Milk River Basin - Water Supply Forecasts

Recorded March to September 2006 natural runoff volume in the Milk River basin was below average, ranging from 59 to 76% of median and ranging in rank between 24th and 26th lowest on record (Table 2). The monthly natural volume forecasts produced for the Milk River basin were higher than recorded values by 13.9%, on average, for the March through September runoff period.

Wet soil conditions existed last fall. Winter precipitation was below normal to normal for the November through April period in most areas of the basin. Initial forecasts in January were for below average to average naural runoff volumes, but much below normal January precipitation resulted in forecasts being revised slightly in February to below average. Forecasts remained relatively constant until May, then forecasts steadily decreased during June and July as precipitation and runoff were less than average during May-June. July and August precipitation was much below normal, and as a result July and August natural volumes were 11th and 12th lowest on record, respectively. September precipitation was below normal to normal. Total runoff volumes for the March-September period ended up below average. Soil moisture is currently near normal and flows are slightly below average as we head into winter.

Milk River at Western Crossing

The March to September 2006 recorded natural volume for the Milk River at Western Crossing was below average at 41,000 dam³ or 76% of median and ranks the 24th lowest on record (based on the 1931-2001 data) (Table 2). This volume is 23% higher 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. As illustrated in Figure 1, the recorded March to September 2006 natural volume for the Milk River at Western Crossing (dark blue line) was above the forecasted values (brown triangles) for all eight forecasts. The recorded volume was within the probable range and greater than the reasonable minimum for all of the eight forecasts.

Forecasts ranged within 2.6% to 15.2% of the recorded volume, on average within 8.2%. The best forecasts were in July and August, at 2.8 and 2.6% of recorded, while January and May were the worst at 15.2 and 14.6%.

Milk River at Milk River

The March to September 2006 recorded natural volume for the Milk River at Milk River was below average at 55,600 dam³ or 59% of median and ranks the 24th lowest on record (based on the 1912-2001 data) (Table 2). This volume is 4% lower than the volume recorded during the same time period last year.

All of the eight forecasts were higher than the recorded value. This is evident in Figure 2, where the recorded March to September 2006 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 within the probable range and greater than the reasonable minimum forecasts for only January, February and March. Recent corrections to flow data for the Milk River at Milk River by Water Survey of Canada resulted in the natural runoff volume for the year being lowered, and volumes as known before the corrections would have been within the forecast range and above the reasonable minimum for all eight forecasts. Runoff downstream of Western Crossing was much less than expected.

Forecasts ranged within 6.0% to 27.5% of the recorded volume, on average within 17.5%. The July and August forecasts were the most accurate, at 8.3 and 6.0% above actual, with the six previous forecasts all within 17.2 to 27.5%.

Milk River at Eastern Crossing

The March to September 2006 recorded natural volume for the Milk River at Eastern Crossing was below normal at 71,200 dam³ or 62% of median and ranks the 26th lowest on record (based on the 1912-2001 data) (Table 2). This volume is 5% 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 those at Milk River. As illustrated in Figure 3, the recorded March to September 2006 natural volume for the Milk River at Eastern Crossing (dark blue line) was below the forecasted values (brown triangles) for the all eight forecasts. The recorded volume was within the forecast probable range for the three forecasts from January through March, and lower thereafter. The recorded volume was greater than the reasonable minimum for six of the eight forecasts, with the exceptions being July and August.

Forecasts ranged within 3.3% to 27.2% of the recorded volume, on average within 16.0%. The July and August forecasts were the most accurate, within 5.9 and 3.3% of actual, with the six previous forecasts all within 18.3 to 27.2%.