Water Supply Outlook for Alberta

Milk River Basin - Water Supply Forecasts

Recorded March to September 2002 natural runoff volume in the Milk River basin was much-above-average, ranging from 184 to 236% of median (Table 2). All three forecast locations in the basin recorded the second highest volume on record for the June to September period. These near record volumes were recorded despite the extremely dry antecedent conditions in the spring.

Precipitation in the headwaters of the Milk River basin ranged from above-normal to much-above-normal for the March to September period. Precipitation values in the basin ranged from 134 to 193 % 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 precipitation in the February to June period. July was extremely dry with August and September recording 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 over that period. 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 Milk River basin were within 123.8% of recorded values, on average for the season.

Milk River at Western Crossing

The March to September 2002 recorded natural volume for the Milk River at Western Crossing was 134,000 dam³ or 236% of median and ranks the fifth highest on record (based on the 1931-95 data) (Table 2).

Eight monthly (January to August) natural volume forecasts were produced for the Milk River at Western Crossing for the March to September period (Figure 1). All eight forecasts were lower than the recorded value and were on average, within 141.0% of the recorded March to September volume. This is evident in Figure 1, where the recorded March to September 2002 natural volume for the Milk River at Western Crossing (dark blue line) was above the forecasted values (brown triangles). The recorded volume was within the probable range on one (August) of the eight forecasts and greater than the reasonable minimum for all eight forecasts. Forecasts ranged within 2.1% to 201.8% 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 seven forecasts. Once the extreme precipitation was accounted for, the recorded volume for the last two forecasts produced (July and August) were on average within 9.2% of the forecasted values.

The closest forecast to the recorded natural volume was produced in August (2.1%) (Figure 1). 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 near record natural runoff volume (fifth highest on record). This is confirmed by the fact that the June to September 2002 natural runoff volume for the Milk River at Western Crossing was the second largest on record (based on the 1931-95 data).

Milk River at Milk River

The March to September 2002 recorded natural volume for the Milk River at Milk River was 189,000 dam³ or 197% of median and ranks the 12th highest on record (based on the 1912-95 data) (Table 2).

Eight monthly (January to August) natural volume forecasts were produced for the Milk River at Milk River for the March to September period (Figure 2). Six of the eight forecasts were lower than the recorded value and two were above (July and August forecasts) the recorded value and were on average, within 115.0% of the recorded March to September volume. This is evident in Figure 2, where the recorded March to September 2002 natural volume for the Milk River at Western Crossing (dark blue line) was above the forecasted values (brown triangles) for the first six forecasts. The recorded volume was within the probable range on two (July and August) of the eight forecasts and greater than the reasonable minimum for all eight forecasts. Forecasts ranged within 4.2% to 163.3% 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 six forecasts. Once the extreme precipitation was accounted for, the recorded volume for the last two forecasts produced (July and August) was in the probable range and were on average, within 5.1% of the forecasted values.

The closest forecast to the recorded natural volume was produced in July (4.2%) (Figure 2). 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 near record natural runoff volume (12th highest on record). This is confirmed by the fact that the June to September 2002 natural runoff volume for the Milk River at Milk River was the second largest on record (based on the 1912-95 data).

Milk River at Eastern Crossing

The March to September 2002 recorded natural volume for the Milk River at Eastern Crossing was 217,000 dam³ or 184% of median and ranks the 17th highest on record (based on the 1912-95 data) (Table 2).

Eight monthly (January to August) natural volume forecasts were produced for the Milk River at Eastern Crossing for the March to September period (Figure 3). Six of the eight forecasts were lower than the recorded value and two were above (July and August forecasts) the recorded value and were on average, within 115.4% of the recorded March to September volume. This is evident in Figure 3, where the recorded March to September 2002 natural volume for the Milk River at Eastern Crossing (dark blue line) was above the forecasted values (brown triangles) for the first six forecasts. The recorded volume was within the probable range on none of the eight forecasts and greater than the reasonable minimum for six of the eight forecasts (July and August did not). Forecasts ranged within 23.6% to 151.4% 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 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 six forecasts. There was a gauge problem in June during the high flows so the July and August forecasts were around 30,000 dam³ too high, as they were based on recorded natural volumes at the time. As a result, the July and August forecasts were too high.

The closest forecast to the recorded natural volume was produced in July (23.4%) (Figure 3). 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 near high natural runoff volumes this year. This is confirmed by the fact that the June to September 2002 natural runoff volume for the Milk River at Eastern Crossing was the second largest on record (based on the 1912-95 data).