Water Supply Forecast Summary

In March of this year, very dry antecedent conditions existed in the province north of Calgary. South of Calgary, soils were wet due to above-normal precipitation last summer and fall.

April snowstorms in the Red Deer and North Saskatchewan River basins resulted in up to three times normal precipitation for the month. As a result, snowpack measured on May 1 was generally above-average in the Red Deer and North Saskatchewan River basins, average in the northern half of the Bow River basin, and below-average to much-below-average elsewhere (snow summary). The large snowpacks in the Red Deer and North Saskatchewan River basins yielded near-record high runoff volumes for the March through May period, ranking from second to fifth highest on record. To the south, the snowpack melted quickly, yielding more runoff than had been anticipated.

The brief wet trend ended abruptly at the end of May, as summer precipitation was below-normal to much-below-normal in the mountains (precipitation summary). Initial indications are that summer precipitation totals were similar to those recorded in the recent drought years of 2001 in the south and 2002 in northern and central Alberta. As a result, July through September runoff volumes ranked from lowest to 12th lowest in up to 85 years of record, with the record lows being recorded at the Brazeau Dam and at Edmonton. The severity of the summer's dryness was even more apparent in August through September runoff volumes, with all but three forecast locations (Bighorn Reservoir, Elbow and Highwood Rivers) within the lowest five volumes on record. Record low runoff volumes for August and September were measured in the Kananaskis River and at the Brazeau Dam. Runoff volumes during the latter part of the summer were similar to those recorded during the same period in recent drought years of 2001 in the south and 2002 in central and northern Alberta.

Recorded natural runoff volumes for the March to September 2003 period in the province were below-average to much-below-average, except in the Red Deer River basin where average runoff was recorded (Table 1). Forecasts produced early in the year tended to underestimate the recorded runoff totals due to the large storms in April. The June, July and August forecasts tended to overestimate the recorded volume totals due to the extremely dry summer. On average, monthly forecasts were very accurate in the Bow, Oldman, and North Saskatchewan River basins. In the Oldman and Bow River basins, a total of 84 forecasts were produced and they were within less than 7%, on average, of the recorded March to September volumes. Twenty-one forecasts in the North Saskatchewan River basin were within 9.4%, on average, of the recorded March to September volumes. Twenty-four forecasts for the Milk River basin were on average within 14.8% of the recorded March to September volumes, and fourteen forecasts in the Red Deer River basin were within 25.4%. Forecasts assume 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 normal. As a result of generally below-normal precipitation being recorded throughout the forecast period, the recorded natural runoff volume tended closer to the probable lower range than the probable forecast point.