Estimate of the Maximum Probable Precipitation for Alberta River Basins

Point measurements of maximum depth showed that over 50% of the rainstorms occur in June and July, with only a small percentage in April (5.6) and September (10. 1). The greatest frequency of occurrence is observed in the Waterton Lakes Park area (just about 1 per year), with relatively high frequencies along the continental divide and decreasing eastward along the foothills and plains of Alberta. Point measurements of maximum depth also showed that the greatest frequencies of occurrences are those in the Waterton Lakes Park area with probability of 2.0 (twice a year) for depths 50 mm and more; 0.38 (1:3 year event) for depths 100 mm and more; and 0.09 (1 :10 year event) for depths 150 mm and more. Seasonally the greatest frequencies are observed in June for southern Alberta and in July for central Alberta. Severe storms (150 mm and greater in depth) are observed to occur in four main regions of the province. Estimates of "Probable Maximum Precipitation" (PMP) using the meteorological approach were made for six river basins for 6-, 12-, 24-, 48-, 72-, 96-hour rainfalls. The maximum estimates of the PMP seem to occur in June for the basins in the southern portion of the province, while in central and northern Alberta the maximum estimates were found to occur in July. Spatial variability of the PMP is also observed in each of the river basins. The largest decrease of the maximum PMP is recorded in the South Saskatchewan River basin. Here at the eastern edges of the basin, the PMP estimates are about 80% lower than those calculated at the western edges. A second method of estimating the PMP, using the statistical technique developed by Hershfield, was also applied to 27 first-order stations. These estimates were about 50 mm higher (for areas of about 250 km2) in the six river basins. The largest estimates by this technique were also obtained in the South Saskatchewan River basin.