1. Frozen Ground Model Cold Regions Workshop 2004 Brian Connelly North Central River Forecast Center Chanhassen, MN

2. The Frozen Ground Scenario Soils are wet as freeze-up begins Soils are wet as freeze-up begins There is little snow cover to insulate the soil There is little snow cover to insulate the soil Hard freeze occurs making the soil less permeable to subsequent snowmelt events Hard freeze occurs making the soil less permeable to subsequent snowmelt events Somewhat rare, but responsible for big floods Somewhat rare, but responsible for big floods Mankato, Minnesota, 1965

3. 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

4. How is index calculated… If T air < 0°C then ΔFI = -C ∙ ( T air 2 + FI 1 2 ) 1/2 - C ∙ FI 1 + H c If T air > 0°C then ΔFI = C ∙ T air + C thaw ∙ P + H c where C = C ground ∙ ( 1 – A snow ) + C ground ∙ A snow ∙ ( 1 – C snow ) W In other words… Frost Index = FI ≤ 0°C When T air < 0°C then the FI decreases ΔFI α (T air – FI) When T air > 0°C then the FI increases ΔFI α T air Snow retards the change in the FI because it insulates the soil. As water enters the soil it increases the FI

5. Percolation & Interflow Reduction

6. For example…

7. Frost Index adjusted to -25 C Frozen Ground model off Baldhill Cr, North Dakota Frost Index = -50 C

8. Frozen Ground model off Forest River, North Dakota Frozen Ground Model on

9. Ensemble Streamflow Prediction 500 1000 1500 2000 2500 3000 3500 Peak Discharge (cfs) 0.90.80.70.60.50.40.30.20.1 Chance of Exceeding Discharges -6% -19% -14% -10% -5% -18% 19% Control FGIX = -15 C Baldhill Creek near Dazey, ND Forecast Period: February - May 0 1000 2000 3000 4000 5000 6000 Peak Discharge (cfs) 0.90.80.70.60.50.40.30.20.1 Chance of Exceeding Discharges -22% 3% 51% 60% 55% 83% 105% 92% 106% Control FGIX = -25 C Baldhill Creek near Dazey, ND Forecast Period: February - May

10. How well does it work? On large events it is quite sensitive to snowmelt volume. 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. 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. 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. Bottom line: We need a frozen ground model to increase runoff ratios in the spring and this model does that.

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