Precipitation Depth
"In watershed studies, it is often necessary to know the average depth of rainfall over an area.”[1]
"Rainfall data and related statistical analyses used to design NRCS engineering measures are generally those amounts measured and published by the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS). The choice of NWS data is due to its availability, length of record, and consistency on a national basis. Numerous other organizations publish data, research reports, and analyses. Use of data from other sources is justified if the data are more recent or more applicable to a specific project purpose, location, or both. Rainfall data sources should be documented and justification for use of non-NWS data should be provided." [1] For dam designs, it is common to analyze the Probable Maximum Precipitation (PMP) event as well as frequency floods. PMP data was historically developed by NOAA as documented in Hydrometeorological Reports (HMR). Many of these studies are dated and have been superseded by site specific or statewide PMP studies.
"The average depth can be determined in various ways, depending on the kind of data used. If the rainfall amount is taken from one of the NWS documents, it is for a specific point and the point-area relationship given in the paper is used to estimate the average depth over the area. It is difficult to obtain an average depth from data of several rain gages because the number and locations of gages and storm variability influence the results. Manual and geospatial methods of using such data are given in this section. The choice of methods depends to some extent on what data are available and where data are available and to some extent on the background and preference of the user."[1] Methods include using a single gage or point value to represent the entire watershed or estimating average rainfall using isohyets or Thiessen polygons.
Best Practices Resources
National Engineering Handbook: Chapter 4 - Storm Rainfall Depth and Distribution, NRCS
Trainings
On-Demand Webinar: PMP and the History of HMRs
On-Demand Webinar: Extreme Precipitation and Dam Safety in a Changing Climate
On-Demand Webinar: Rainfall and Dam Safety-From PMP to the 100-Year Storm
Citations:
Revision ID: 7349
Revision Date: 07/21/2023