THE HYDROLOGIC CYCLE AND ITS RELATION TO METEOROLOGY— 
RIVER FORECASTING 
By RAY K. LINSLEY 
U. S. Weather Bureau, Washington, D. C. 
THE HYDROLOGIC CYCLE AND ITS RELATION 
TO METEOROLOGY 
Hydrology is that branch of physical geography 
which deals with the waters of the earth exclusive of 
the oceans. As a central focus for their efforts, hydrolo- 
gists have adopted what is known as the hydrologic 
cycle (Fig. 1). This concept describes the circulation of 
water as it evaporates from the oceans and enters the 
atmosphere, is precipitated to the earth, and ultimately 
returns to the oceans by surface and underground 
channels. The interest of meteorology in the atmos- 
pheric phase of this cycle is self-evident. 
No schematic diagram can do justice to the complex- 
ities of the hydrologic cycle. Instead of a simple step- 
by-step cycle, all phases may occur simultaneously. 
Large volumes of water may remain in storage at the 
earth’s surface as snow or soil moisture. Some of this 
stored moisture is evaporated and may be reprecipi- 
tated over land. Many writers [4] have attempted to 
evaluate the quantities of water involved in the mete- 
orological phases of the cycle. From the viewpoint of 
the hydrologist such evaluations are largely academic, 
for he is interested in the disposition of the water which 
reaches a specific area, and its ultimate source is of 
little concern. There seems little doubt that moisture 
evaporated from the land is an unimportant source of 
continental precipitation and, hence, that such ac- 
tivities as land drainage or reservoir construction can 
have little effect on the precipitation regime of an area. 
Because of the importance of water in human life, 
hydrology in a simple form has been practiced since 
the dawn of history. Archeological evidence indicates 
that many successful water-supply and drainage proj- 
ects were constructed at early dates. Scientific hy- 
drology got its start in the 15th century a.D., when it 
was demonstrated that precipitation over land areas 
was adequate to supply the flow of streams. Relatively 
little further progress was made in the science until 
the current century, when engineering requirements 
forced the development of techniques for the design of 
major water-control projects. 
Fields of Hydrology. O. E. Meinzer has suggested 
the division of hydrology into four fields representing 
Stages in the surface phase of the hydrologic cycle: 
potamology, study of surface streams; limnology, study 
of lakes; cryology, study of snow and ice; and geohy- 
drology, study of ground water. 
Lakes are an important influence on the climate of 
their immediate vicinity and may contribute substan- 
tial amounts of water vapor to the atmosphere. The 
limnologist is vitally interested in long-term trends in 
precipitation and evaporative potential, as these factors 
affect the progressive rise and fall of lake levels. A 
study of lake-level trends may ultimately prove valu- 
able in interpreting long-period changes in climate. 
While the field of cryology is, in the strict sense, 
limited to snow and ice on the earth’s surface, the 
meteorological conditions during snowfall greatly in- 
fluence the physical characteristics of the snow which 
accumulates on the ground. This snow subsequently 
undergoes a continuous metamorphosis in response to 
the effects of temperature, humidity, wind, and pre- 
cipitation. Studies of the imfluence of meteorological 
factors on snow may eventually contribute to our 
knowledge of the changes in air-mass characteristics 
resulting from passage over extensive snow-covered 
areas. 
Of the four branches of hydrology, geohydrology is 
probably least closely related to meteorology. The 
ground-water specialist is mterested in precipitation 
and precipitation trends in the intake areas which feed . 
the major water-bearing formations. Beyond this point, 
however, the geological features of the ground-water 
problem are paramount. In turn there appears to be 
little of interest to the meteorologist in the study of 
ground water. 
Potamology is the largest of the branches of hy- 
drology and to some extent embraces the other three 
branches. The surface-water hydrologist is mterested 
in lakes as sources of streams or as factors in the char- 
acteristics of streams which flow through them. He is 
greatly interested in snow and ice as an important 
source of water for stream flow. Finally, the dry- 
weather flow of most streams is derived largely from 
ground water and the potamologist must therefore con- 
cern himself with the ground-water features of his 
area. He is more interested in meteorological conditions 
than are his colleagues, for the flow of surface streams 
is directly related to the prevailing weather situation. 
In the material to follow, some of the more important 
problems in surface-water hydrology in which the me- 
teorologist may be interested, or to which he may 
ultimately contribute a solution, are discussed briefly. 
Precipitation. Since precipitation is the source of 
all stream flow (and ground water) with the exception 
of small quantities of water of internal origin, a basic 
problem in hydrology is the evaluation of the precipi- 
tation regime over an area. The hydrologist finds it 
necessary to deal with areas defined in terms of natural 
drainage boundaries, that is, river basins. His aim is 
the quantitative solution of the equation defining the 
hydrologic balance of the area, 
P= EAS = IR (1) 
1048 
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