1. WinTR-20 Computation Differences March 2009 1 WinTR-20 Project Formulation Hydrology Computer Program Will TR-20 and WinTR-20 give the same results ?? Presented by: WinTR-20 Development Team

2. WinTR-20 Computation Differences March 2009 2 WinTR-20 Computation Differences Runoff hydrographs Adding hydrographs Diverting or dividing hydrographs Structure Routing Reach Routing

3. WinTR-20 Computation Differences March 2009 3 Runoff Hydrographs With the same delta t, TR-20 and WinTR- 20 will produce equal hydrographs. However, the delta t is rarely the same. Delta t is set by the user in TR-20. Delta t is 6 % of the Tc in WinTR-20.

4. WinTR-20 Computation Differences March 2009 4 Runoff Hydrographs (continued) The minimum practical delta t in TR-20 is 0.1 hour. There is no practical limit to delta t in WinTR- 20 (delta t is set within WinTR-20). For example, if Tc is 0.1 hour then delta t will be 0.006 hour. This will cause peak discharges to be slightly higher (generally 5% or less) in WinTR-20 for small Tc’s.

5. WinTR-20 Computation Differences March 2009 5 Runoff Hydrographs (cont) The peak in TR-20 is determined based on a parabolic fit to the highest three consecutive hydrograph coordinates. This is not a good mathematical approach. It is better to make the delta t smaller instead. This may cause the TR-20 peak discharge to be significantly higher than the largest hydrograph coordinate for small Tc’s.

6. WinTR-20 Computation Differences March 2009 6 Runoff Hydrographs (cont) To see the internal delta t in WinTR-20, leave the Print Time Increment blank on the Global Output window.

7. WinTR-20 Computation Differences March 2009 7 Adding Hydrographs If two hydrographs with the same time interval are added, then TR-20 and WinTR- 20 will produce equal hydrographs. Again, the time intervals are rarely the same. WinTR-20 will combine hydrographs with different delta t’s.

8. WinTR-20 Computation Differences March 2009 8 Adding Hydrographs (cont) For example, let us add hydrographs A and B. Hydrograph A has a time interval of 0.1 hour and Hydrograph B has an interval of 0.33 hour. Hydrograph A is interpolated to a time interval of 0.33 hour and added to Hydrograph B. Hydrograph B is interpolated to an interval of 0.1 hour and added to Hydrograph A.

9. WinTR-20 Computation Differences March 2009 9 Adding Hydrographs (continued) Whichever hydrograph has the higher peak is saved for further operations downstream.

10. WinTR-20 Computation Differences March 2009 10 Diverting or dividing hydrographs Given the same cross section elevation- discharge data and inflow hydrograph at the same delta t, the TR-20 and WinTR-20 will produce equal hydrographs. Again, the time intervals are rarely the same.

11. WinTR-20 Computation Differences March 2009 11 Structure Routing For the same Structure elevation-discharge-storage data and inflow hydrograph, TR-20 and WinTR-20 will produce equal hydrographs.

12. WinTR-20 Computation Differences March 2009 12 Maximum number of Hydrograph Points The maximum number is 100,000. This allows WinTR-20 to save hydrographs at extremely short time intervals. Sometimes with structure routings with very low release rates, this 100,000 point limit can be exceeded.

13. WinTR-20 Computation Differences March 2009 13 Difference in Reach Routing The Modified Att-Kin method in TR-20 was replaced with the Muskingum-Cunge method. In general, the Muskingum-Cunge will give a higher peak discharge at the end of the reach when considering the same rating table, reach length, etc.

14. WinTR-20 Computation Differences March 2009 14 The End

15. WinTR-20 Computation Differences March 2009 15