Water Supply Forecast Summary

In March of this year, very dry antecedent conditions existed in the province due to two years of consecutive drought. Many locations last year recorded near record low volumes over the latter part of the summer and did not receive significant precipitation over the winter. Mountain snowpack was above-average at the higher elevations (snow summary) with very little snowpack on the plains.

Recorded natural runoff volumes for the March to September 2002 period in the province was a tale of two extremes (Table 1). In the south, the Milk and Oldman River basins experienced one of the largest runoffs on record this year as a result of record precipitation in June (precipitation summary). All forecast locations in the Oldman River basin were in the top eleven volumes on record, with two locations (St. Mary and Belly Rivers) in the top three volumes on record. In the Milk River basin, all three forecast locations recorded the second highest volume on record for the June to September period. These near record volumes were recorded despite the extremely dry antecedent conditions in the spring.

Forecasts for the Oldman and Milk River basins underestimated the recorded volume this year as a result of the extreme precipitation recorded, particularly in the first part of the forecast season (February to June). All forecasts produced assume normal precipitation for the remainder of the forecast season and therefore underestimated the recorded volume by a significant amount.

In the Bow River basin, recorded March to September 2002 natural runoff volume in the Bow River basin ranged from below-average in the northern portion of the basin to above-average in southern areas. This difference resulted from the southern sub-basins being were hit by two major storms, one in late May and one in early June, which resulted in much-above-normal precipitation being recorded for the season. Northern sub-basins (mainstem of the Bow, Spray and Cascade) did not get hit by the storms and therefore recorded below-normal to normal precipitation for the season. Despite the wide variance in precipitation within the basin, the natural runoff forecasts produced for the Bow River at Calgary performed well, as the natural volume forecasts were within 4% of the recorded values, on average (Table 1).

As you continue north, precipitation totals and recorded natural runoff volumes decreased significantly. In the Red Deer River basin, recorded March to September 2002 natural runoff volume was much-below-average and ranked in the lowest 13 values on record and is only 4 to 5% higher than the recorded volume over the same time period last year (Table 1). Precipitation in the headwaters of the Red Deer River basin was below-normal to much-below-normal. As a result of the below-normal precipitation, the recorded volume came in near the lower quartile of the forecasts produced throughout the season.

Recorded March to September 2002 natural runoff volume in the North Saskatchewan River basin was average at Lake Abraham and much-below-average at Brazeau Reservoir and at Edmonton. Precipitation in the North Saskatchewan River basin was below-normal to much-below-normal for the March to September period, with the exception of March and April, where the headwaters received above-normal precipitation. The lack of runoff from spring snowpack and summer precipitation in the plains area (between Bighorn Dam and Edmonton) is clearly evident when comparing the natural volumes as a percent of average. This year's volume ranks eighth lowest on record (based on the 1912-95 data) at Edmonton and is only 5% higher than the recorded volume over the same time period last year (Table 1). As a result of 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 and was within 7% of the forecasted values, on average.