HYDROMETEOROLOGY IN THE UNITED STATES 
By ROBERT D. FLETCHER 
U. S. Weather Bureau, Washington, D. C. 
INTRODUCTION 
Scope of Hydrometeorology. Meteorological forecasts 
and advices, though varying greatly as to type, tend 
to fall into more or less identifiable groups. One such 
group has come to be known as hydrometeorology, that 
branch of meteorology which deals with the rainfall 
statistics of storms in order to determine, theoretically 
or empirically, relationships between meteorological 
factors and the precipitated moisture which reaches 
the ground. It is especially concerned with those atmos- 
pheric processes which yield the water dealt with in 
hydrology. In practice, the hydrometeorologist de- 
velops estimates—usually in terms of the ‘maximum 
possible” —of the space-time distribution of rain over 
specified drainage areas. The hydrologist uses these 
values in estimating the flood hydrographs which would 
result. 
The boundaries of hydrometeorology are not sharply 
defined. The hydrometeorologist, like the hydrologist, 
is concerned with snow melt, since it is intimately in- 
volved with meteorological processes. The cloud physi- 
cist and hydrometeorologist, on microscopic and mac- 
roscopic scales, respectively, deal with the formation of 
rain and snow in clouds. The weather forecaster and 
hydrometeorologist both prepare quantitative pre- 
dictions of precipitation, although the hydrometeorolo- 
gist usually does not forecast when precipitation of a 
given magnitude will occur, but rather that certain 
maximum intensities can occur over particular areas. 
Wherever problems concerned with the accumulation 
of water from rainfall or snow melt are found, there is a 
need for hydrometeorological advices or forecasts. In 
recent years, however, systematic hydrometeorological 
services have been established largely to provide bases 
for the design of flood-control and water-usage struc- 
_ tures. Among the better known of the hydrometeorologi- 
cal services in the United States are those of the U.S. 
Weather Bureau. That agency’s Hydrometeorological 
Section was organized to provide hydrometeorological 
advice for the Department of the Army, Corps of 
Engmeers, which is concerned with the design of flood- 
control structures. The Weather Bureau’s Cooperative 
Studies Section was established primarily to perform a 
similar function for the Department of the Interior, 
Bureau of Reclamation, which is concerned with water 
conservation and irrigation. The regular duties of the 
Weather Bureau forecast centers include the quantita- 
tive prediction of precipitation intensities over specified 
areas. Such information is applicable to the operations | 
of flood-control and water-usage structures. 
1. Consult “‘The Hydrologic Cycle and Its Relation to 
Meteorology—River Forecasting” by R. K. Linsley, pp. 1048- 
1054 in this Compendium. 
Maximum Possible Precipitation. For a given area 
and duration, the depth of precipitation which can be 
reached, but not exceeded, is defined as the maximum 
possible precipitation. Since the laws governing precipita- 
tion rates are not completely known, this figure is an 
estimate which implies a range of tolerance, the extent 
of which depends upon limitations in technical knowl- 
edge, data, and thoroughness of analysis. The values 
computed from the analysis are considered to be the 
maximum possible since, within the confines of current 
theory and available data, they are derived from the 
most productive combinations of the factors controlling 
rainfall intensities. 
In general, three steps are followed in the derivation 
of maximum possible precipitation values. First a storm 
model is established—usually in the form of an equation 
—which expresses rainfall as a function of independent 
meteorological variables which are not only measurable 
but also well represented as to quantity and length of 
record. Theoretical meteorology is basic in the estab- 
lishment of the equation form: empiricism is utilized in 
the selection of proportionality factors, constants of 
integration, and the like. Second, the model is tested 
through the introduction of observed values for the 
independent variables. Comparison between observed 
and computed rainfall rates provides a basis for the 
estimation of the reliability of the model. Third, the 
independent variables are maximized statistically. In- 
troduction of the maximum values of these variables, 
or the most effective combination of them, into the 
successful rainfall equation produces the estimate of 
maximum possible precipitation. 
BASIC DATA AND GRAPHICAL PRESENTATIONS 
Rainfall Intensity, Depth, and Volume. The history 
of hydrometeorological investigation has demonstrated 
the need by the field, at one time or another, for nearly 
every element of the standard meteorological observa- 
tion. Of considerable importance are (1) humidity, to 
determine the quantity of moisture which can be pre- 
cipitated, (2) pressure, to determine the mass of air 
which can enter into the precipitation process, and (3) 
wind, to determine the rate at which precipitation can 
take place. The fundamental element, however, is the 
precipitation itself. This measurement is one of intensity 
of precipitation, or rate of accumulation of precipitation 
in the measuring device. It is thus a measurement of the 
average depth of precipitation through duration, the 
minimum length of duration obtainable being a function 
of the time-sensitivity of the measurement. It is also a 
measure of the average volume of precipitation through 
duration, the volume equalling the product of the 
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