Water Supply Outlook for Alberta

October 2006

Oldman River Basin - Water Supply Forecasts

Recorded March to September 2006 natural runoff volumes in the Oldman River basin ranged from below average in the Belly River and the Oldman River near Brocket to average into the St. Mary Reservoir. Recorded volumes ranged from 83 to 95% of average, ranking between 30th and 40st lowest on record (Table 3). The monthly natural volume forecasts produced for the Oldman River basin were within 5.0% of recorded values, on average, for the March through September runoff period.

Entering last winter, soil conditions were saturated and river flows were high due to much above normal summer and fall precipitation. Winter snowpacks were generally below average to average, and initial forecasts were for near average natural runoff volumes. Forecasts remained steady until July, after much above normal June precipitation resulted in a significant rise in forecasts for the southern tributaries (Belly, Waterton, St. Mary Rivers). Lesser June precipitation in the Oldman River basin upstream of the Oldman Dam resulted in a drop in forecasts there. Forecasts for the Oldman River at Lethbridge rose slightly in July.

Final forecasts (in August) for all areas dropped significantly after much below normal precipitation during July. Pincher Creek, for example, recorded only 29% of normal July precipitation. Another dry month followed in August (Pincher Creek precipitation was 29% of normal). September precipitation was generally below normal to normal. Natural runoff volumes for the month of August ranged from lowest on record in the Belly River to 9th lowest in the Waterton River. As a result, natural runoff volumes for the year (March through September) ended up between August's forecasted and lower quartile forecast values in most areas. Recent corrections to flow data in the Waterton River basin by Water Survey of Canada resulted in the natural runoff volume for the year being lower than forecasted.

Volumes were much lower than in 2005 at Brocket and Lethbridge due to flooding last year, but higher than last year in the southern tributaries, where flooding was much less prevalent.

Soil moisture is currently near normal but flows are generally below average as we head into winter.

St. Mary River at St. Mary Reservoir

The March to September 2006 recorded natural volume for the St. Mary River was average at 709,000 dam3 or 95% of average and ranks the 39th lowest on record (based on 1912-2001 data) (Table 3). This volume is 18% higher than the volume recorded during the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced for the St. Mary River for the March to September period. As illustrated in Figure 4, the recorded March to September 2006 natural volume for the St. Mary River (dark blue line) was similar to the forecasted value (brown triangles) for all but July's forecast, when it was lower. The July forecast ended up being high as precipitation that followed in July was minimal. As a result, conditions followed more closely the decile and lower quartile forecasts, and the forecast was lowered in August. The total recorded volume was greater than the reasonable minimum for all of the seven forecasts and within the probable range for all forecasts but July's.

Forecasts ranged from 0.5% to 8.6% of the recorded volume, on average within 2.3%. All but the July forecast were within three percent of the recorded value.

Belly River at Mountainview

The March to September 2006 recorded natural volume for the Belly River was below-average at 230,000 dam3 or 94% of average and ranks the 35th lowest on record (based on the 1912-2001 data) (Table 3). This volume is 4% higher than the volume recorded during the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced for the Belly River for the March to September period. As illustrated in Figure 5, the recorded March to September 2006 natural volume for the Belly River (dark blue line) was similar to the forecasted value (brown triangles) for all but July's forecast, when it was lower. The July forecast ended up being high as precipitation that followed in July was minimal. As a result, conditions followed more closely the decile and lower quartile forecasts, and the forecast was lowered in August. The total recorded volume was greater than the reasonable minimum for all of the seven forecasts and within the probable range for all forecasts but July's.

Forecasts ranged within 0.3% to 7.3% of the recorded volumes, on average within 2.1%. All but the July forecast were within 2.5% of the recorded value.

Waterton River

The March to September 2006 recorded natural volume for the Waterton River was below-average to average at 565,000 dam3 or 93% of average and ranks 38th lowest on record (based on the 1912-2001 data) (Table 3). This volume is 8% higher than the volume recorded during the same time period last year.

Seven monthly (February to August) natural volume forecasts were produced for the Waterton River for the March to September period. As illustrated in Figure 6, the recorded March to September 2006 natural volume for the Waterton River (dark blue line) was below the forecasted values (brown triangles) for all seven forecasts. Recent corrections to flow data in the Waterton River basin by Water Survey of Canada resulted in the natural runoff volume for the year being lower than forecasted. The recorded volume was within the probable range and greater than the reasonable minimum for five of seven forecasts, with the exceptions being August and September.

Forecasts ranged within 2.2% to 14.7% of the recorded volumes, within 6.5% on average. All but the July forecast were within 8% of the recorded value.

Oldman River at Brocket

The March to September 2006 recorded natural volume for the Oldman River near Brocket was below average at 900,000 dam3 or 83% of average and ranks the 30th lowest on record (based on the 1912-2001 data) (Table 3). This volume is much lower than the volume recorded during the same time period last year, which was 132% of average due to flooding.

Seven monthly (February to August) natural volume forecasts were produced for the Oldman River near Brocket for the March to September period. As illustrated in Figure 7, the recorded March to September 2006 natural volume for the Oldman River near Brocket (dark blue line) was below the forecasted values (brown triangles) for all seven forecasts. Forecasts ranged within 1.7% to 15.4% of the recorded volume, on average within 11.3%. July and August were the only forecasts within 7%, and the previous five were all over 11% off. The total March-September recorded volume was within the probable range on the first three (February - April) of seven forecasts and greater than the reasonable minimum on all but July's forecast which was 0.9% higher. Below to much below average precipitation during the March - September period meant that lower quartile forecasts were most accurate, within 2.8% on average.

Oldman River at Lethbridge

The March to September 2006 recorded natural volume for the Oldman River at Lethbridge was below average to average at 2,670,000 dam3 or 89% of average and ranks the 40th lowest in 91 years of record (1912-2001) (Table 3). This volume is 17% lower than the volume recorded during the same time period last year when flooding occurred.

Seven monthly (February to August) natural volume forecasts were produced for the Oldman River at Lethbridge for the March to September period. As illustrated in Figure 8, the recorded March to September 2006 natural volume for the Oldman River near Brocket (dark blue line) was similar to the forecasted values (brown triangles) for all seven months. In July, a drop in the forecast for the Oldman Dam combined with a rise in forecasts for the southern tributaries resulted in only a slight rise in the Lethbridge forecast. The most accurate forecasts were in February and August, both within less than 0.5% of the final recorded natural volume. The middle five forecasts were within 2.8 to 5.6% of the recorded values. Forecasts were within 2.7% of the recorded volumes, on average. The recorded March-September volume total was within the probable range and greater than the reasonable minimum on all of the seven forecasts.


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