Fundamentals of Engineering Design
5.1.5.1.9 Weather and Climatological Data
An enormous variety of weather and climatological data are available from the National Oceanic
and Atmospheric Administration's (NOAA) National Climatic Data Center (NCDC) on the world wide
web at http://www.ncdc.noaa.gov/. NCDC is the world's largest active archive of weather data. In
addition to producing numerous climate publications and responding to data requests from all over the
world, NCDC supports a three tier national climate services support program that includes NCDC,
Regional Climate Centers (RCC's), and State Climatologists. Weather and climatological data are useful
in identifying the magnitude and rate of precipitation associated with a particular design storm, temperature,
humidity and other data used in modeling evapotranspiration processes, and long-term records of rainfall
for locations of interest.
5.1.5.1.10 Watershed Climate and Hydrology
For many years, the USGS has been involved in the development of regional regression equations
for estimating flood magnitude and frequency at ungaged sites. These regression equations are used to
transfer flood characteristics from gaged to ungaged sites through the use of watershed and climatic
characteristics as explanatory or predictor variables. The USGS-developed regression equations are
generally unbiased, reproducible, and easy to apply. The standard errors of estimate or prediction
generally range from 30-60 percent, with 21 states having standard errors in this range. There are fourteen
states where there is at least one hydrologic region within the state with a standard error less than 30
percent. The largest standard errors are for equations applying the western portion of the nation where the
at-site variability of the flood records is greater, where the network of unregulated gaging stations is less
dense and there are more difficulties in regionalizing flood characteristics, and the flood records are
generally shorter. The smallest standard errors are generally for equations developed for the eastern
portion of the country where the converse of the above conditions is generally true. Regionalized discharge
is discussed in Section 3.1.2.
In geomorphic or hydraulic analyses of fluvial systems, it is often necessary to identify a "channel-
forming" or "effective" discharge that exerts the most morphologic influence over channel geometry. The
smaller of the bankfull or estimated two year discharge (Q2) is frequently used as surrogate value of
effective discharge when flow and sediment monitoring data are insufficient to directly compute the actual
effective discharge. Estimates of Q2 derived from the above equation may be particularly useful in
assessment and design under these common circumstances. Since hydrology determines the boundary
conditions for hydraulic analyses, the accuracy of any subsequent hydraulic analyses will only be as good
as the hydrologic information upon which they are based.
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