Summary

Hydrologic modeling is far from a perfect science and really is as much art as it is science.  Whether you are using regression equations based on years of measured data, or a deterministic model like HEC-1/HMS there will be discrepancies in computed results because of the uncertainty of the input parameters.  However, if you are careful and understand how the hydrologic models function you should be able to develop confidence in your results and be able to successfully apply them to the design of hydraulic structures.  Where possible it is good practice to use more than one method or model and compare in order to gain a greater appreciation of the behavior of the watershed or drainage being modeled.

Hopefully this short lesson has adequately reviewed the basic principles in typical hydrologic models, and specifically the Army Corps of Engineers' HEC-1/HMS models.  A primary intent of this lesson is to review basic hydrology prior to attending a short course on the use of the Watershed Modeling System (WMS), an advanced hydrologic modeling graphical user interface that uses GIS data like digital elevations, land use, and soils to build and run hydrologic models.  If you still feel lost and unsure about hydrologic modeling you may wish to review a textbook that covers these topics in more detail and/or attend one of many training courses, such the National Highway Institute's course "number" on Introduction to Hydrology.