Oldman River Basin - Water Supply Forecasts

Recorded March to September 2008 natural runoff volumes in the Oldman River basin varied widely from above to much above average for the St. Mary River to below average to average at the Belly River near Mountainview. Much above normal precipitation in May and June raised volumes significantly in the Waterton and St. Mary River basins, but the Belly River did not respond as much. Areas of the Oldman River basin above Brocket and Lethbridge recorded lesser precipitation during this period, resulting in lower volumes as well, compared to the Waterton and St. Mary Rivers. Natural runoff volumes this year varied from 96% to 111% of average (Table 1).

The six monthly (February to August, excluding June) forecasts of March through September 2008 natural volumes produced for the Bow River basin were within 7.2% of recorded values, on average. No forecasts were produced in June this year due to flood events. Measured volumes were within the forecast probable range except for two locations in August, and greater than the reasonable minimum for all forecasts and locations.

Natural runoff volumes measured during the month of September were below average at Brocket, and average for all other locations (Table 1).

St. Mary River at St. Mary Reservoir

The March to September 2008 recorded natural volume for the St. Mary River at St. Mary Dam was above to much above average at 829,200 dam³ or 111% of average and ranks 66th lowest (26th highest) in 91 years of record (1912-2001 data) (Table 1).

Six monthly (February to August, excluding June) natural volume forecasts were produced for the March to September 2008 runoff period. Forecasts ranged within 12.4% to 18.4% of the recorded volume for the first four monthly forecasts, and 4.9% to 7.7% for July and August's forecasts. Volume was underestimated in August, as the recorded volume was greater than the upper quartile forecast. Significant corrections to the flow data by Water Survey of Canada for May, June and July resulted in the March - September volume increasing by 6%.

Belly River at Mountainview

The March to September 2008 recorded natural volume for the Belly River was below average to average at 234,800 dam³ or 96% of average and ranks the 39th lowest on record (based on the 1912-2001 data) (Table 1).

Six monthly (February to August, excluding June) natural volume forecasts were produced for the March to September 2008 runoff period. The monthly forecast accuracies ranged from 0.6% to 4.7%, for an overall average of 2.1%. The recorded volume was within the probable forecast range and greater than the reasonable minimum forecast for all six forecasts.

Waterton River

The March to September 2008 recorded natural volume for the Waterton River at the Waterton Dam was above average at 647,200 dam³ or 107% of average and ranks 57th lowest (35th highest) in 91 years of record (1912-2001 data) (Table 1).

Six monthly (February to August, excluding June) natural volume forecasts were produced for the March to September 2008 runoff period. Forecasts ranged within 12.8% to 17.6% of the recorded volume for the first four monthly forecasts, and 1.4% to 2.7% for July and August's forecasts. Much above normal precipitation in May and June raised volumes significantly. Volume was overestimated in August, as the recorded volume tracked just below the lower quartile forecast.

Oldman River at Brocket

The March to September 2008 recorded natural volume for the Oldman River at Brocket was average at 1,055,800 dam³ or 97% of average and ranks the 47th lowest on record (based on the 1912-2001 data) (Table 1).

Six monthly (February to August, excluding June) natural volume forecasts were produced for the March to September 2008 runoff period. The monthly forecast accuracies ranged from 0.0% to 6.2%, for an overall average of 3.0%. The recorded volume was within the probable forecast range and greater than the reasonable minimum forecast for all six forecasts.

Oldman River at Lethbridge

The March to September 2008 recorded natural volume for the Oldman River at Lethbridge was average to above average at 3,042,100 dam³ or 102% of average and ranks the 51st lowest on record (41st highest) (based on the 1912-2001 data) (Table 1).

Six monthly (February to August, excluding June) natural volume forecasts were produced for the March to September 2008 runoff period. Forecasts ranged within 9.0% to 13.2% of the recorded volume for the first four monthly forecasts, and 0.7% to 2.3% for July and August's forecasts. The recorded volume was within the probable forecast range and greater than the reasonable minimum forecast for all six forecasts.