Missouri River Basin Water Supply Forecast Methods
Water within the Missouri River Basin is often stored in reservoirs, diverted unto fields, diverted into ditches, or diverted across basin boundaries by inter-basin tunnels rather than following nature’s course downstream. Reservoirs alter the seasonal timing of streamflow. Evaporation from reservoirs, irrigation, and other influences can reduce the total amount of water available to flow downstream. Streamflow gages measure the flow that is present in the river after all of these factors have exerted their influence.
The Missouri Basin River Forecast Center (MBRFC) attempts to forecast the streamflow that would have occurred without reservoirs. This is in part due to the fact that our forecasts may be used to guide the operation of reservoirs and irrigation systems; operators of those systems want to know how much water to expect, not how or when we think they will release it or use it. In any case, we have elected to not attempt to forecast human behavior, and our forecasts are for natural flows, at least where use comes from stored water. Therefore, if we forecast 100,000 AF of natural inflow to a reservoir but the 60,000 AF will likely be held by the reservoir, then the expectation for downstream flows would be 40,000 AF. The average individual user will probably find it more useful to look only at how this year’s forecast compares to the historical average, or to flows from a particular year. Both of these types of data, and more, are available at this website.
Not all diversions, returns, and uses of water are accounted for in our natural flow determinations. This may be due to the fact that many of these uses are insignificant, but in most cases it is due to the fact that data are not available to make the adjustments. As technology, time, and inter-agency coordination permit it may be possible to make more adjustments to better estimate natural flows. On the other hand, an argument could be made that it is better to stay the course for the sake of consistency between year-to-year forecasts. For example, the upper Yellowstone has large areas of land irrigated by unmeasured, run-of-river diversions without reservoir storage. Historically, these have not been accounted for in statistical water supply forecasting, i.e. regression equations predict the flow expected after these losses. Downstream water users have been accustomed to observing the flows after these uses have been subtracted for many decades, so it is unclear who would benefit from a natural flow forecast in areas like this.
Forecasts are issued on or before the eighth working day each month, starting in January and ending after the June forecast has been published. The forecast period is usually April 1 through September 30 up through the April forecast issuance. Subsequent forecasts are generally from the first of the current month through September 30, i.e. the forecast period in June is June 1 through September 30.
Water Supply forecasts typically include current snow water equivalent from one or more SNOTEL sites, and often include October through December precipitation as measured at one or more SNOTEL sites, too.
Ensemble Water Supply Forecasts
As described in much greater detail elsewhere, ensemble forecasts are made by running available, historical years of climatological data through a hydrologic model, starting each historical-year’s run with today’s initial (model) conditions. The historical period of record is constrained to post 1978 by the availability of NRCS - SNOTEL data. Ensemble forecasts are generated once a month, on the Tuesday after the third Thursday of the month, the third Thursday being when the NOAA National Weather Service Climate Prediction Center issues their one and three-month outlooks. In March there are two ensemble runs made, the second required for Spring Outlook (snowmelt crest prediction) products.
Ensemble forecast model runs starting in January are run to the end of September, and from July through December they are run for 95 days.
Ensemble forecasts are run with adjustment for consumptive water use where those uses are upstream of reservoirs. Thus, the ensemble forecasts are similar in that way to SWS forecasts for areas like the upper Yellowstone. The influence of reservoirs has been removed from ensemble runs by routing water through them as though they do not exist. Consumptive use below reservoirs is not modeled. So, the ensemble forecasts must be interpreted slightly differently depending on whether the irrigated areas are above or below reservoirs.
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Table 1: Streamflow Adjustments for Statistical Water Supply by State |
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MONTANA |
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Sorted by downstream order |
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Adjustment Key: |
'G' Main gage, start with this |
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'-' subtract this gage |
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'+' add this gage |
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USGS ID |
Nominal station |
Computational Stations |
Adjustment |
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5017500 |
ST MARY R NR BABB, MT |
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5017500 |
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ST MARY R NR BABB |
G |
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6015500 |
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SHERBURNE LAKE (STORAGE CHANGE) |
+ |
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6012500 |
RED ROCK RIVER NEAR MONIDA, MT (Lima Reservoir Inflow ) |
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6012500 |
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RED ROCK R NR MONIDA |
G |
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6012000 |
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LIMA RESERVOIR (STORAGE CHANGE) |
+ |
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6015400 |
BEAVERHEAD R NR GRANT, MT (Clark Canyon Reservoir Inflow) |
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6015400 |
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BEAVERHEAD R NR GRANT |
G |
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6012000 |
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LIMA RESERVOIR (STORAGE CHANGE) |
+ |
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6016000 |
BEAVERHEAD RIVER NEAR BARRATTS, MT |
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6016000 |
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BEAVERHEAD R NR BARRATTS |
G |
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6012000 |
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LIMA RESERVOIR (STORAGE CHANGE) |
+ |
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6015300 |
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CLARK CANYON RESERVOIR (STORAGE CHANGE) |
+ |
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6019500 |
RUBY R ABV RESV NEAR ALDER, MT (Ruby Reservoir Inflow) |
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6019500 |
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RUBY R ABV RESV NR ALDER |
G |
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6025500 |
BIG HOLE R NR MELROSE, MT |
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6025500 |
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BIG HOLE R NR MELROSE |
G |
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6038500 |
MADISON RIVER NEAR GRAYLING, MT (Hebgen Reservoir Inflow) |
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6038500 |
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MADISON R NR GRAYLING |
G |
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6038000 |
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HEBGEN LAKE (STORAGE CHANGE) |
+ |
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6041000 |
MADISON RIVER NEAR McALLISTER, MT (Ennis Reservoir Inflow) |
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6041000 |
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MADISON R NR McALLISTER |
G |
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6038000 |
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HEBGEN LAKE (STORAGE CHANGE) |
+ |
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6040500 |
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ENNIS LAKE (STORAGE CHANGE) |
+ |
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6043500 |
GALLATIN RIVER NEAR GALLATIN GATEWAY, MT |
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6043500 |
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GALLATIN R NR GALLATIN GATEWAY |
G |
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6052500 |
GALLATIN RIVER AT LOGAN, MT |
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6052500 |
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GALLATIN R AT LOGAN |
G |
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6054500 |
MISSOURI RIVER AT TOSTON, MT |
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6054500 |
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MISSOURI R AT TOSTON |
G |
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6015300 |
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CLARK CANYON RESERVOIR (STORAGE CHANGE) |
+ |
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6038000 |
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HEBGEN LAKE (STORAGE CHANGE) |
+ |
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6040500 |
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ENNIS LAKE (STORAGE CHANGE) |
+ |
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6077000 |
SHEEP CREEK NEAR WHITE SULPHUR SPRINGS, MT |
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6077000 |
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SHEEP CK NR WHITE SULPHUR SPRINGS |
G |
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6078600 |
SUN RIVER AT GIBSON DAM NR AUGUSTA, MT (Gibson Reservoir Inflow) |
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6078600 |
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SUN R AT GIBSON DAM NR AUGUSTA |
G |
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6090800 |
MISSOURI RIVER AT FORT BENTON, MT |
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6090800 |
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MISSOURI R AT FORT BENTON |
G |
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6058500 |
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CANYON FERRY LAKE (STORAGE CHANGE) |
+ |
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6099500 |
MARIAS RIVER NEAR SHELBY, MT |
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6099500 |
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MARIAS R NR SHELBY |
G |
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6090900 |
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LOWER TWO MEDICINE LAKE (STORAGE CHANGE) |
+ |
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6093000 |
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FOUR HORNS LAKE (STORAGE CHANGE) |
+ |
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6094000 |
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SWIFT RESERVOIR (STORAGE CHANGE) |
+ |
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6095500 |
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LAKE FRANCES (STORAGE CHANGE) |
+ |
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6109500 |
MISSOURI RIVER AT VIRGELLE, MT |
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6109500 |
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MISSOURI RIVER AT VIRGELLE, MT |
G |
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6058500 |
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CANYON FERRY LAKE (STORAGE CHANGE) |
+ |
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6101300 |
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LAKE ELWELL (TIBER) (STORAGE CHANGE) |
+ |
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6115200 |
MISSOURI RIVER NEAR LANDUSKY, MT |
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6115200 |
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MISSOURI R NR LANDUSKY |
G |
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6058500 |
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CANYON FERRY LAKE (STORAGE CHANGE) |
+ |
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6101300 |
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LAKE ELWELL (TIBER) (STORAGE CHANGE) |
+ |
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6120500 |
MUSSELSHELL RIVER AT HARLOWTON |
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6120500 |
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MUSSELSHELL R AT HARLOWTON |
G |
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6116500 |
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BAIR RESERVOIR (STORAGE CHANGE) |
+ |
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6119000 |
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MARTINSDALE RESERVOIR (STORAGE CHANGE) |
+ |
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6132000 |
MISSOURI RIVER BELOW FT PECK DAM (Ft Peck Reservoir Inflow) |
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6132000 |
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MISSOURI R BLW FT PECK DAM |
G |
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6058500 |
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CANYON FERRY LAKE (STORAGE CHANGE) |
+ |
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6131500 |
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FORT PECK LAKE (STORAGE CHANGE) |
+ |
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6131501 |
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FORT PECK LAKE EVAPORATION |
+ |
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6133000 |
MILK R AT WESTERN CROSSING |
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6133000 |
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MILK R AT WESTERN CROSSING |
G |
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6134500 |
MILK RIVER AT MILK RIVER, ALBERTA, CANADA |
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6134500 |
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MILK RIVER AT MILK RIVER, ALBERTA, CANADA (11AA005) |
G |
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5018500 |
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ST MARY CANAL AT ST MARY XING NR BABB |
- |
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6135100 |
MILK RIVER AT EASTERN CROSSING – ADJUSTED |
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6135000 |
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MILK R AT EASTERN CROSSING (11AA031) |
G |
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5018500 |
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ST MARY CANAL AT ST MARY XING NR BABB |
- |
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YELLOWSTONE LAKE OUTLET (SEE WYOMING, BELOW) |
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6191500 |
YELLOWSTONE RIVER AT CORWIN SPRINGS, MT |
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6191500 |
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YELLOWSTONE R AT CORWIN SPRINGS |
G |
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6192500 |
YELLOWSTONE AT LIVINGSTON |
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6192500 |
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YELLOWSTONE AT LIVINGSTON |
G |
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6200000 |
BOULDER R AT BIG TIMBER |
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6200000 |
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BOULDER R AT BIG TIMBER |
G |
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6205000 |
STILLWATER RIVER NEAR ABSAROKEE, MT |
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6205000 |
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STILLWATER R NR ABSAROKEE |
G |
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6204000 |
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MYSTIC LAKE (STORAGE CHANGE) |
+ |
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6207500 |
CLARKS FORK YELLOWSTONE RIVER NR BELFRY, MT |
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6207500 |
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CLARKS FK RIVER NR BELFRY |
G |
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6214500 |
YELLOWSTONE R AT BILLINGS |
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6214500 |
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YELLOWSTONE R AT BILLINGS |
G |
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6287000 |
BIGHORN RIVER NEAR ST XAVIER, MT |
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6287000 |
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BIGHORN R NR ST XAVIER |
G |
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6224500 |
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BULL LAKE (STORAGE CHANGE) |
+ |
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6258900 |
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BOYSEN RESERVOIR (STORAGE CHANGE) |
+ |
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6281500 |
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BUFFALO BILL RESERVOIR (STORAGE CHANGE) |
+ |
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6286400 |
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BIGHORN LAKE (STORAGE CHANGE) |
+ |
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6294000 |
LITTLE BIGHORN RIVER NEAR HARDIN, MT |
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6294000 |
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LITTLE BIGHORN R NR HARDIN |
G |
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6306300 |
TONGUE R AT STATE LINE NR DECKER, MT (same site listed under Wyoming) |
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6306300 |
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TONGUE R AT STATE LINE NR DECKER |
G |
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6297500 |
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HIGHLINE DITCH |
+ |
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6309000 |
YELLOWSTONE RIVER AT MILES CITY, MT |
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6309000 |
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YELLOWSTONE R AT MILES CITY |
G |
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6258900 |
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BOYSEN RESERVOIR (STORAGE CHANGE) |
+ |
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6281500 |
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BUFFALO BILL RESERVOIR (STORAGE CHANGE) |
+ |
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6286400 |
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BIGHORN LAKE (STORAGE CHANGE) |
+ |
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6307000 |
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TONGUE RIVER RESERVOIR (STORAGE CHANGE) |
+ |
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6324500 |
POWDER RIVER AT MOORHEAD, MT |
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6324500 |
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POWDER R AT MOORHEAD |
G |
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6326500 |
POWDER RIVER NEAR LOCATE, MT |
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6326500 |
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POWDER R NR LOCATE |
G |
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6329500 |
YELLOWSTONE RIVER NEAR SIDNEY, MT |
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6329500 |
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YELLOWSTONE R NR SIDNEY |
G |
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6258900 |
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BOYSEN RESERVOIR (STORAGE CHANGE) |
+ |
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6281500 |
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BUFFALO BILL RESERVOIR (STORAGE CHANGE) |
+ |
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6286400 |
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BIGHORN LAKE (STORAGE CHANGE) |
+ |
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6338490 |
MISSOURI RIVER AT GARRISON DAM, ND (Lake Sakakawea Inflow) |
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6338490 |
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MISSOURI RIVER AT GARRISON DAM, ND |
G |
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6058500 |
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CANYON FERRY LAKE (STORAGE CHANGE) |
+ |
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6131500 |
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FORT PECK LAKE (STORAGE CHANGE) |
+ |
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6131501 |
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FORT PECK LAKE EVAPORATION |
+ |
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6258900 |
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BOYSEN RESERVOIR (STORAGE CHANGE) |
+ |
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6281500 |
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BUFFALO BILL RESERVOIR (STORAGE CHANGE) |
+ |
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6286400 |
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BIGHORN LAKE (STORAGE CHANGE) |
+ |
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Table 1 (continued): Streamflow Adjustments for Statistical Water Supply by State |
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WYOMING |
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Sorted by downstream order |
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Adjustment Key: |
'G' Main gage, start with this |
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'-' subtract this gage |
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'+' add this gage |
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USGS ID |
Nominal station |
Computational Stations |
Adjustment |
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6186500 |
YELLOWSTONE RIVER AT YELLOWSTONE LAKE OUTLET, WY |
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6186500 |
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YELLOWSTONE R AT YELLOWSTONE LK OUTLET |
G |
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6259000 |
WIND RIVER BELOW BOYSEN RES, WY (Boysen Reservoir Inflow) |
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6259000 |
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WIND R BLW BOYSEN RES |
G |
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6224500 |
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BULL LAKE (STORAGE CHANGE) |
+ |
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6258900 |
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BOYSEN RESERVOIR (STORAGE CHANGE) |
+ |
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6279500 |
BIGHORN RIVER AT KANE, WY |
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6279500 |
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BIGHORN R AT KANE |
G |
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6224500 |
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BULL LAKE (STORAGE CHANGE) |
+ |
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6258900 |
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BOYSEN RESERVOIR (STORAGE CHANGE) |
+ |
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6282000 |
SHOSHONE RIVER BELOW BUFFALO BILL RES (Buff. Bill Inflow) |
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6282000 |
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SHOSHONE BLW BUFFALO BILL RES |
G |
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6281500 |
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BUFFALO BILL RESERVOIR (STORAGE CHANGE) |
+ |
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6281900 |
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HEART MTN CANAL DIVERSION |
+ |
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6298000 |
TONGUE RIVER NEAR DAYTON, WY |
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6298000 |
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TONGUE RIVER NR DAYTON |
G |
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6297500 |
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HIGHLINE DITCH |
+ |
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6306300 |
TONGUE R AT STATE LINE NR DECKER, MT (same site also listed under Montana) |
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6306300 |
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TONGUE R AT STATE LINE NR DECKER |
G |
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6297500 |
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HIGHLINE DITCH |
+ |
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