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.

 

The MBRFC uses two different techniques to forecast natural streamflows, statistical water supply forecasts and ensemble water supply forecasts.

 

STATISTICAL WATER SUPPLY FORECASTS

Statistical water supply forecasts (SWS) are produced using regression equations fit to available data. The period of record used most commonly starts with the availability of data from U. S. Department of Agriculture’s Natural Resources Conservation Service (NRCS) SNOTEL data (usually 1979) and extends at present to generally year 2000. These dates vary somewhat from site to site; in some cases, SNOTEL records can be extended by correlation with nearby snow course data, and in other cases equations have been updated using more recent data. Regular updating of equations to reflect new data occurs every ten years. The streamflow data used are adjusted to better approximate natural flows as described below (Table 1). Forecasts are issued on or before the fifth 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.

The SWS regression equations 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. 

 SWS forecasts are presented at three levels of probability as follows:

Most Probable Forecast. Given the current hydrometeorological conditions to date, this is the best estimate of what the actual runoff volume will be this season.

Reasonable Maximum Forecast. Given current hydrometeorological conditions, the seasonal runoff that has a ten (10) percent chance of being exceeded.

Reasonable Minimum Forecast. Given current hydrometeorological conditions, the seasonal runoff that has a ninety (90) percent chance of being exceeded.

The MBRFC coordinates its SWS forecasts with the NRCS. By agreement, the NRCS is the final authority for published SWS forecasts for most Missouri headwater basins, while the MBRFC is the final authority for relatively fewer, mostly downstream SWS forecast points. In most cases, almost all of the flow at the downstream points is generated from the NRCS’s headwater basins, and headwater forecasts drive the SWS equations that generate forecasts for the downstream locations.  Where MBRFC has the authority to determine the forecast flow, most of our forecasts are created using the NRCS’s upstream flow forecasts as the independent variables in regression equations. Second, the regression equations used to produce MBRFC’s downstream forecasts are fit to observed, historical upstream flows as the independent variables. These equations are then used to forecast downstream flow using forecast, rather than observed, upstream flow as the independent variable. Probable high and probable low flow ranges are calculated using regionalized standard errors from headwater basin data.

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 1979 through (currently) 2003 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.

 
       
       

Table 1: Streamflow Adjustments for Statistical Water Supply

by State
 
       
MONTANA
       
Sorted by downstream order
 
              
     
 
Adjustment Key:
 'G'    Main gage, start with this
 
   
 '-'     subtract this gage
 
   
 '+'     add this gage
 
       
     
USGS ID
Nominal station
Computational Stations
Adjustment
       
5017500
ST MARY R NR BABB, MT                         
 
5017500
 
ST MARY R NR BABB                        
G
6015500
 
SHERBURNE LAKE (STORAGE CHANGE)          
+
       
6012500
RED ROCK RIVER NEAR MONIDA, MT (Lima Reservoir Inflow )
 
6012500
 
RED ROCK R NR MONIDA                     
G
6012000
 
LIMA RESERVOIR (STORAGE CHANGE)          
+
       
6015400
BEAVERHEAD R NR GRANT, MT (Clark Canyon Reservoir Inflow)
 
6015400
 
BEAVERHEAD R NR GRANT
G
6012000
 
LIMA RESERVOIR (STORAGE CHANGE)          
+
       
6016000
BEAVERHEAD RIVER NEAR BARRATTS, MT              
 
6016000
 
BEAVERHEAD R NR BARRATTS                 
G
6012000
 
LIMA RESERVOIR (STORAGE CHANGE)          
+
6015300
 
CLARK CANYON RESERVOIR (STORAGE CHANGE)  
+
       
6019500
RUBY R ABV RESV NEAR ALDER, MT (Ruby Reservoir Inflow)
 
6019500
 
RUBY R ABV RESV NR ALDER                 
G
       
6025500
BIG HOLE R NR MELROSE, MT                   
 
6025500
 
BIG HOLE R NR MELROSE                    
G
       
6038500
MADISON RIVER NEAR GRAYLING, MT (Hebgen Reservoir Inflow) 
6038500
 
MADISON R NR GRAYLING
G
6038000
 
HEBGEN LAKE (STORAGE CHANGE)             
+
       
6041000

MADISON RIVER NEAR McALLISTER, MT (Ennis Reservoir Inflow)

6041000
 
MADISON R NR McALLISTER
G
6038000
 
HEBGEN LAKE (STORAGE CHANGE)             
+
6040500
 
ENNIS LAKE (STORAGE CHANGE)              
+
       
6043500
GALLATIN RIVER NEAR GALLATIN GATEWAY, MT
 
6043500
 
GALLATIN R NR GALLATIN GATEWAY           
G
       
6052500
GALLATIN RIVER AT LOGAN, MT
 
6052500
 
GALLATIN R AT LOGAN                      
G
       
6054500
MISSOURI RIVER AT TOSTON, MT
 
6054500
 
MISSOURI R AT TOSTON                     
G
6015300
 
CLARK CANYON RESERVOIR (STORAGE CHANGE)  
+
6038000
 
HEBGEN LAKE (STORAGE CHANGE)             
+
6040500
 
ENNIS LAKE (STORAGE CHANGE)              
+
       
6077000
SHEEP CREEK NEAR WHITE SULPHUR SPRINGS, MT         
 
6077000
 
SHEEP CK NR WHITE SULPHUR SPRINGS        
G
       
6078600
SUN RIVER AT GIBSON DAM NR AUGUSTA, MT (Gibson Reservoir Inflow)
6078600
 
SUN R AT GIBSON DAM NR AUGUSTA           
G
       
6090800
MISSOURI RIVER AT FORT BENTON, MT
 
6090800
 
MISSOURI R AT FORT BENTON                
G
6058500
 
CANYON FERRY LAKE (STORAGE CHANGE)       
+
       
6099500
MARIAS RIVER NEAR SHELBY, MT
 
6099500
 
MARIAS R NR SHELBY                       
G
6090900
 
LOWER TWO MEDICINE LAKE (STORAGE CHANGE) 
+
6093000
 
FOUR HORNS LAKE (STORAGE CHANGE)         
+
6094000
 
SWIFT RESERVOIR (STORAGE CHANGE)         
+
6095500
 
LAKE FRANCES (STORAGE CHANGE)            
+
       
 
 
 
 
6109500
MISSOURI RIVER AT VIRGELLE, MT
 
6109500
 
MISSOURI RIVER AT VIRGELLE, MT
G
6058500
 
CANYON FERRY LAKE (STORAGE CHANGE)       
6101300
 
LAKE ELWELL (TIBER) (STORAGE CHANGE)     
       
6115200
MISSOURI RIVER NEAR LANDUSKY, MT
 
6115200
 
MISSOURI R NR LANDUSKY                   
G
6058500
 
CANYON FERRY LAKE (STORAGE CHANGE)       
+
6101300
 
LAKE ELWELL (TIBER) (STORAGE CHANGE)     
+
       
6120500
MUSSELSHELL RIVER AT HARLOWTON                
 
6120500
 
MUSSELSHELL R AT HARLOWTON               
G
6116500
 
BAIR RESERVOIR (STORAGE CHANGE)          
+
6119000
 
MARTINSDALE RESERVOIR (STORAGE CHANGE)   
+
       
6132000
MISSOURI RIVER BELOW FT PECK DAM (Ft Peck Reservoir Inflow)
6132000
 
MISSOURI R BLW FT PECK DAM               
G
6058500
 
CANYON FERRY LAKE (STORAGE CHANGE)       
+
6131500
 
FORT PECK LAKE (STORAGE CHANGE)          
+
6131501
 
FORT PECK LAKE EVAPORATION               
+
       
6133000
MILK R AT WESTERN CROSSING                
 
6133000
 
MILK R AT WESTERN CROSSING               
G
       
6134500
MILK RIVER AT MILK RIVER, ALBERTA, CANADA
 
6134500
 
MILK RIVER AT MILK RIVER, ALBERTA, CANADA (11AA005)
G
5018500
 
ST MARY CANAL AT ST MARY XING NR BABB    
-
       
6135100
MILK RIVER AT EASTERN CROSSING - ADJUSTED
 
6135000
 
MILK R AT EASTERN CROSSING (11AA031)             
G
5018500
 
ST MARY CANAL AT ST MARY XING NR BABB    
-
       
 
YELLOWSTONE LAKE OUTLET (SEE WYOMING, BELOW)
 
 
 
 
 
6191500
YELLOWSTONE RIVER AT CORWIN SPRINGS, MT
 
6191500
 
YELLOWSTONE R AT CORWIN SPRINGS          
G
       
6192500
YELLOWSTONE AT LIVINGSTON                 
 
6192500
 
YELLOWSTONE AT LIVINGSTON                
G
       
6200000
BOULDER R AT BIG TIMBER                   
 
6200000
 
BOULDER R AT BIG TIMBER                  
G
       
6205000
STILLWATER RIVER NEAR ABSAROKEE, MT
 
6205000
 
STILLWATER R NR ABSAROKEE                
G
6204000
 
MYSTIC LAKE (STORAGE CHANGE)             
+
       
6207500
CLARKS FORK YELLOWSTONE RIVER NR BELFRY, MT
 
6207500
 
CLARKS FK RIVER NR BELFRY                
G
       
6214500
YELLOWSTONE R AT BILLINGS                 
 
6214500
 
YELLOWSTONE R AT BILLINGS                
G
       
6287000
BIGHORN RIVER NEAR ST XAVIER, MT
 
6287000
 
BIGHORN R NR ST XAVIER                   
G
6224500
 
BULL LAKE (STORAGE CHANGE)               
+
6258900
 
BOYSEN RESERVOIR (STORAGE CHANGE)        
+
6281500
 
BUFFALO BILL RESERVOIR (STORAGE CHANGE)  
+
6286400
 
BIGHORN LAKE (STORAGE CHANGE)            
+
       
6294000
LITTLE BIGHORN RIVER NEAR HARDIN, MT
 
6294000
 
LITTLE BIGHORN R NR HARDIN               
G
       
6306300

TONGUE R AT STATE LINE NR DECKER, MT (same site listed under Wyoming)

6306300
 
TONGUE R AT STATE LINE NR DECKER         
G
6297500
 
HIGHLINE DITCH                           
+
       
6309000
YELLOWSTONE RIVER AT MILES CITY, MT               
 
6309000
 
YELLOWSTONE R AT MILES CITY              
G
6258900
 
BOYSEN RESERVOIR (STORAGE CHANGE)        
+
6281500
 
BUFFALO BILL RESERVOIR (STORAGE CHANGE)  
+
6286400
 
BIGHORN LAKE (STORAGE CHANGE)            
+
6307000
 
TONGUE RIVER RESERVOIR (STORAGE CHANGE)  
+
       
6324500
POWDER RIVER AT MOORHEAD, MT
 
6324500
 
POWDER R AT MOORHEAD                     
G
       
6326500
POWDER RIVER NEAR LOCATE, MT
 
6326500
 
POWDER R NR LOCATE                       
G
       
6329500
YELLOWSTONE RIVER NEAR SIDNEY, MT
 
6329500
 
YELLOWSTONE R NR SIDNEY                  
G
6258900
 
BOYSEN RESERVOIR (STORAGE CHANGE)        
+
6281500
 
BUFFALO BILL RESERVOIR (STORAGE CHANGE)  
+
6286400
 
BIGHORN LAKE (STORAGE CHANGE)            
+
       
6338490
MISSOURI RIVER AT GARRISON DAM, ND (Lake Sakakawea Inflow)
6338490
 
MISSOURI RIVER AT GARRISON DAM, ND
G
6058500
 
CANYON FERRY LAKE (STORAGE CHANGE)       
+
6131500
 
FORT PECK LAKE (STORAGE CHANGE)          
+
6131501
 
FORT PECK LAKE EVAPORATION               
+
6258900
 
BOYSEN RESERVOIR (STORAGE CHANGE)        
+
6281500
 
BUFFALO BILL RESERVOIR (STORAGE CHANGE)  
+
6286400
 
BIGHORN LAKE (STORAGE CHANGE)            
+
       
       
       
       

Table 1 (continued): Streamflow Adjustments for Statistical Water Supply by State

       
WYOMING
       
Sorted by downstream order
 
              
     
 
Adjustment Key:
 'G'    Main gage, start with this
 
   
 '-'     subtract this gage
 
   
 '+'     add this gage
 
       
     
USGS ID
Nominal station
Computational Stations
Adjustment
6186500
YELLOWSTONE RIVER AT YELLOWSTONE LAKE OUTLET, WY
 
6186500
 
YELLOWSTONE R AT YELLOWSTONE LK OUTLET   
G
       
6259000
WIND RIVER BELOW BOYSEN RES, WY (Boysen Reservoir Inflow)
6259000
 
WIND R BLW BOYSEN RES
G
6224500
 
BULL LAKE (STORAGE CHANGE)               
+
6258900
 
BOYSEN RESERVOIR (STORAGE CHANGE)        
+
       
6279500
BIGHORN RIVER AT KANE, WY
 
6279500
 
BIGHORN R AT KANE                        
G
6224500
 
BULL LAKE (STORAGE CHANGE)               
+
6258900
 
BOYSEN RESERVOIR (STORAGE CHANGE)        
+
       
6282000
SHOSHONE RIVER BELOW BUFFALO BILL RES (Buff. Bill Inflow)
 
6282000
 
SHOSHONE BLW BUFFALO BILL RES
G
6281500
 
BUFFALO BILL RESERVOIR (STORAGE CHANGE)  
+
6281900
 
HEART MTN CANAL DIVERSION                
+
       
6298000
TONGUE RIVER NEAR DAYTON, WY
 
6298000
 
TONGUE RIVER NR DAYTON                   
G
6297500
 
HIGHLINE DITCH                           
+
       
6306300

TONGUE R AT STATE LINE NR DECKER, MT (same site also listed under Montana)

6306300
 
TONGUE R AT STATE LINE NR DECKER         
G
6297500
 
HIGHLINE DITCH                           
+
       
           
 

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