Frozen Ground Model

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Frozen Ground Model

• Cold Regions Workshop 2004
• Brian Connelly
• North Central River Forecast Center
• Chanhassen, MN

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The Frozen Ground Scenario

• Soils are wet as freeze-up begins
• There is little snow cover to insulate the soil
• Hard freeze occurs making the soil less permeable
  to subsequent snowmelt events
• Somewhat rare, but responsible for big floods

Mankato, Minnesota, 1965
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Modeling Frozen Ground Runoff
The frozen ground model reduces the percolation
and interflow rates.
From Center for Hydrometeorology and Remote
Sensing, University of California, Irvine
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How is index calculated

• In other words
• Frost Index FI 0C
• When Tair lt 0C then the FI decreases ?FI a
  (Tair FI)
• When Tair gt 0C then the FI increases ?FI a
  Tair
• Snow retards the change in the FI because it
  insulates the soil.
• As water enters the soil it increases the FI

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Percolation Interflow Reduction
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For example
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Baldhill Cr, North Dakota
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Forest River, North Dakota
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Ensemble Streamflow Prediction
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How well does it work?

• On large events it is quite sensitive to snowmelt
  volume.
• Since the Frost Index depends on air temperatures
  and simulated SWE, QC of these variables is
  extremely important.
• Sometimes it works well, sometimes not, but it
  almost always requires runtime modifications.
• Bottom line We need a frozen ground model to
  increase runoff ratios in the spring and this
  model does that.

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Final Thoughts

• The Frozen Ground Model allows us to increase
  runoff ratios in the spring.
• QC of temperatures and SWE is very important.
• The model is very sensitive to snowmelt volume
  Use model output and ESP results with care.
• Forecaster can use run-time modifications to
  affect results.
• It cannot simulate ice layers at the snow/soil
  interface (Variable Impervious Area mod?).