Modelling and mapping critical loads and exceedances for the Georgia Basin, British Columbia, using a zero base-cation depletion criterion

Critical load (CL) and exceedance maps of sulphur (S) and nitrogen (N) for upland soils were generated for the Georgia Basin, British Columbia, Canada, by synthesizing available data layers for atmospheric deposition, climate (precipitation, temperature), soil, site classification and elevation. Critical loads were determined using the steady-state mass-balance model and a criterion based on zero-tolerance for further base-cation depletion. The resulting CL values were generally lowest on ridge tops and increased towards valleys. Critical load exceedance ranged from 13% of the Georgia Basin under wet deposition to 32% under modelled total (wet and dry) deposition. Moreover, exceedance increased by an additional 10% when considering upland areas only for the Georgia Basin. Significant portions of the Georgia Basin are predicted to experience exceedance-enhanced base-cation depletion rates above 200 eq ha–1 y–1 and turn-over times to a final new base saturation state within 200 years under continued atmospheric S and N deposition.