DES 606: Watershed Modeling with HEC-HMS

1. DES 606: Watershed Modeling with HEC-HMS Historical Rainfall Theodore G. Cleveland, Ph.D., P.E. 6 OCT 11

2. Using Historical Rainfall • Mechanics of data entry have already been visited in earlier modules • Historical rainfall is generally non-uniform spaced data • DSS utilities exist to help enter such data, but are genuinely difficult to operate. • A viable alternative is to use the historical data to generate a uniformly spaced time series

3. Rainfall Data Preparation • Rainfall data is usually tabulated (either on paper or electronically) with date and time and the associated accumulated rainfall • Uniform in-time tabulations are uncommon: • convert a rainfall time-series where the time intervals are varied into uniformly spaced data • This derivative time-series can then be used in HEC-HMS as just another time series.

4. A Procedure • Obtain data from source(s) such as: NWS, HCOEM, USGS • Convert the date time format into a common unit; for example seconds since start of event. • Once data are in common time unit (not date-time), then the analyst can interpolate onto uniformly spaced time units.

5. Some Details • Convert the date time format into a common unit; for example seconds since start of event. • Be aware that gages reset and the analyst may have to manually deal with this issue. • Typically a user-written utility program will take data in the following two-column format and convert it into elapsed time since start of file (i.e. first date-time stamp in the file). • This procedure can be done in Excel, although for large files and/or many files Excel is not an ideal tool.

6. Some Details • Convert the date time format into a common unit; for example seconds since start of event. • Be aware that gages reset and the analyst may have to manually deal with this issue. • Typically a user-written utility program will take data in the following two-column format and convert it into elapsed time since start of file (i.e. first date-time stamp in the file). • This procedure can be done in Excel, although for large files and/or many files Excel is not an ideal tool.

7. Example • Consider the first HMS model presented. • In that model a “curve-fit” was used to interpolate the rainfall hyetograph onto uniform time intervals. • Later an interpolation approach was presented – in principle, that same spreadsheet can be used

8. Example • Consider a data file such as

9. Example • Some data are every 15 mintes, then to every 30

10. Example • Manual correction is a nuisance

11. Example • Lets modify the file, then pass through a program that fixes time and time steps

12. Example • Copy-Paste into Excel

13. Example • Keep the DATE_TIME and ACC_WTD_PRECIP

14. Example • Add a row to have time start at 00:00 on 5/20/1978

15. Example • Copy-Paste into a Text file for an interpolator program.

16. Example • Open a CMD window. • CD to the location of the file • Run the program “rain_elapsed_time.exe” • Note use of “redirection” program< input > output • The data of interest are now in file named “junk.txt”

17. Example • The data of interest are now in file named “junk.txt” • Can then put the time and depth into an interpolation spreadsheet

18. Example • Now the series can be put into HMS as just another time series, starting at the user specified date • Need to use calculator to determine 2580 minutes is 1 day + 19 hours.

19. Example • Now the series can be put into HMS as just another time series, starting at the user specified date

20. Example • Now the series can be put into HMS as just another time series, starting at the user specified date

21. Example • Now the series can be put into HMS as just another time series, starting at the user specified date

22. Example • Run the model (after changing gages)

23. Summary • Used program to uniformly space time series for HMS use • Could use the Excel sheet for interpolation, but still need the data converted into elapsed time. • Alternate approach is to use the R package

24. Summary • Historical storms are time-series entered as gages in HEC-HMS • Data need to be uniformly spaced in time • Can experiment with DSSIRIT utility • Prepare data externally to the program • Once the derivative time series is built, then remainder is same as other time series.