Interpreting Water Supply Forecasts

A water supply forecast is a prediction of
streamflow volume that will flow past a point on a stream during a
specified season, typically in the spring and summer. These
forecasts are given not as a single number, but as a range of numbers to
reflect risk and forecast uncertainty. The information provided
below is intended to help users interpret these forecasts. 
...To Water Supply Forecasting Products 
Each month, five forecasts are issued for each forecast point and each
forecast period. Unless otherwise specified, all forecasts are
for streamflow volumes that would occur naturally without any upstream
influences. Water users need to know what the different forecasts represent
if they are to use the information correctly when making operational
decisions. The following is an explanation of each of the forecasts:
90 Percent Chance of Exceedance Forecast. There is a 90
percent chance that the actual streamflow volume will
exceed this forecast value, and there is a 10 percent chance that the actual streamflow
volume will be less than this forecast value.
70 Percent Chance of Exceedance Forecast. There is a 70
percent chance that the actual streamflow volume will
exceed this forecast value, and there is a 30 percent chance that the actual streamflow
volume will be less than this forecast value.
50 Percent Chance of Exceedance Forecast. There is a 50 percent chance that the actual streamflow volume will
exceed this forecast value, and there is a 50 percent chance that the actual streamflow
volume will be less than this forecast value. Generally, this forecast
is the middle of the range of possible streamflow volumes that can be
produced given current conditions.
30 Percent Chance of Exceedance Forecast. There is a 30
percent chance that the actual streamflow volume will
exceed this forecast value, and there is a 70 percent chance that the actual streamflow
volume will be less than this forecast value.
10 Percent Chance of Exceedance Forecast. There is a 10
percent chance that the actual streamflow volume will
exceed this forecast value, and there is a 90 percent chance that the actual streamflow
volume will be less than this forecast value.
*Note: There is still a 20 percent chance that actual streamflow volumes will fall either below the 90 percent exceedance forecast
or above the 10 percent exceedance forecast.
These forecasts represent the uncertainty inherent in making streamflow
predictions. This uncertainty may include sources such as: unknown
future weather conditions, uncertainties associated with the various
prediction methodologies, and the spatial coverage of the data network in a
given basin.
These forecasts are given to users to help make riskbased decisions.
Users can select the forecast corresponding to the level of risk they are
willing to accept in order to minimize the negative impacts of having more
or less water than planned for.
To Decrease the Chance of Having Less Water than Planned for
A user might determine that making decisions based on a 50 percent chance of
exceedance forecast is too much risk to take (there is still a 50% chance
that the user will receive less than this amount). To reduce the risk
of having less water than planned for, users can base their operational
decisions on one of the forecasts with a greater chance of being exceeded
such as the 90 or 70 percent exceedance forecasts.
To Decrease the Chance of Having More Water than Planned for
A user might determine that making decisions based on a 50 percent chance of
exceedance forecast is too much risk to take (there is still a 50% chance
that the user will receive more than this amount). To reduce the risk
of having more water than planned for, users can base their operational
decisions on one of the forecasts with a lesser chance of being exceeded
such as the 30 or 10 percent exceedance forecasts.
Using the forecasts  an Example
Using the 50 Percent Exceedance Forecast. Using the
example forecasts shown below, there is a 50% chance that actual
streamflow volume at the Yellowstone River at Livingston will be less than
1320 KAF (1000 acrefeet) between April 1 and
July 31. There is also a 50% chance that actual streamflow
volume will be greater than 1320 KAF.
Using the 90 and 70 Percent Exceedance Forecasts. If an unexpected shortage of water could
cause problems (such as irrigated agriculture), users might want to plan on
receiving 1240 KAF (from the 70 percent exceedance forecast).
There is a 30% chance of receiving less than 1240 KAF.
Alternatively, if users determine the risk of using the 70 percent
exceedance forecast is too great, then they might plan on receiving 1120 KAF (from the 90 percent exceedance forecast). There is a 10% chance
of receiving less than 1120 KAF.
Using the 30 or 10 Percent Exceedance Forecasts. If an
unexpected excess of water could cause problems (such as operating a flood
control reservoir), users might plan on
receiving 1400 KAF (from the 30 percent exceedance forecast). There is
a 30% chance of receiving more than 1400 KAF.
Alternatively, if users determine the risk of using the 30 percent
exceedance forecast is too great, then they might plan on receiving 1840 KAF
(from the 10 percent exceedance forecast). There is a 10% chance of
receiving more than 1840 KAF.
Users could also choose a volume in between any of these values to
reflect their desired risk level.

UPPER YELLOWSTONE RIVER BASIN 
Streamflow Forecasts 
April 1, 2005 

============ Chance of Exceeding *
=========== 

50% 
(1000AF) 
(1000AF) 
(1000AF) 
(1000AF) 
(1000AF) 

YELLOWSTONE at
Lake Outlet 
APRJUL 
285 
345 
385 
65 
425 
485 
590 
APRSEP 
385 
460 
510 
63 
560 
635 
805 
YELLOWSTONE RIVER
at Corwin Springs 
APRJUL 
915 
1060 
1160 
70 
1260 
1400 
1650 
APRSEP 
1110 
1270 
1390 
71 
1510 
1670 
1970 
YELLOWSTONE RIVER
near Livingston 
APRJUL 
1120 
1240 
1320 
70 
1400 
1520 
1900 
APRSEP 
1360 
1500 
1600 
70 
1700 
1840 
2280 
SHIELDS RIVER nr
Livingston 
APRJUL 
14 
59 
90 
62 
121 
166 
145 
APRSEP 
13 
66 
102 
63 
138 
193 
162 

* 90%, 70%, 30%, and 10% chances of exceeding are
the probabilities that the
actual flow will exceed the volumes in the table. 

