Water Supply Outlook for Alberta

North Saskatchewan River Basin - Water Supply Forecasts

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 (Table 6). Despite the low runoff this year, the natural volume forecasts produced for the North Saskatchewan River basin were within 8.4% of recorded values, on average.

Precipitation in the North Saskatchewan River basin was below-normal to much-below-normal for the March to September period. Precipitation values in the basin ranged from 66 to 96 % of normal. Most months recorded much-below-normal to below-normal precipitation 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
(Table 6).

North Saskatchewan River at Bighorn Dam (Lake Abraham)

The March to September 2002 recorded natural volume for the North Saskatchewan River at Bighorn Dam was 2,175,000 dam³ or 102% of average and ranks 11th lowest on record (based on the 1973-90 data) (Table 6).

Seven monthly (February to August) natural volume forecasts were produced for Bighorn Dam for the March to September period (Figure 18). Six of the seven forecasts were lower than the recorded value and were on average, within 6.9% of the recorded March to September volume. This is evident in Figure 18, where the recorded March to September 2002 natural volume at Bighorn Dam (dark blue line) was above the forecasted values (brown triangles). The recorded volume was within the probable range and greater than the reasonable minimum for all seven forecasts. Forecasts ranged within 3.5% to 9.5% of the recorded volumes, with two of the forecasts within five percent of the recorded values.

The difference between the forecast and the recorded value is attributed to the fact the actual recorded precipitation was below-normal in the basin while the forecast volumes assume normal precipitation for the forecast period. The precipitation in the headwater portion of the basin during May and June (not shown) was much-below-normal and as a result, impacted the recorded runoff volumes considerably, particularly the July to September and August to September periods. Recorded volume for the July to September period was the tenth lowest (31 years of record) on record while the August to September volumes were the third lowest on record.

The closest forecast to the recorded natural volume was produced in June (3.5%) (Figure 18). The forecasts remained fairly steady throughout the forecast season (brown triangles), with minor fluctuations from month to month depending on recorded precipitation. The August 1 forecast increased due to the above-normal precipitation in the headwaters during July.

Brazeau River at Brazeau Dam

The March to September 2002 recorded natural volume for the Brazeau River at Brazeau Dam was 1,022,000 dam³ or 71% of average and ranks fifth lowest on record (based on the 1962-90 data) (Table 6). The 2002 recorded volume is only 5% higher than the recorded volume over the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced for Brazeau Dam for the March to September period (Figure 19). All seven forecasts were higher than the recorded value and were on average, within 11.4% of the recorded March to September volume. This is evident in Figure 19, where the recorded March to September 2002 natural volume at Dickson Dam (dark blue line) was below the forecasted values (brown triangles). The recorded volume was within the probable range five out of seven forecasts and greater than the reasonable minimum in all seven forecasts. Forecasts ranged within 1.8% to 19.6% of the recorded volumes, with one of the forecasts within five percent of the recorded value.

The difference between the forecast and the recorded value is attributed to the fact the actual recorded precipitation was much-below-normal in the basin while the forecast volumes assume normal precipitation for the forecast period. This is evident as the precipitation in the headwater portion of the basin during June and July (not shown), ranged between 40 and 50 % of normal. These two months are normally the highest in terms of monthly precipitation and as a result, impacted the recorded runoff volumes considerably, particularly the July to September and August to September periods. Recorded volume for the July to September period was the second lowest on record while the August to September volumes were the fifth lowest on record despite above-normal precipitation during August.

All seven forecasted volumes were higher than the actual recorded value as a result of the below-normal precipitation (Figure 19). The closest forecast to the recorded natural volume was produced in August (1.8%). The forecasts remained fairly steady throughout the first part of the forecast season, with minor fluctuations from month to month depending on recorded precipitation. The July and August 1 forecasts were lowered considerably due to the much-below-normal precipitation in the foothills during June and July. The lack of precipitation in June and July impacted recorded natural volumes considerably as the May and June forecasts did not fall within the probable range. 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.

North Saskatchewan River at Edmonton

The March to September 2002 recorded natural volume for the North Saskatchewan River at Edmonton was 4,266,000 dam³ or 72% of average and ranks eighth lowest on record (based on the 1912-95 data) (Table 6). The 2002 recorded volume is only 5% higher than the recorded volume over the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced at Edmonton for the March to September period (Figure 20). All seven forecasts were higher than the recorded value and were on average, within 7.0% of the recorded March to September volume. This is evident in Figure 20, where the recorded March to September 2002 natural volume at Edmonton (dark blue line) was below the forecasted values (brown triangles). The recorded volume was within the probable range six out of seven forecasts and greater than the reasonable minimum in all seven forecasts. Forecasts ranged within 1.2% to 14.0% of the recorded volumes, of which, three of the forecasts were within five percent of the recorded values and all but one were within ten percent of observed values.

The difference between the forecast and the recorded value is attributed to two factors. The actual recorded precipitation was much-below-normal in the basin (particularly in the plains area) while the forecast volumes assume normal precipitation for the forecast period and this, combined with the basin being extremely dry as a result of drought, impacted volumes between Bighorn and Brazeau Dams and Edmonton. Despite near normal precipitation in August and September, the dry May, June and first half of July combined with the extremely dry conditions that had existed previously, resulted in recorded volume for the July to September period being the lowest on record while the August to September volumes were the second lowest on record.

All seven forecasted volumes were higher than the actual recorded value as a result of the below-normal precipitation
(Figure 20). The closest forecast to the recorded natural volume was produced in February (1.2%). The forecasts increased slightly throughout the first part of the forecast season (due to above-normal precipitation in February and March) before dropping sharply for the July and August 1 forecasts. The lack of precipitation in June and July impacted recorded natural volumes considerably as the June forecast did not fall within the probable range. 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.