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Frozen Ground Frozen Ground ModelModel
Cold Regions Workshop 2004Cold Regions Workshop 2004Brian ConnellyBrian Connelly
North Central River Forecast CenterNorth Central River Forecast CenterChanhassen, MNChanhassen, MN
The Frozen Ground The Frozen Ground ScenarioScenario
Soils are wet as Soils are wet as freeze-up beginsfreeze-up begins
There is little snow There is little snow cover to insulate the cover to insulate the soilsoil
Hard freeze occurs Hard freeze occurs making the soil less making the soil less permeable to permeable to subsequent snowmelt subsequent snowmelt eventsevents
Somewhat rare, but Somewhat rare, but responsible for big responsible for big floodsfloods
Mankato, Minnesota, 1965
Modeling Frozen Ground Modeling Frozen Ground RunoffRunoff
The frozen ground model reduces the percolation and interflow rates.
From Center for Hydrometeorology and Remote Sensing, University of California, Irvine
How is index calculated…How is index calculated…
If Tair < 0°C then ΔFI = -C ∙ ( Tair2 + FI1
2)1/2 - C ∙ FI1 + Hc
If Tair > 0°C then ΔFI = C ∙ Tair + Cthaw ∙ P + Hc
where C = Cground ∙ ( 1 – Asnow ) + Cground ∙ Asnow ∙ ( 1 – Csnow )W
In other words…• Frost Index = FI ≤ 0°C• When Tair < 0°C then the FI decreases ΔFI α (Tair – FI)• When Tair > 0°C then the FI increases ΔFI α Tair
• Snow retards the change in the FI because it insulates the soil.• As water enters the soil it increases the FI
Percolation & Interflow Percolation & Interflow ReductionReduction
0.0
0.2
0.4
0.6
0.8
1.0
0 0.2 0.4 0.6 0.8 1
Soil Moisture (0.0=wet; 1.0=dry)
Re
du
ctio
n in
Pe
rco
latio
n &
Inte
rflo
wFI = -5 C FI = -10 C FI = -15 C FI = -25 C
For example…For example…
Frost Index adjusted to -25 CFrozen Ground model off
Baldhill Cr, North Baldhill Cr, North DakotaDakota
Frost Index = -50 C
Frozen Ground model off
Forest River, North Forest River, North DakotaDakota
Frozen Ground Model on
Ensemble Streamflow PredictionEnsemble Streamflow Prediction
500
1000
1500
2000
2500
3000
3500
Pe
ak
Dis
ch
arg
e (c
fs)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1Chance of Exceeding Discharges
-6%-19%-14%-10%-10%
-10%-5%
-18%
19%
Control
FGIX = -15 C
Baldhill Creek near Dazey, NDForecast Period: February - May
0
1000
2000
3000
4000
5000
6000
Pe
ak
Dis
ch
arg
e (c
fs)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1Chance of Exceeding Discharges
-22%3%
51%60%55%
83%105%
92%
106%
Control
FGIX = -25 C
Baldhill Creek near Dazey, NDForecast Period: February - May
How well does it work?How well does it work? On large events it is quite sensitive to On large events it is quite sensitive to
snowmelt volume.snowmelt volume. Since the Frost Index depends on air Since the Frost Index depends on air
temperatures and simulated SWE, QC of temperatures and simulated SWE, QC of these variables is these variables is extremelyextremely important. important.
Sometimes it works well, sometimes not, Sometimes it works well, sometimes not, but it almost always requires runtime but it almost always requires runtime modifications.modifications.
Bottom line: We need a frozen ground Bottom line: We need a frozen ground model to increase runoff ratios in the model to increase runoff ratios in the spring and this model does that.spring and this model does that.
Final Thoughts…Final Thoughts… The Frozen Ground Model allows us to The Frozen Ground Model allows us to
increase runoff ratios in the spring.increase runoff ratios in the spring. QC of temperatures and SWE is very QC of temperatures and SWE is very
important.important. The model is very sensitive to snowmelt The model is very sensitive to snowmelt
volume; Use model output and ESP results volume; Use model output and ESP results with care.with care.
Forecaster can use run-time modifications Forecaster can use run-time modifications to affect results.to affect results.
It It cannotcannot simulate ice layers at the snow/soil simulate ice layers at the snow/soil interface (interface (Variable Impervious Area modVariable Impervious Area mod?).?).
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