Oldman River Basin - Water Supply Forecasts

Below-average to average natural runoff volumes are forecast for the March to September 2002 period in the Oldman River basin (Table 3a). Current forecasted values for the March to September period for the Oldman River at Lethbridge would rank 27th lowest in 84-years of record (1912-95). The May 1 forecasts are 3 to 9 % higher than last month's forecast due to above-normal precipitation being recorded during the month of April. May's forecasts are 15 to 19 % higher than those produced on February 1 due to the above-normal precipitation this spring. Current runoff volume forecasts for this year are 21 to 44% higher than recorded runoff volumes last summer.

Despite above-average snow accumulations at the higher elevations, below-average to average runoff is forecast for the March to September period because the snow below 6000 feet is average and the extreme soil moisture deficits that exist. This year, the March to September water supply forecasts are heavily dependent on rainfall during the spring and early summer (May and June) months. Water supply from the higher elevations, given the current snow accumulations, is assured. If above-normal precipitation occurs at the lower elevations, the forecast water supply will continue to improve.

The first two months of the forecast period have been completed, with March-April 2002 recorded runoff volumes being much-below-average to average, ranging from 52 to 112 % of average. The extremely dry antecedant conditions are evident in the flow for the Oldman River near Brocket, as the March-April 2002 recorded volume is the ninth lowest on record. The remaining volume forecast, which is the March to September 2002 forecast minus the recorded volume to date, is available in Table 3b.