Water Supply Forecast Summary

Going into last winter, dry antecedent conditions existed in the province, as soil moisture ranged from below to much below average. Winter snowpack varied from much below average to average in the upper Red Deer and North Saskatchewan River basins, and above to much above average in the plains areas of these basins. Snowpacks were either below or much below average in most basins to the south. Despite the dry conditions, melt occurred about a month ahead of usual, resulting in generally average natural runoff volumes during March and/or April.

Precipitation was below to much below normal in most forecast basins during March and April, and into May in the Oldman River basin, resulting in very low runoff totals during the spring. The March through May recorded natural runoff volume was the lowest on record for Brazeau Reservoir inflow, second lowest for the North Saskatchewan River at Edmonton, and 10th lowest for the Red Deer River at Dickson Dam and at Red Deer. March through June volume totals were generally near the 15th lowest on record in most areas of the Oldman River basin.

May began a reversal of this dry trend in many areas. Precipitation during the May-July period generally ranged from near normal in the Oldman River basin to above to much above normal in most of the Bow River basin. As a result, soil moisture and runoff volumes in most areas began to improve. August precipitation was generally above normal or better, creating runoff during the August-September period which ranged from above to much above average, except at Banff and the Milk River, where most of this precipitation continued to go into the soil. Runoff during this August-September period ranged from 9th to 15th highest on record for all forecast locations in the Bow River basin except Banff.

Total recorded natural runoff volumes for the March to September 2004 period in the province ranged from below-average to much-below-average, except for four locations in the Bow River basin and at the Bighorn Reservoir, where runoff ranged from average to above average. Most locations ranked between the 11th and 35th lowest on record, with four locations in the Bow ranking higher (Table 1). March-September 2004 volumes were 3 to 22% higher than during the same period last year in the Bow River basin and in most areas of the Oldman and North Saskatchewan River basins, but slightly lower at the Oldman Dam and significantly lower (by 11 to 33%) at Edmonton and in the Red Deer and Milk River basins.

Forecasts generally dropped from January/February to May due to the low precipitation totals, but held steady or improved slightly in most areas from June through August with normal to above normal precipitation. On average, monthly forecasts were very accurate in the Red Deer and Oldman River basins. In the Red Deer River basin, fourteen forecasts produced during the year were on average within 5.4% of the recorded March-September totals, while in the Oldman River basin, a total of 35 forecasts were on average within 6.5%. Twenty-one forecasts in the North Saskatchewan River basin were within 9.5%, on average, of the recorded March to September volumes. Twenty-four forecasts for the Milk River basin were on average within 8.9% of the recorded March to September volumes, and 49 forecasts in the Bow River basin were within 11.9%. Forecasts assumed normal precipitation for the remainder of the forecast season and so the difference between the forecasts and the recorded values is attributed to the fact the actual recorded precipitation was not always normal. As a result of heavy precipitation during August, after the final forecasts for the year had been issued, the recorded natural runoff volume tended closer to the probable upper range than the probable forecast value for the August-September period, which raised the March-September totals beyond several of the forecasts.