In meeting its hydrologic forecast responsibilities, the National
Weather Service (NWS) Missouri Basin River Forecast Center (MBRFC)
simulates soil moisture across the Missouri River basin in real-time.
The Modeled Saturation Index (MSI) quantifies the relative degree of
tension water saturation present within the soil column, and is
calculated on a sub-basin level. The above graphic displays the MSI
across the Missouri River basin. *"Click here to read more...."*

(The following info would be what the user would see if they clicked on
the "read more" button.)

**EXPLANATION OF THE MSI GRAPHIC**

The National Weather Service River Forecast System (NWSRFS) river
forecast model is the primary tool used by MBRFC to produce river stage
and river flow forecasts. The precipitation-runoff component of the
river forecast model is the Sacramento Soil Moisture Accounting Model
(SAC-SMA). The SAC-SMA conceptually simulates the physical mechanisms
driving water movement through the soil column. The ability of the soil
to absorb and hold water is one such physical mechanism. "Tension water"
is that volume of water which is absorbed and held by the soil column.
Tension water is not available to produce immediate runoff from a
precipitation event, or to produce longer-term base flow from water
which has percolated downward in the soil column. Tension water deficits
within the soil column are replenished by free water, such as is made
available during precipitation events. The larger the tension water
deficit, the greater the reduction of free water available for runoff or
base flow. The modeled tension water within the soil column is
represented by two portions, an upper portion which represents the
amount of precipitation which has been absorbed and held (not available
for runoff), and a lower portion which represents the amount of
percolated water which has been absorbed and held (not available for
base flow). The MSI is a mathematical representation of the degree to
which tension water is present in the entire soil column, and is
calculated by the following equation:

MSI = [TW~upper~ + TW~lower~]{.underline}

TW~up\ cap~ + TW~low\ cap~

where: TW~upper~ is the volume of upper portion tension water

TW~lower~ is the volume of lower portion tension water

> TW~up\ cap~ is the capacity available for upper portion tension water
>
> TW~low\ cap~ is the capacity available for lower portion tension water

The 530,000 square mile Missouri River basin has been subdivided into
approximately 1200 watersheds. These 1200 watersheds vary in size from
about 30 square miles to over 8500 square miles, with the majority of
basins being a few hundred square miles in size. The above computation
is performed on an individual watershed basis to produce the MSI
graphic. The MSI graphic is a geographic display of the individual MSI's
as viewed across the Missouri River basin. The MBRFC updates the MSI
daily.

Tension water is only one of the several soil moisture types which the
SAC-SMA models in simulating the natural precipitation-runoff process.
These various soil moisture types and the mechanisms which determine
their quantities interrelate and influence one another in a very complex
manner. Viewed alone, the degree to which tension water is present in
the soil column (the "MSI") does not give the complete picture of the
hydrologic state of a watershed. While not conclusive, the MSI can give
the user a sound indication of the potential hydrologic response of a
watershed to a precipitation event, or its ability to sustain current
streamflow conditions.