Frozen Ground Model Cold Regions Workshop 2004 Brian Connelly North Central River Forecast Center Chanhassen, MN
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
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
If Tair < 0°C then ΔFI = -C ∙ ( Tair2 + FI12)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 How is index calculated… • 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
Frozen Ground model off Frost Index adjusted to -25 C Frost Index = -50 C Baldhill Cr, North Dakota
Frozen Ground Model on Frozen Ground model off Forest River, North Dakota
Ensemble Streamflow Prediction Baldhill Creek near Dazey, ND Forecast Period: February - May 3500 3000 19% 2500 Control Baldhill Creek near Dazey, ND 2000 Forecast Period: February - May Peak Discharge (cfs) FGIX = -15 C -18% 1500 6000 -5% 106% -10% 1000 -10% 5000 -10% -14% -19% 92% 500 -6% 4000 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Control 105% Chance of Exceeding Discharges 3000 83% Peak Discharge (cfs) 55% FGIX = -25 C 2000 60% 51% 3% 1000 -22% 0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Chance of Exceeding Discharges
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.
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?).