Environmental Engineering Reference
In-Depth Information
Recording gages measure rainfall in inches or millimeters on an hourly basis. In many
locations, rain gages may be read only on a daily basis or only during storm activity. Snow
accumulation is measured visually on a periodic basis and snowmelt estimated.
Daily rainfall records are kept by most countries although locations may be widely dis-
persed and specific area coverage may be poor. Monthly rainfall charts providing total
accumulation are the normal forms of presentation in many countries. Mean annual pre-
cipitation for countries or other large areas is provided on maps such as that for the United
States (Figure 8.1). Figure 8.2 presents the mean annual pan evaporation for the United
States.
Storm data, or rainfall intensity and duration measured during periods of maximum
downfall, are also important data. Storm data are a significant element of rainfall
data because maximum runoff and flood flows are likely to occur during storms, with
maximum impact on drainage systems and spillways; the occurrence of erosion,
mudflows, avalanches, and slides, moreover, increases enormously with intensity and
duration.
Data are procured from local weather stations and state and federal agencies. In the
United States, the federal agency is the U.S. Department of Commerce, National Oceanic
and Atmospheric Administration (NOAA), National Weather Service, Washington DC.
The publication Climatological Data provides daily precipitation data for each month for
each state. The publication Hourly Precipitation Data provides hourly data for each month
for each state. Locations of the precipitation gaging stations are shown on a series of maps
titled “River Basin Maps Showing Hydrological Stations.”
FIGURE 8.1
Average annual precipitation (in in.), in the United States (U.S. Weather Bureau).
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