Variability in spring snowpack and winter atmospheric circulation pattern frequencies in the Peace River Basin
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Prior to its regulation in 1968, the Peace River had historically been a significant contributor to ice-jam flooding of the Peace-Athabasca Delta. Following regulation, however, there has been a twenty-two year period, 1974-1996, devoid of ice-jam flooding and significant drying of the delta has ensued. This lack of flooding has been linked to the effects of higher freeze-up levels produced by regulation and the inability of the Peace River to generate large spring flows. Between 1974 and 1996, hydrometeorological conditions in the basin have not been conducive to the production of large spring runoff. This study reports on historical changes (both spatially and temporally) in spring snowpacks of the Peace River Basin over the period 1947-1992. Snowpack measurements were only available for the years 1960-1992, therefore other climate variables were used as a surrogate for snowpack to extend the analysis for the full 46-year period. Changes in snowpack are important because the occurrence of ice-jam floods in the Peace-Athabasca Delta is known to be at least partly dependent on the spring runoff that they produce. Years of spring ice-jam floods in the Peace-Athabasca Delta coincide with above average seasonal snowpack values for most areas in the basin. The spatial distribution of the April 1 snowpack in the Peace River Basin was found to be below average over the entire basin from 1960 to 1962. Snowpacks were either greater than average over the entire region, or greater than average in the south and below average in the north from 1963 to 1976. Greater than average snowpacks in the north and lower than average in the south or below average snowpacks over the entire region were dominant from 1977 to 1992. Comparison analysis of spatial patterns of precipitation identified two distinct precipitation regimes, one in the north and one in the southern portion of the basin. Precipitation trends at Grande Prairie (south) indicate that a significant shift occurred after 1976 towards decreased values. The decrease in precipitation and the size of spring snowpacks at most locations in the Peace River Basin were found to be controlled by decreases in the frequency of certain synoptic types in the mid 1970s. These decreases were associated with changes in prevailing atmospheric circulation as shown by the decrease of snow producing types resulting from the application of the Kirchhofer climate classification technique to surface pressure and 50 kPa height data. The lack of spring ice-jam flooding in the Peace-Athabasca since 1974 is partially explained by a shift in climatic and atmospheric patterns in the mid 1970s. This shift caused the decreased winter precipitation and snowpack needed to produce large spring runoff.