Water Supply Outlook for Alberta

Oldman River Basin - Water Supply Forecasts

Recorded March to September 2002 natural runoff volume in the Oldman River basin was much-above-average ranging from 143 to 155% of average (Table 3). All forecast locations in the basin were in the top eleven volumes on record, with two locations (St. Mary and Belly Rivers) in the top three volumes on record. These near record volumes were recorded despite the extremely dry antecedent conditions in the spring.

Precipitation in the headwaters of the Oldman River basin ranged from above-normal to much-above-normal for the March to September period. Precipitation values in the basin ranged from 104 to 160 % of normal. The early months of the season (March, April, May and June) recorded much-above normal precipitation in the basin (>200% of normal). Many locations in the basin recorded record or near record precipitation through June (February to June precipitation rankings). July was extremely dry with August and September recording near below-normal to normal precipitation totals.

The near record precipitation in the first part of the forecast season (February to June) impacted the natural runoff volume forecasts produced over that period as the early forecasts assumed normal precipitation. The result was that the early forecasts (February, March, April, May and June) underestimated the actual natural runoff volumes because of the record high precipitation and the end result was that the natural volume forecasts produced for the Oldman River basin were within 48% of recorded values, on average for the season.

St. Mary River

The March to September 2002 recorded natural volume for the St. Mary River was 1,163,000 dam3 or 155% of average and ranks the third highest 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 (Figure 4). Six of the seven forecasts (exception was the August forecast) were lower than the recorded value and were on average, within 47.7% of the recorded March to September volume. This is evident in Figure 4, where the recorded March to September 2002 natural volume for the St. Mary River (dark blue line) was above the forecasted values (brown triangles). The recorded volume was within the probable range on two (July and August) of the seven forecasts and greater than the reasonable minimum for all seven forecasts. Forecasts ranged within 2.5% to 76.7% of the recorded volumes, with one of the forecasts within five percent of the recorded values.

The difference between the forecast and the recorded value is attributed to the fact that the February to June precipitation was the highest on record while the forecast volumes assume normal precipitation for the forecast period. Three storm systems (one in April, one in late May and the other in early June) brought significant precipitation to the basin. As a result, the recorded natural volume tended above the probable range for the first five forecasts. Once the extreme precipitation was accounted for, the recorded volume for the last two forecasts produced (July and August) was within the forecast probable range and were on average within 4.3% of the forecasted values.

The closest forecast to the recorded natural volume was produced in August (2.5%) (Figure 4). The forecasts increased steadily throughout the forecast season (brown triangles), with a substantial increase in July as a result of the record precipitation in June.

Despite the extremely dry conditions entering this spring, near record precipitation in the early portion of the forecast period (February to June) resulted in near record natural runoff volume (third highest on record). This is confirmed by the fact that the June to September 2002 natural runoff volume in the St. Mary River system was the largest on record (based on the 1912-95 data).

Belly River

The March to September 2002 recorded natural volume for the Belly River was 367,000 dam3 or 150% of average and ranks the third highest 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 (Figure 5). Six of the seven forecasts (exception was the August forecast) were lower than the recorded value and were on average, within 45.3% of the recorded March to September volume. This is evident in Figure 5, where the recorded March to September 2002 natural volume for the Belly River (dark blue line) was above the forecasted values (brown triangles). The recorded volume was within the probable range on one (July) of the seven forecasts and greater than the reasonable minimum for all seven forecasts. Forecasts ranged within 2.6% to 70.7% of the recorded volumes, with one of the forecasts within five percent of the recorded values.

The difference between the forecast and the recorded value is attributed to the fact that the February to June precipitation was near the highest on record while the forecast volumes assume normal precipitation for the forecast period. Three storm systems (one in April, one in late May and the other in early June) brought significant precipitation to the basin. As a result, the recorded natural volume tended above the probable range for the first five forecasts. Once the extreme precipitation was accounted for, the recorded volume for the last two forecasts produced (July and August) was on average within 4.8% of the forecasted values.

The closest forecast to the recorded natural volume was produced in July (2.6%) (Figure 5). The forecasts increased steadily throughout the forecast season (brown triangles), with a substantial increase in July as a result of near record precipitation in June.

Despite the extremely dry conditions entering this spring, near record precipitation in the early portion of the forecast period (February to June) resulted in near record natural runoff volume (third highest on record) for the March to September 2002 period.

Waterton River

The March to September 2002 recorded natural volume for the Waterton River was 947,000 dam3 or 143% of average and ranks the sixth highest 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 (Figure 6). Five of the seven forecasts (except the July and August forecasts) were lower than the recorded value and were on average, within 41.9% of the recorded March to September volume. This is evident in Figure 6, where the recorded March to September 2002 natural volume for the Waterton River (dark blue line) was above the forecasted values (brown triangles) for the first five forecasts. The recorded volume was within the probable range on one (July) of the seven forecasts and greater than the reasonable minimum for all seven forecasts. Forecasts ranged within 2.8% to 68.3% 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 that the February to June precipitation was near the highest on record while the forecast volumes assume normal precipitation for the forecast period. Three storm systems (one in April, one in late May and the other in early June) brought significant precipitation to the basin. As a result, the recorded natural volume tended above the probable range for the first five forecasts. Once the extreme precipitation was accounted for, the recorded volume for the last two forecasts produced (July and August) was on average, within 3.7% of the forecasted values.

The closest forecast to the recorded natural volume was produced in July (2.8%) (Figure 6). The forecasts increased steadily throughout the forecast season (brown triangles), with a substantial increase in July as a result of the record precipitation in June.

Despite the extremely dry conditions entering this spring, near record precipitation in the early portion of the forecast period (February to June) resulted in near record natural runoff volume (sixth highest on record) for the March to September 2002 period.

Oldman River at Brocket

The March to September 2002 recorded natural volume for the Oldman River near Brocket was 1,616,000 dam3 or 147% of average and ranks the 11th highest 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 (Figure 7). Five of the seven forecasts (exceptions were the July and August forecasts) were lower than the recorded value and were on average, within 52.4% of the recorded March to September volume. This is evident in Figure 7, where the recorded March to September 2002 natural volume for the Oldman River near Brocket (dark blue line) was above the forecasted values (brown triangles) for the first five forecasts. The recorded volume was within the probable range on two (July and August) of the seven forecasts and greater than the reasonable minimum for all seven forecasts. Forecasts ranged within 0.6% to 83.2% of the recorded volumes, with one of the forecasts within five percent of the recorded values.

The difference between the forecast and the recorded value is attributed to the fact that the February to June precipitation was near the highest on record while the forecast volumes assume normal precipitation for the forecast period. Three storm systems (one in April, one in late May and the other in early June) brought significant precipitation to the basin. As a result, the recorded natural volume tended above the probable range for the first five forecasts. Once the extreme precipitation was accounted for, the recorded volume for the last two forecasts produced (July and August) was within the forecast probable range and were on average within 3.6% of the forecasted values.

The closest forecast to the recorded natural volume was produced in August (0.6%) (Figure 7). The forecasts increased steady throughout the forecast season (brown triangles), with a substantial increase in July as a result of the record precipitation in June.

Despite the extremely dry conditions entering this spring, near record precipitation in the early portion of the forecast period (February to June) resulted in the 11th highest volume on record. If normal precipitation would of occurred in July and August (recorded precipitation was below-normal), it is highly probable that the recorded volume would have been the one of the highest on record this year.

Oldman River at Lethbridge

The March to September 2002 recorded natural volume for the Oldman River at Lethbridge was 4,480,000 dam3 or 151% of average and ranks the eighth highest 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 (Figure 8). All seven forecasts were lower than the recorded value and were on average, within 52.8% of the recorded March to September volume. This is evident in Figure 8, where the recorded March to September 2002 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 on two (July and August) of the seven forecasts and greater than the reasonable minimum for all seven forecasts. Forecasts ranged within 5.7% to 81.9% of the recorded volumes, with none 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 February to June precipitation was near the highest on record while the forecast volumes assume normal precipitation for the forecast period. Three storm systems (one in April, one in late May and the other in early June) brought significant precipitation to the basin. As a result, the recorded natural volume tended above the probable range for the first five forecasts. Once the extreme precipitation was accounted for, the recorded volume for the last two forecasts produced (July and August) was within the forecast probable range and were on average within 6.6% of the forecasted values.

The closest forecast to the recorded natural volume was produced in August (5.7%) (Figure 8). The forecasts increased steadily throughout the forecast season (brown triangles), with a substantial increase in July as a result of the record precipitation in June.

Despite the extremely dry conditions entering this spring, near record precipitation in the early portion of the forecast period (February to June) resulted in near record natural runoff volume (eight highest on record). This is confirmed by the fact that the June to September 2002 natural runoff volume for the Oldman River at Lethbridge was the fourth largest on record (based on the 1912-95 data). Below-normal precipitation in July and August kept the seasonal volume (March to September) from being one of the highest on record.


For technical enquires about this web page please contact Alberta Environment - Environmental Management Water Management Operations Branch at AENV-WebWS@gov.ab.ca