Frequently Asked Questions - Water Supply Outlook

Q. What is the Water Supply Outlook?

The Water Supply Outlook is a report containing forecasts of runoff for southern and central mountain rivers, and in spring, snowmelt runoff in all non-mountainous areas. It also contains a summary of current snowpack, precipitation, river flow volumes, reservoir storage and soil moisture, and data for these is published in the Maps and Data Summaries section. Runoff from the mountains is important for the major rivers in the province where reservoirs store water supplies for irrigation, hydroelectricity and community & municipal purposes. Plains area runoff is important for replenishing soil moisture and water storage in local storage facilities, such as dugouts

Q. When is the Water Supply Outlook issued?
A. Once all data values are compiled and analyzed from the previous month, Water Supply Outlooks are issued incrementally, as parts are completed, over the first one to two weeks of January, March, April, May and July. The February Water Supply Outlook is issued during the second or third week. August and September Outlooks are issued only during below normal runoff years. A year in review report is issued in November. No reports are issued in June, October, and December.
Q. How are water supply forecasts produced? Why do forecasts change from month to month, sometimes dramatically?

Forecasts are produced by assuming normal weather (precipitation, temperature) in the future. Forecast runoff is modeled using the current antecedent conditions and future scenarios. The further the departure from normal weather, the larger the change. Monthly updates usually result in more accurate forecasts as the year progresses since the period of known data increases and the period of unknown (forecasted) data decreases.

In dealing with future scenarios, assumptions are made to how/when weather patterns occur. If the weather does not follow its "normal" pattern, forecasts can change dramatically. For example, a major storm will create much greater runoff than normal. Similarily, a major hot spell will create much-above-average runoff from snowmelt.

The forecast based on normal weather is the probable forecast. Other forecasts are produced using lower quartile, upper quartile, and lower decile (10% chance of precipitation, being lower) precipitation. These forecasts provide a reasonable range of (lower to upper quartile) volume and a reasonable minimum (lower decile).

Q. What information can I expect within the monthly Water Supply Outlooks?
A. Information that you would likely find within the Water Supply Outlook from month to month is:
  • The January report provides comments on the previous month's precipitation but also reports and comments on the previous fall (September-October), and winter precipitation to date (November and December). This is the first report of the water year to report and comment on the preliminary status of snow conditions (such as snow water equivalent in the mountains), and provides a water supply forecast for the most southerly drainage basin, the Milk River Basin.
  • The February to September Water Supply Outlooks:
    • Report and comment on snow, precipitation, soil moisture, natural river flow volumes as applicable.
    • Forecast mountain runoff volumes for the March to September period and subsets thereof for 20 river locations. Issued February-April, and July; May & August if conditions warrant.
    • Forecast spring plains runoff, or describes the spring runoff as it is occurring. Plains spring runoff is usually complete by the end of April. Issued March and April.
  • The November Water Supply Outlook is unique in that it is not only the last Outlook in the water year reporting Fall precipitation, but also reviews recorded and forecasted water supplies in the province for the water year.
Q. If I know what the snowpack conditions are like, why do I need streamflow forecasts to evaluate my future water supplies?
A. Although snow water content is a major contributing factor to the runoff forecast, other components such as rainfall and soil moisture conditions affect the amount of spring and summer streamflows. Streamflow forecasts, which consider snowpack plus other factors related to runoff, provide a better overall picture of streamflow conditions than just using snowpack. This is true whether the user wants specific flow values or an indication of general conditions. (United States Dep't Agriculture, National Water and Climate Center, Publications)

Q. What is meant by normal precipitation?
A. Normal precipitation for a specific location is determined by computing the average precipitation for a specific time period over the last 30 years. Typically most normals are based on the 1971-2000 data. The percent of normal precipitation is equal to: (the present precipitation total/ normal precipitation) x 100.

For most sites the following percentage ranges are used for precipitation:
  • much-above-normal : values 130% and greater
  • above-normal : values between 110% and 130%
  • normal : values 90% to 110%
  • below-normal : values between 70% and 90%
  • much-below-normal : values 70% and below
Q. How is normal defined for runoff and snowpack conditions?
A. Runoff and snowpack are compared to the average of the entire dataset. The following ranges are used for comparing runoff and snowpack to historical values:
  • much-above-average : ranked in the upper 15% of historically recorded values
  • above-average : ranked between the upper 15% and 35% of historically recorded values
  • average : ranked between the upper 35% and lower 35% of historically recorded values
  • below-average : ranked between the lower 15% and 35% of historically recorded values
  • much-below-average: ranked in the lower 15% of historically recorded values.

Q. What is the difference between natural (adjusted) flows and observed flows?
A. "Observed flows are the flows measured at a given point on a stream, regardless of the effect of upstream water management on streamflows. Natural flows are the flows that would have occurred without human influence. Natural (adjusted) flow is calculated by adjusting observed flows for changes in storage and gauged diversions that affect streamflow.