Water Supply Outlook for Alberta

October 2004

Milk River Basin - Water Supply Forecasts

Recorded March to September 2004 natural runoff volume in the Milk River basin was much-below-average, ranging from 31 to 37% of median and ranging in rank between 11th and 15th lowest on record (Table 2). The monthly natural volume forecasts produced for the Milk River basin were within 8.9% of recorded values, on average, for the March through September runoff period.

Very dry soil conditions existed entering last winter. Winter precipitation was roughly half of normal for the November through April period in most areas of the basin, with the winter season ending on a very dry note as February-April precipitation was generally less than one third of normal. The exception was the Cypress Hills area where a much above average snowpack existed into March. Spring snowmelt occurred about a month ahead of schedule, and runoff during March was above median in the western upper elevations and in the eastern Cypress Hills area. Much above normal precipitation occurred in May and August while normal precipitation fell during June and July. Despite this new wet trend, only the natural volume in the Milk River at the eastern U.S. border crossing showed significant improvement as the August-September volume total rose to approximately 75% of the median. Total runoff volumes for the March-September period remained much below average as most of the precipitation went into the soil.

Milk River at Western Crossing

The March to September 2004 recorded natural volume for the Milk River at Western Crossing was much-below-average at 20,100 dam3 or 35% of median and ranks the 11th lowest on record (based on the 1931-2001 data) (Table 2). This volume is 27% lower than the volume recorded during the same time period last year.

Eight monthly (January to August) natural volume forecasts were produced for the Milk River at Western Crossing for the March to September period. The first two forecasts were for approximately half the median natural runoff volume. Minimal precipitation during February then caused the forecast to drop, but better than expected runoff in March brought the forecast back up. April provided little precipitation however, so the forecast again dropped. The June 1 forecast rose again slightly due to the much above normal precipitation during May. The final two forecasts saw a gradual downward trend, as runoff volumes did not improve despite near normal precipitation during June and July. Heavy precipitation again in August produced little runoff in August and September, but typically runoff during this period is a minimal portion of the March-September total. As illustrated in Figure 1, the recorded March to September 2004 natural volume for the Milk River at Western Crossing (dark blue line) was below the forecasted values (brown triangles) for the first seven forecasts, and slightly above for August. The recorded volume was within the probable range and greater than the reasonable minimum for all of the eight forecasts.

Forecasts ranged within 0.2% to 19.1% of the recorded volume, on average within 9.0%. The best forecast was in August, while three of eight forecasts (May, July, August) were within 6%.

Milk River at Milk River

The March to September 2004 recorded natural volume for the Milk River at Milk River was much-below-average at 29,400 dam3 or 31% of median and ranks the 12th lowest on record (based on the 1912-2001 data) (Table 2). This volume is 33% lower than the volume recorded during the same time period last year.

Eight monthly (January to August) natural volume forecasts were produced for the Milk River at Milk River for the March to September period. Natural flow forecasts mirrored the trends of those in the upper basin at Western Crossing through the year, but with even lesser runoff from the area between the two locations, the forecasts here were slightly less accurate. All of the eight forecasts were higher than the recorded value. This is evident in Figure 2, where the recorded March to September 2004 natural volume for the Milk River at Milk River (dark blue line) was below the forecasted values (brown triangles) every month. The recorded volume was greater than the reasonable minimum for all eight forecasts, and within the probable range on three of the eight forecasts (lower end of range slightly high from January to March and in May and August.

Forecasts ranged within 0.6% to 22.3% of the recorded volume, on average within 11.1%. The August forecast was the most accurate, with the final two forecasts both within 4% and the final four all within 8% of the recorded values.

Milk River at Eastern Crossing

The March to September 2004 recorded natural volume for the Milk River at Eastern Crossing was much-below-normal at 43,600 dam3 or 37% of median and ranks the 15th lowest on record (based on the 1912-2001 data) (Table 2). This volume is 33% lower than the volume recorded during the same time period last year.

Eight monthly (January to August) natural volume forecasts were produced for the Milk River at Eastern Crossing for the March to September period. Natural flow forecasts mirrored the trends of those upstream, however the better snowpack on the eastern side of the basin created better soil moisture here when it melted, allowing for slightly better runoff generation from above normal summer rains. As a result, runoff from the eastern side of the basin prevented forecasts from dropping by as wide a margin as they had upstream. As illustrated in Figure 3, the recorded March to September 2004 natural volume for the Milk River at Eastern Crossing (dark blue line) was below the forecasted values (brown triangles) for the first seven forecasts and above for the final forecast in August. The recorded volume was within the forecast probable range on all of the eight forecasts and greater than the reasonable minimum for all of the eight forecasts.

Forecasts ranged within 0.6% to 17.0% of the recorded volume, on average within 6.7%. The May forecast was the most accurate, five of the eight forecasts were within 6% of the recorded volume (all but January, February and April), and three of these were within 1% (May, July, August).


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