Influences of Main Large-Scale Climate Indices on Canada’s Streamflow Trends over the Past Several Decades

Abstract:

The main objective of this study is to examine the influence of three important large-scale climate indices influencing the Canadian climate on Canada’s monthly streamflow trends. These main climate indices are El-Niño Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and Pacific Decadal Oscillation (PDO). This study combines the use of wavelet transform techniques and the Mann-Kendall (MK) trend test in order to achieve its main objective. The MK test was used to analyze trends and determine the starting point of trends in streamflow data. The continuous wavelet transform (CWT) was used to extract the time-frequency information of the streamflow and climate index data used. Only streamflow data from gauging stations that are categorized as natural and have a minimum of 40 years of complete records were used in the study – there are 73 gauging stations that met these requirements. Starting points of trends varied from station to station, and the CWT obtained show variabilities at the intra-annual, inter-annual and inter-decadal scales. Wavelet coherence spectra indicate that the influences of ENSO and the NAO indices occur at the 2-6-year time scales, and the influences of the PDO index are more apparent at time scales of up to 8 years and greater than 16 years. In particular, all CWT spectra of streamflow consistently show significant variability at the 6-12 months scales (which may be related to seasonality events such as spring snowmelt). The 6-12 month frequencies were isolated from the streamflow data and Spearman’s correlation coefficients were used to identify the amount of time streamflow activities lagged behind the influence of each climate index. This study observed that dominant sources of streamflow variations at periodic scales of 6-12 months, are more significantly influenced by NAO and PDO indices compared with ENSO. The results obtained from this study can be used by water managers and planners in different regions of Canada to improve streamflow predictions, which in turn help to better manage water resources. Furthermore, it is possible to apply the procedures used in this study in other regions of the world.