WinTR-20 SensitivityMarch WinTR-20 Sensitivity to Input Parameters

WinTR-20 SensitivityMarch Lesson Objectives 1. Identify the various WinTR-20 Input Parameters that affect the volume of runoff and peak discharge predictions. 2. Identify the relative sensitivity of WinTR-20 to its input parameters in predicting the peak and/or volume of runoff. 3. Identify the relative sensitivity of WinTR- 20 to its input parameters in relation to channel routing.

WinTR-20 SensitivityMarch WinTR-20 Hydrology Model Predicts Volume of Runoff Predicts Peak Rate of Runoff Predicts Entire Hydrograph of Runoff Based on Watershed and Rainfall Characteristics Modeled as Input Parameters Changes to Input Parameters Will Change the Volume and Rate of Runoff Predicted

WinTR-20 SensitivityMarch WinTR-20 Watershed Input Variables Drainage Area Runoff Curve Number (RCN) Time of Concentration (t c ) Unit Peak Factor (UPF) of Dimensionless Unit Hydrograph (DUH) Antecedent Runoff Condition (ARC)

WinTR-20 SensitivityMarch WinTR-20 Rainfall Input Variables Depth of Rainfall Rainfall Distribution (includes duration)

WinTR-20 SensitivityMarch Effects of Variation in Drainage Area % Change in DA results in comparable change to predicted volume and peak of runoff. Be sure DA is being properly identified (be aware of non-contributing areas).

WinTR-20 SensitivityMarch Effects of Variation in RCN % Change in RCN results in exaggerated change to predicted volume and peak of runoff. RCN can be influenced by stage of vegetal growth and/or antecedent rainfall at time of storm event.

WinTR-20 SensitivityMarch Effects of Variation in t c % Change in t c results in decreased change to predicted peak rate of runoff (no change in volume). A decrease in t c results in an increase in predicted peak discharge.

WinTR-20 SensitivityMarch Effects of Variation in Unit Peak Factor % Change in UPF results in nearly similar change to predicted peak rate of runoff (no change in volume). UPF is a watershed based response to excess rainfall assumed to be similar per inch of runoff.

WinTR-20 SensitivityMarch Effects of Variation in Antecedent Runoff Condition (ARC) ARC values of 1 or 3 alter the RCN selected for assumed ARC 2 conditions. ARC 2 is normally assumed for design. ARC 1 can be used to help calibrate for a known “drought” condition prior to the target storm event (not necessarily accurate). ARC 3 can be used to help calibrate for a known “saturated soil” condition prior to the target storm event (not necessarily accurate).

WinTR-20 SensitivityMarch ARC Adjustments (Continued) For this example: DA = 1.0 mi 2, t c = 1 hr, RCN = 70, 4.0 inch 24 hr Type II Rainfall ARC 2 – (RCN 70), Q v = 1.33”, Q p = 437 cfs ARC 1 – (RCN 51), Q v = 0.37”, Q p = 65 cfs ARC 3 – (RCN 85), Q v = 2.46”, Q p = 874 cfs WinTR-20 results are very sensitive to changes in ARC. Be sure that assumed change is appropriate or alter RCN within ARC 2 conditions for finer adjustment.

WinTR-20 SensitivityMarch Effects of Variation in Rainfall Depth % Change in Rainfall Depth results in exaggerated change to predicted volume and peak of runoff. Be sure that the actual Rainfall that has occurred and is being calibrated to is properly identified for the entire watershed.

WinTR-20 SensitivityMarch Effects of Variation in Rainfall Distribution Design rainfall distributions normally set by criteria (e.g. Type I, IA, II, or III). Can attempt to calibrate to a historical rainfall event of known varying intensity (recording rain gage). Rainfall distribution alone (not depth) only effects the rate of runoff, not the volume.

WinTR-20 SensitivityMarch Effects of Variation in Rainfall Distribution (Continued) For this example: DA = 1.0 mi 2, t c = 1 hr, RCN = 70, 4.0 inch 24 hr Type II Rainfall Type II - Q p = 437 cfs Type I - Q p = 221 cfs Type IA - Q p = 106 cfs Type III - Q p = 383 cfs WinTR-20 peaks are very sensitive to selection of rainfall distribution. Calibrate with the best known rainfall distribution.

WinTR-20 SensitivityMarch Parameter Selection for Desired Change in WinTR-20 Runoff Volume WinTR-20 Parameter to be Changed, Independent of Others Desired Change in Runoff Volume (%) -50%-25%-10%-5%+5%+10%+25%+50% Required Change in Drainage Area-50%-25%-10%-5%+5%+10%+25%+50% Required Change in Rainfall-26%-13%-5%-2.5%+2.5%+5%+12.5%+23% Required Change in RCN-17%-8%-2%-1%+1%+2%+7%+13% Required Change in Time of ConcentrationN/C Required Change in Unit Peak FactorN/C N/C signifies, No Change possible to alter volume. This parameter does not effect volume prediction.

WinTR-20 SensitivityMarch Parameter Selection for Desired Change in WinTR-20 Peak Runoff WinTR-20 Parameter to be Changed, Independent of Others Desired Change in Runoff Peak (%) -50%-25%-10%-5%+5%+10%+25%+50% Required Change in Drainage Area-50%-25%-10%-5%+5%+10%+25%+50% Required Change in Rainfall-24%-12%-5%-2.50%+2.5%+5%+11%+21% Required Change in RCN-13.5%-6%-2%-1%+1%+2%+5.5%+11% Required Change in Time of Concentration+150%+50%+15%+7%-6%-12%-26.5%-44% Required Change in Unit Peak Factor-54%-29%-12%-6%+6%+13%+33%+72%

WinTR-20 SensitivityMarch Combined Parameter Impacts Assumed Normal Run DA = 1 mi 2, RCN =70, t c = 1.0 hr, UPF = 484 Runoff Volume = 1.33”, Peak Rate = 437 cfs Low Run DA = 1 mi 2, RCN =63, t c = 1.25 hr, UPF = 300 Runoff Volume = 0.92”, Peak Rate = 148 cfs High Run DA = 1 mi 2, RCN =77, t c = 0.75 hr, UPF = 600 Runoff Volume = 1.81”, Peak Rate = 904 cfs

WinTR-20 SensitivityMarch WinTR-20 Channel Routing Model Predicts hydrograph (including peak) at downstream end of reach. Based on cross section and reach characteristics modeled as input parameters. Changes to input parameters will change the peak discharge and hydrograph shape predicted at the end of the reach.

WinTR-20 SensitivityMarch WinTR-20 Channel and Reach Input Variables Selection of representative cross section Cross section rating table (slope and “n”) Channel length Flood plain length Shape of inflow hydrograph Base flow (if significant)

WinTR-20 SensitivityMarch WinTR-20 Channel Routing Sensitivity Test Trapezoidal cross section, BW = 15, SS = 2:1 Slope = and Manning n = 0.03, 0.04, 0.05 Channel length, 0.8 to 1.2 mile Inflow hydrograph, DA = 1, CN = 80, Tc = 0.5 and 1.0, RF = 3.2 inches, Type II storm Base flow = WinTR-20 runs

WinTR-20 SensitivityMarch Two Inflow hydrographs Red (higher) is the hydrograph for Tc = 0.5 hour. Green (lower) is the hydrograph for Tc = 1.0 hour.

WinTR-20 SensitivityMarch Effects of Variation in Length and “n” % Change in length results in less change to predicted peak outflow. % Change in Manning “n” results in less change to predicted peak outflow.

WinTR-20 SensitivityMarch Effects of Variation in Length and “n” % Change in length and “n” results in less change to predicted peak outflow. Length and “n” less sensitive for Tc = 1.0 hydrograph.

WinTR-20 SensitivityMarch Effects of Variation in Length and “n” % Change in length and “n” results in less change to predicted peak outflow. Results for steep slope are less sensitive.

WinTR-20 SensitivityMarch Effects of Variation in Length and “n” % Change in length and “n” results in less change to predicted peak outflow. Results for Tc = 1.0 hydrograph are even less.

WinTR-20 SensitivityMarch Porcupine Mountains State Park, Michigan

WinTR-20 SensitivityMarch Questions???

WinTR-20 SensitivityMarch The End