Building on the Water Supply Assessment, this study provides an initial assessment of climate    change effects on water supply to the NSR. The study predicts temperature and rainfall patterns for the period 2021 to 2050.

North Saskatchewan Watershed Alliance (NSWA) contracted Golder Associates Ltd. (Golder) to assess the water yield from the North Saskatchewan River Basin (NSRB) and its variability under natural hydrologic conditions and present climatic conditions. An additional request was to carry out data analyses and hydrologic modeling to assess potential changes in the water yield under forecasted future climatic conditions. The effects of climate change on the water yield from the NRSB will affect water uses and water management in the basin. There is a need to assess the potential effects so that watershed planners can adapt their plans to take advantage of positive effects and implement mitigation measures to minimize the negative effects. The scope of the study also included a review of the literature on climate change as it pertains to the prairie regions and an assessment of trends in observed temperature, precipitation and stream flow data in the NSRB. This report summarizes the assessment of the potential effects of climate change on the water yield from the NSRB.

Trend analyses on air temperature data at the selected climate stations in the NSRB, namely, Nordegg, Rocky Mountain House, Edmonton and Vermilion, suggest that there is a generally increasing trend in air temperature. The general trend seems to be towards increasing precipitation, but the trends in monthly, seasonal and annual precipitation data at the four selected locations in the NSRB are not statistically significant.

The annual mean stream flow data at selected hydrometric stations in the headwater basins of the Athabasca River and western portion of the NSRB generally show a decreasing trend in recent years. Trend lines fitted to recent flow data are not necessarily accurate predictors of future increases or decreases in flows. Notwithstanding the foregoing statement, linear trend lines fitted to the data suggest that the predicted annual mean flows would decrease by between 4% and 9%, depending on station location, by the year 2035 compared to the baseline period of 1961-1990. These predicted changes in annual yield by 2035 are, however, well within the variability in annual yield from year to year.

The 1961 to 1990 period was selected as the climatological baseline period for the modelling work. The future conditions have been represented by the 30-year period between 2021 and 2050, which would be representative of the mid-2030s. The ECHAM50M, NCARCCSM3, GFDLC2.1 and CGCM3T47 General Circulation Models, also known as Global Climate Models, (GCMs) were selected for assessing the effects of climate change on the water yield in the NSRB based on a comparison of GCM predictions with observed climate data in the NSRB. The A1B, A2 and B1 climate scenarios were selected for each GCM. Scenario A1B represents future balanced socio-economic and environmentally-based development; scenario A2 assumes that the current global socio-economic situation will continue in the future; and, scenario B1 represents future development that is more environmentally-based than at present.

The forecasts indicate that NCARCCSM3-SRA1B predicts the largest increase in temperature (about 2.2^oC), while ECHAM50M-SRB1 predicts the smallest increase (about 0.3^oC). Predictions of changes in precipitation tend to vary significantly between GCMs and even between scenarios for a given GCM. The change in mean annual total precipitation for the forecast period of 2021-2050 from the baseline period of 1961-1990 ranges from a decrease of about 8% (GFLDLC2.1-SRA2) to an increase of about 19% (NCARCCSM3-SRA2), with 10 of the 12 scenarios predicting an increase in precipitation. The forecasted increasing trend in precipitation appears to be consistent with trends in observed data at the climate stations in the NSRB.

The European Centre for Mid-Range Weather Forecast global re-analysis (ERA-40) climate data from 1961 to 1990 was used to represent the baseline climate conditions in the NSRB.  The modified Interactions Soil-Biosphere-Atmosphere land surface model (MISBA) of Météo France was set up for the North Saskatchewan River Basin (NSRB). The study area was limited to the portion of the NSRB west of Edmonton because ERA-40 data east of Edmonton was not available for this study.  

For the purposes of this study, simulated water yield in the NSRB for the baseline period was assessed against natural flows recorded at the Environment Canada WSC Hydrometric Station 05DF001 at Edmonton. The simulated flows from the MISBA model with ERA-40 data are reasonably close to the observed flows at 05DF001. However, while the observed maximum monthly yields tend to occur in June and July, the highest monthly simulated yields occur earlier in May and June. Notwithstanding the differences, the simulated flows were considered reasonable for the purposes of this study and are used to assess the relative effects (simulated 2021-2050 model outputs compared with simulated 1961-1990 model outputs) of climate change on water yield in the NSRB at WSC Station 05DF001.

The European Centre for Mid-Range Weather Forecast global re-analysis (ERA-40) baseline (1961-1990) climate data was adjusted to reflect the changes forecasted by the combination of the four selected GCMs and three scenarios.  Five of the six ECHAM50M and NCARCCSM3 GCM-scenario combinations are predicting increases in annual yield from the baseline 1961-1990 period to the 2021-2050 forecast period that range from 5% to 15%. Only the ECHAM50M-SRA1B combination predicts a decrease of about 11%. The CGCM3T47 and GFLDC21 GCM-scenario combinations are predicting decreases in annual yield that range from 3% to 23%. The predictions of the CGCM3T47 and GFLDC21 GCMs tend to follow trends in observed flow data. However, the ECHAM50M and NCARCCSM3 GCMs are the more representative GCMs of the baseline climate of the runoff-producing headwater basins of the NSRB, and the results of the simulations using these models may indicate the more likely trend in future yield from the NSRB. 

The simulations of the forecasted climate scenarios result in a range of possible impact on water yield from the NSRB. Notwithstanding that the GCMs most representative of baseline climate in the NSRB predict increases in future annual yield, the range of possible impacts should be considered in watershed planning because the model predictions have some degree of uncertainty associated with them.