critical loadings assessment: comparisons of gauging methods versus an isotopic approach

An isotope mass balance technique is applied to quantify water yield and refine a steady-state critical acid loadings assessment for 49 lakes in hydrologically complex, wetland-rich terrain of northeastern Alberta. The approach uses physical and climatological data combined with site-specific measurements of evaporative isotopic enrichment of H-2 and O-18 in take water to measure, lake residency and ungauged runoff to lakes. Mean water yields to individual lakes across the region over a 3-year period are estimated to range from 5 to 395 mm.year(-1), with a standard deviation of two times the predicted estimates based on interpolation of gauged stream flow from broad-scale watersheds in the area. Comparison of the method with longer-term Water Survey of Canada hydrometric data suggests very similar-average water yields for moderate- to large-sized watersheds. However, the isotope-based estimates appear to capture extreme low water yields in flat, disconnected areas and extreme high water yields in other areas thought to be related to stronger connections to regional groundwater flow systems. For aquatic ecosystems of northeastern Alberta, an area expected to be affected by acid deposition from regional oil sands development, continued refinement of the technique is important to accurately assess critical loads for ungauged systems, particularly those in low-yield settings.