Ventura County Design Hydrology and VCRat Design Program


Design Hydrology Methodology

New: The 2017 Hydrology Manual Update and the VCRat 2.64 are now available below.


Please see the VCWPD Hydrology Manual (downloadable below) for information on our design hydrology methods.  In general, peak flows from urban and small undeveloped watersheds for use in channel design can be evaluated using our modified rational method and programs (VCRat2.6, TC Calculator- available on this website, including training materials).  Peak flows and hydrographs for regional facility design can be evaluated using the calibrated S-Graph developed from 2005 storm data.  Other peak flow design methods that have been used to evaluate floodplains include the results from flow frequency analyses (for gaged watersheds) and inserting design storm rainfall in continuous models (HSPF).  For information on these advanced methods, please contact the Hydrology Section at (805) 654-2069 or email your request to

VCRat Program Downloads


Notice: VCRat 2.64 Program UPDATE (BETA)!!!

On June 30, 2017, the design hydrology VCRat program was updated to version 2.64.  The new version runs only on 64-bit computers and contains new soild and rain zone curves.  The program can still run legacy models using the older soil and rain zone curves. This program does not run on older 32-bit computers.


WARNING: VCRat 2.64 was tested successfully on a computers with the Windows 7 and Windows 10 64-bit Operating System. VCRat 2.64 and the TC Calculator is still in development. VCRat 2.64 WILL NOT run on 32-bit computers.

VCRat 2.64 ZipVCRat 2.64 Beta (ZIP, 13.4 MB)

Older VCRat 2.6 Program

On December 2, 2008, the design hydrology program VCRat2.5 was updated to version 2.6.  The Tc Calculator program was also updated at the same time. You can download and install the updated programs following the instructions provided below.



VCRat 2.6 was tested successfully on a computer with the Windows Vista Operating System. VCRat 2.6 and the TC Calculator have not been verified to run on all versions and configurations of the Vista Operating System. We have also received reports that the programs WILL NOT run on 64-bit computers as of December 2010.


VCWPD has developed the Hydrology Manual and the Hydrology Manual Appendix E files as a resource for computing design hydrology. Although it is anticipated that the overall approach described in this manual will not significantly change, any hydrologic process should be considered a dynamic process and subject to periodic refinement as additional input data become available. Techniques described in this manual will be continuously reviewed and updated, as necessary, to provide a thorough and complete package for hydrologic design. The VCRat 2.6.2008.11 and Tc Calculator Programs are available for download as of December 2, 2008. You may also need the Tc Calculator Manual and the VCRat 2.6 and Tc Calculator Training Materials to help you in understanding the applications.


Tc Calculator Manual (PDF, 579 KB)Tc Calculator Manual (PDF, 579 KB)
2006 Hydrology Manual (PDF, 10.8 MB superseded by 2010 Manual Below)VCRat 2.6 and Tc Calculator Programs (ZIP, 35.3 MB)
VCRat2.6 and Tc Calculator Training Materials (zip, 3.5 mb)VCRat2.6 and Tc Calculator Training Materials (zip, 2.8 mb)
Training materials for VCRat2.6 and Tc Calculator Programs developed for 2009 Training Classes.