Rainfall-Runoff Modeling

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"Rainfall-runoff modeling can be used to simulate the processes involved with converting rainfall and/or snowmelt into a hydrograph representing the upper level or maximum flood runoff that a drainage basin might reasonably be expected to produce. The unit hydrograph approach is the basic tool or “model” to convert rainfall to runoff after abstracting suitable infiltration losses. However, it should be noted that there are a number of other techniques available for making this conversion, including highly complex computerized watershed models."

"In 1932, Leroy K. Sherman initially proposed the unit hydrograph approach to convert rainfall occurring over a drainage basin to flood runoff from that basin. Sherman’s approach, which was formally presented in the April 7, 1932, issue of Engineering News Record, has undergone considerable refinement over the years. The advent of high speed electronic computers has led a number of hydrologists to devise approaches using complex watershed models, as alternatives to the unit hydrograph model, to predict a drainage basin’s runoff response to rainfall. Many of these watershed models are an appropriate basis for simulating a continuous series of runoff responses to normal precipitation events. However, in the Bureau of Reclamation’s application, the primary interest is in simulating a basin’s runoff response to extreme rainfall events. Because these complex watershed models generally require extensive calibration to adequately represent a drainage basin’s physical properties, considerable-effort must be expended in the field and office in acquisition of data relative to these properties. In the final analysis, the relative “goodness” of an approach is measured by how well that approach reproduces actual recorded flood events. Comparative studies have indicated that both approaches are able to satisfactorily reproduce these events with neither one being notably superior to the other. Accordingly, the Bureau has, over the years, retained the unit hydrograph approach because of its simplicity, reliability, and the relatively low costs associated with its application to flood hydrology studies."

Required Data

• Watershed Delineation
• Precipitation Depth
• Precipitation Temporal Distribution
• Precipitation Spatial Distribution
• Rainfall Losses
• Unit Hydrograph
• Reach Routing

Best Practices Resources

National Engineering Handbook: Chapter 4 - Storm Rainfall Depth and Distribution, NRCS

Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 8- Determination of the Probable Maximum Flood, FERC

Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420), USACE

Flood Hydrology Manual, USBR

Trainings

On-Demand Webinar: Introduction to Hydrologic Modeling Using Geospatial Information

Citations:

Revision ID: 7351
Revision Date: 07/21/2023