Oldman River Basin - Water Supply Forecasts

Recorded March to September 2003 natural runoff volume in the Oldman River basin was much-below-average ranging from 63 to 81% of average and ranking between 12th and 21st lowest on record (Table 3). The monthly natural volume forecasts produced for the Oldman River basin were within 6.6% of recorded values, on average, for the March through September runoff period.

Entering last winter, antecedent conditions were wet. However, precipitation totals in the headwaters of the basin were generally below-normal to much-below-normal during winter and summer. Normal to much-above normal precipitation was recorded in the basin during March and April, in the upper basin of the mainstem Oldman River, temporarily raising forecasts somewhat. July through September was then very dry, with volumes during this period ranging in rank from second to twelfth lowest on record (based on the 1912-95 data). August through September was even drier, with volumes during this period ranking among the lowest five in 85 years of record, near or above volumes recorded during the same period in 2001. For the runoff year (March through September), volumes in 2003 were much higher than in 2001 except in the Waterton River where volumes were only slightly better than in 2001.

St. Mary River

The March to September 2003 recorded natural volume for the St. Mary River was much-below-average at 535,900 dam³ or 71% of average and ranks the fifteenth lowest on record (based on the 1912-95 data) (Table 3).

Seven monthly (February to August) natural volume forecasts were produced for the St. Mary River for the March to September period. All of the seven forecasts were higher than the recorded value and were on average within 5.9% of the recorded March to September volume. This is evident in Figure 4, where the recorded March to September 2003 natural volume for the St. Mary River (dark blue line) was below the forecasted values (brown triangles). The recorded volume was greater than the reasonable minimum and within the probable range on five of the seven forecasts (all but July and August). Forecasts ranged from 1.9% to 10.5% of the recorded volumes, with the best forecast in March. Three of the forecasts were within five percent of the recorded value.

Adjustments of volume records led to 10,000 dam³ less volume than had been known. Had this not changed, the recorded volume would have exceeded the minimum for six of seven forecasts (all but August).

Despite the wet conditions entering this winter, monthly forecasts were fairly consistent as below-normal to much-below-normal precipitation was recorded through most of the year. All forecasts overestimated volumes as normal precipitation was anticipated. July through September recorded volume ranked second lowest on record (based on the 1912-95 data), but volume during the same period in both 2000 and 2001 was also lower. These extremely low values led to recorded volumes being outside the forecasted probable range and less than the forecasted minimum during July and August.

Belly River

The March to September 2003 recorded natural volume for the Belly River was much-below-average at 198,200 dam³ or 81% of average and ranks the twenty-first lowest on record (based on the 1912-95 data) (Table 3).

Seven monthly (February to August) natural volume forecasts were produced for the Belly River for the March to September period. Two of the seven forecasts (February and March) were lower than the recorded value. This is evident in Figure 5, where the recorded March to September 2003 natural volume for the Belly River (dark blue line) was above the forecasted values (brown triangles) for February and March. The recorded volume was within the probable range and greater than the reasonable minimum on six of seven forecasts (all but August). Forecasts ranged within 0.6% to 6.5% of the recorded volumes, with the best forecast in June, and were on average within 3.6% of the recorded March to September volume. Four of the forecasts were within five percent of the recorded values.

Despite the wet conditions entering this winter, monthly forecasts were fairly consistent as below-normal to much-below-normal precipitation was recorded through most of the year. August through September recorded volume ranked fourth lowest on record (based on the 1912-95 data), and volume during the same period in 2001 was similar. These extremely low values led to recorded volumes being outside the forecasted probable range and less than the forecasted minimum during August.

Waterton River

The March to September 2003 recorded natural volume for the Waterton River was much-below-average at 419,700 dam³ or 63% of average and ranks the twelfth lowest on record (based on the 1912-95 data) (Table 3).

Seven monthly (February to August) natural volume forecasts were produced for the Waterton River for the March to September period. All of the seven forecasts were lower than the recorded value and were on average, within 8.0% of the recorded March to September volume. This is evident in Figure 6, where the recorded March to September 2003 natural volume for the Waterton River (dark blue line) was below the forecasted values (brown triangles) for all seven forecasts. The recorded volume was within the probable range on the first three (February, March, April) of the seven forecasts and greater than the reasonable minimum for six of seven forecasts (all but August). Forecasts ranged within 2.3% to 13.3% of the recorded volumes, with the best forecast in August and four of the forecasts within six percent of the recorded values.

Despite the wet conditions entering this winter, monthly forecasts were fairly consistent as below-normal to much-below-normal precipitation was recorded through most of the year. July through September recorded volumes ranked third lowest on record (based on the 1912-95 data), but volume during the same period in 2001 was lower. These extremely low values led to recorded volumes being outside the forecasted probable range during August and September and less than the forecasted minimum during August.

Oldman River at Brocket

The March to September 2003 recorded natural volume for the Oldman River near Brocket was much-below-average at 774,500 dam³ or 70% of average and ranks the 18th lowest on record (based on the 1912-95 data) (Table 3).

Seven monthly (February to August) natural volume forecasts were produced for the Oldman River near Brocket for the March to September period. The first two monthly forecasts were lower than the recorded value. This is evident in Figure 7, where the recorded March to September 2003 natural volume for the Oldman River near Brocket (dark blue line) was above the forecasted values (brown triangles) for the first two forecasts. The recorded volume was within the probable range and greater than the reasonable minimum on five of the seven forecasts (all but July and August). Forecasts ranged within 1.8% to 11.0% of the recorded volume, with the best forecast in April and two of the forecasts within five percent of the recorded value. Forecasts were on average within 7.7% of the recorded March to September volume.

Despite wet conditions entering last winter, initial forecasts were low due to the lack of snow accumulation. Forecasts increased substantially in April and May as a result of storms during March and April. The forecasts decreased steadily through the summer with summer precipitation being below-normal to much-below-normal. However, the recorded volume ended up outside the forecast probable range and below the reasonable minimum during July and August as the July through September recorded volume was seventh lowest on record.

A correction of volume data resulted in 36,000 dam³ less runoff this year. Had the volumes been accurate during our forecasts, all forecasts would have been above the reasonable minimum, and forecasts would have been within 6.7%, on average, of the recorded March to September volume. The volume would have been at the lower quartile forecast of July and August.

Oldman River at Lethbridge

The March to September 2003 recorded natural volume for the Oldman River at Lethbridge was much-below-average at 1,899,000 dam³ or 64% of average and ranks the 13th lowest on record (based on the 1912-95 data) (Table 3).

Seven monthly (February to August) natural volume forecasts were produced for the Oldman River at Lethbridge for the March to September period. All seven forecasts were lower than the recorded value. This is evident in Figure 8, where the recorded March to September 2003 natural volume for the Oldman River near Brocket (dark blue line) was above the forecasted values (brown triangles). The recorded volume was within the probable range and greater than the reasonable minimum on five of the seven forecasts (all but July and August). Forecasts ranged within 0.7% to 12.6% of the recorded volumes, with the best forecast in March and two of the forecasts within five percent of the recorded values. Forecasts were on average within 7.8% of the recorded March to September volume.

Despite wet conditions entering last winter, initial forecasts were low due to the lack of snow accumulation. Forecasts increased in April and May as a result of storms in the basin during March and April. The forecasts decreased steadily through the summer with summer precipitation being below-normal to much-below-normal. However, the recorded volume ended up outside the forecast probable range and below the reasonable minimum during July and August as the July through September recorded volume was third lowest on record, similar to the volume recorded during the same period in 2001.

A correction of volume data resulted in 71,000 dam³ less runoff this year. Had the volumes been accurate during our forecasts, six of seven forecasts would have been above the reasonable minimum, and forecasts would have been within 6.7%, on average, of the recorded March to September volume.