Changes in Mountain Hydrology from 1962 to 2100: The Past, Present and Future of a Canadian Rockies Headwaters Basin Subject to Climate Change and Forest Management

Abstract:

Marmot Creek Research Basin in the Canadian Rockies headwaters of the South Saskatchewan River, Alberta, Canada has been intensely instrumented since 1962. The research basin includes high alpine rock and meadows, sub-alpine forests, montane forests and forest clearings over 1500 m of relief. Precipitation in the upper elevations is dominated by snowfall, whilst rainfall dominates in the lower elevations and both reach a maximum during the spring freshet. The experimental basin was developed initially to study the impacts of sub-alpine forest management on streamflow generation. Forest management practices in the 1970s and 1980s included large clearcuts and small forest gap thinning. However, climate warming since the early 1960s has been well above the global average and the flood of record (2013) and some exceptionally warm years with low snowpacks (2015) have occurred recently. Basin observations have been assembled and used to drive diagnostic modelling using the Cold Regions Hydrological Model from the 1960s to present and an atmospheric model was used to drive the hydrological model for future climates of the late 21^st C. Over the historical period, and despite substantial forest cover reduction, warming and declining low elevation snowpacks, there are no trends in precipitation or streamflow timing, seasonality or volume nor associations with teleconnections. Diagnosis of this insensitivity to change suggests compensatory processes at various elevations dampen the streamflow response to changing meteorology – e.g. increases in precipitation were countered by increasing evapotranspiration. Forest cover reduction desynchronized runoff generation despite increasing snowpacks. However, this resilience appears to have limits. Using a dynamically downscaled future climate model (4 km WRF RCP8.5) to drive the hydrological model, a substantial decline in snowpacks at all elevations, a forward shift in in snowmelt freshet and peak streamflow, decline in late season flows and an increase in both evapotranspiration and annual streamflow volume emerge as the basin shifts from a snowmelt dominated to a rainfall-runoff regime. Despite these hydrological regime changes, the relative resilience of this basin's streamflow generation to substantial climate and land cover changes is remarkable.