JANUARY, 1933 MONTHLY WEATHER REVIEW T
Drain-
age area, Date ;quare miles
6. Percentage of run-ofl.-(a) I t is fortunate for agncul-
ture in Iowa t,hat the percentage of run-off from storm rainfall is small. In the August, 1929, flood, the percent-
age of the precipitation reaching the stream, amounted to
any, of the midsummer precipitation percolates deep enough into the soil to replenish the ground water and
TABLE 1.-Percentage run-off of typical Iowa summer .fbods-C!~~i.
only 18.3 per cent at Iowa Cit and 33.7 per cent at River
Marshalltown. Experiments in B icate that very little, if
Total
tion, inches
$;&
repleiish the supply available for sGbsequent, run-off.
Hence, the balance of the water was either evaporated immediately or stored in the upper layers of the soil for
future evaporation and plant use. The graph showing the comparison of the cumulative run-off and precipitation is
shown in Figure 4.
(b ) In the following table, the percentage storm rain-
fall appearing as flood run-off is listed for other midsummer
floods on Iowa streams. On the larger basins, the flood
run-off rarely exceeds 30 per cent of the precipitation, although occasionally on some smaller watersheds almost
70 per cent of the storm rainfall has reached the streams.
Des ~o i n e s --- -.
Do .........
Raccoon ........
Skunk .... ______
Do---------.
Do-.------..
Syuaa Cree%----
TABLE 1.-Percentage run-off of typical Iowa summer floods
"*
14.080
3,460
2,915
I-
May-June 1915 ....
{July, 1920; -....-...
May-June, 1903-..-
June. 1905 _.._.. ~...
May-June. 1917.. . .
May-June. 191L..
September, 1928.. -.
May-June, 191i. -.
May-June, 1917.-..
May-June, 1918-...
June-July. 19U.-.-
I
September-Octo-
1 1
Do. - - - - - __ - .- Marshalltown Cedar ____________ Cedar Rapids.
5.34
4.53
9.53
1.92
5.88
6.46
5.9s
4.62
7. 16
7. 16
3. i S
12. 66
Drain-
age area, square miles
1.56
1.38
3.59
.49 1.79
1. R1
1. R8
1. 79
2.69
2.00
1. 36
4.58
315
M5
-I I-
'J&bGy,- 1924.. -. 2.64
.___do _..____..____ 2.28
September-Octo- 12.85
June, 1928 __._____.. 2.65
September, 1928.. __ 1.98
__._do _____..______ 1.84
ber, 1928. {
Mey-June, 1903..-- 7.10
August, 3928 _._____ 4.48
1.500 __.--do- - ___. .. - __ _. 3.07
8.570 May-June, 1903 .___ 5.75
May, 1927 ____ ______ I I
.-- -do _____________
off, ofpre- inches clpita- Ralston Creek-.. tion
1.04
1.23
,I, 2.36 33.8
,111 g;
1.09 19.0
River
Iowa -__.___.__---
Gaging statio!
Kalo-. __ . . -.
Keosaurlua.. .
Van Meter-..
Coppock.. .-.
Augusta.. - - -
Ames ____ __ __
---.do-
~
- - - -.
Iowa City.-..
Gaging station
Iowa City
Date
Total
inches
run- on.
Total
tion, inches
$;ti
____do- - _______.___ June, 1927 __...__...
Jme, 1928 _______.._
July. 1928 ._________
__._do.. . ._ ____ __. - August, 1828 _..____
.... do. -. __. - .-. . ..
.59
4.88
2.08 1.35
1. O i
1.61
3.30
1.77
2.54
2.76
4. 17
1.40
1. M
3.35
.49
. i 8
.82
.32
.59
.41 .05
.52
.19
.4 s
.61
.90 .34
.71
.76
1.52
~
Per cent UnQff )f pre- :ipita- tion
29.2
3.00
30.4
25.6
30.5
28.0
31.6
38.8 40.0
28.0
36.0
38.4
42. 0
28.9 41. 1
42. n
55. 1
67.3
26. 1
18. 5
39.4
48.0 69. 5
82.4
25.0
13. 41. I
37.9 27. 0
25.4
27.9
27.5
36.3
17.2
25.2
E5
THE TECHNICAL USE THAT ENGINEERS MAKE OF UNITED STATES WEATHER
BUREAU OBSERVATIONS
By CHARLES H. LEE
[Member Am. SOC. C. R. and Consulting Hydraulic Engineer. San Francisool
The work of the civil engineer embraces the planning, construction, and operation of both private and pubhc
enterprises and improvements. In connection with these activities he probably makes use of a greater
variety of meteorological data than any other class of
citizens, and in so doing affects the welfare of many
people. It should therefore be of interest to meteorolo-
gists to know of some of the technical uses that engineers make of United States Weather Bureau observations. These Itre outlined in this paper together with suggestions
as to ways in which the value of the observations could
be increased.
PRECIPITATION
Precipitation is one of the most important classes of
meteorological data which the engineer, and especially the hydraulic engineer, uses. It is fundamental to the
planning of many projects and is very important in the
design of hydraulic structures. It is necessary to know not alone the quantity of precipitation during long periods of time such as months, years, and groups of years, but
also the rate or intensity for short periods of nimutes and
hours. The character of the precipitation, whether rain
or snoG, is also important. Quantity.-Knowledge regarding the quantity of pre-
cipitation or depth in inches falling during periods of 24
hours or more, is of greatest importance in innking water
supply estimates for new hydraulic projects and for en- largement of existing systems. The proposed use of
water may be for one of a great variety of purposes, such
as domestic, industrial, municipal, irrigation, hydroelec- tric power, navigation, mining, or recreation. In every
instance, however, it is necessary to know definitely whether sufficient water is available for the pro osed use,
flow measurements are of course depended upon to the
extent available, but must usually be supplemented by studies of precipitation. The character of records re-
quired for this purpose range from long term average
monthly and annual precipitation extending over periods
of 50 or more years, to the actual precipitation which
occurred on a specific day or during a certain storm. The data published in the monthly issues of Chiatologi-
cal Data are as much sought after in this connection as are the annual summaries or the occasional long-term
summaries issued by the bureau.
In regions where there is a definite winter rainy seaaon, such as on the Pacific coast of the United States, the com-
pilation of precipitation da;ta on the basis of the season instead of the calendar year is of reat benefit to engineers,
studies without the necessity of laborous preliminary computations. Precipitation records have been published
both in the dr est season of the year and also B wing the
dryest year w L 'ch may be expected to occur. Stream-
for it permits of immediate use o K the data in water supply
8 MONTHLY WEATHER REVIEW JANUARY, 1933
Yrs at
by the Weather Bureau in this form in recent least for the California section, using the period uly 1 to
June 30, and have been a source of great satisfaction to engineers. Seasonal compilations show clearly the char-
acte,r of the rainy season, whether wet or dry, and permit of the direct comparison of stream flow with the precipi-
tation which produces it. There is great variety in the application which is made of quantitative precipitation dat,a in water-supply
studies. The principal use is in supplementing stream-
flow records. By their use i t is possible to estimate the
normal or flood flow of surface streams supplied by run-off from precipitation occurnng during periods or n.t points
for which stream-flow observations are not, available.
These studies enable the engineer to determine whether storage is necessary to produce the required wate,r supply,
and if so, the reservoir capacity which will he necessary. In recent years more and more attention has been given to ground water as a source of supply. As precipitation
is an imporbant source of ground water, this has led to
t8he investigation of the absorption from precipit,at,ion by
the soil and other natural formations. This involves the study of precipitation from each individual storm in connection with soil moisture and evaporation. Studies
of this nature enable the engineer to determine the extent a.nd reliability of the flow Irom springs and the amount of
ground water which is permane,ntlg recoverable by pump- ing from wells. Precipitation, in addition to being
the source of strea.m flow or ground .water used in irriga- tion, is frequently a direct source of soil moisture for
growing agricultural plants. The ascertainment of what proportion of the annual plant requirements will be thus
obta.ined directly, is of great import.ance in certain locali- ties in determining the supply to be provided for irriga-
tion works. Another use of precipitation data is in forecasting the current seasons supply as an aid to operation of existing
water systems. This is of part,icular value where precipi- tation occurs as snow at high elevation and the melting
period carries over into the summer. Elaborate snow surveys are now made during the winter and spring
months of each year by various State, Federal, and private agencies throughout the West. In compiling the results
of such surveys, extensive use is made of Weather Bureau precipitation data in comparing the observations with
normal and in preparing the forecast for areas of early snow melting. The results of the surveys are widely
sought by municipalities, water districts, and water and power companies, as the basis for making water contracts and planning each years operations. An entirely different use of quantitative precipitation
data is that in connection with the design of land-drainage projects. In this case it is desired to determine the
quantity of water absorbed by the soil from precipitation and which is to be removed by draina e works. This
pipe and draina e outlets, the capacity of drainage pumps, etc. D d y and storm records are sought for
this purpose as well as the amount of precipitation during longer periods. Isohyetose maps.--It is frequently necessary in making
water-supply studies to prepare isohyetose maps, or maps showing lines of equal depth of precipitation, either for a single storm, a whole season, or as an annual average during a long period. For general study such a map
might. cover some large geographicnl or political unit, but
for specific studies it would be confined to n single water-
information is essential in determining t E e sizes of drain
shed or group of adjacent watersheds. A proper geo-
Faphic distribution of precipitation stations is of great importance in the preparation of such a map, since topog-
raphy and slope is a primary factor in local precipitation.
If the watershed is one of low relief and well settled, there
may be but litt,le 1oca.l variation and existing stations may be adequate. If, however, part or all of the water-
shed is mountainous and sparsely settled, the existing
stations will probably be inadequate for the purpose. There are frequent instances in the mountainous portions
of the West where differences in average annual precipita.- tion of 500 to 1,000 per cent occur within distances of from 10 to 40 miles. A careful local study of the relations
of precipitation nnd elemtion on windward and leeward
slopes of mountain ranges may sometimes help to supple- ment a deficiency of mountain stations, but on the Pacific
slope there are many very important watersheds which have too few stsations within or even near them to make
femible the construction of isohyet.ose maps. The value
of these maps is so great in a State such as California, where every drop of water is being inventoried and plans
and cost estimates prepared for its ultimate use, t.hat a very definite effort would be justified to overcome this
deficiency. It would not necessn.rily involve a great increase in the total number of precipitation sta.tions, but
certainly a redistribution of stations with respect to critical topographic position would be required. Such a
program might well include regular n.nd cooperative
stations maintained by the Weather Bureau as well as
those maintained by private or municipal agencies.
Intensity of precipitation.-Increasing density of popu- lation and the development of the automobile have created a great popular demand for paved streets and
highways, which in turn have given rise to the necessity of providing storm drainage. The latter may be accom-
plished by the construction of a simple culvert under a surfaced thoroughfare, or may involve the construction
of an extensive system of storm sewers, or even the intricate combination of flood control reservoirs and arti-
ficial channels costing millions of dollars, such as being built for protection of the thickly populated areas of Los
Angeles County, Calif. Regardless of the magnitude of the improvement, the determination of the quantity of
water to be carried, and hence the required size of the
pipe, conduit, open channel, or capacity of flood control reservoir, can best be determined from records of maxi- mum rainfall intensity, that is, the greatest quantity
falling during periods of five minutes and more up to sev- eral hours but usually less than 24 hours. By study of the flood run-off cha,racteristics of the tributary drainage
area, together with actual correlation of time of occur- rence and intensities of heavy rainfalls and resulting
flood flows a t any point, it is possible to establish definite relations between rainfall and flood run-off intensities.
Such data can only be obtained b installation of an
sheets. In order to be of greatest usefulness such a record
should include the storm of maximum intensity to be anticipated in the re on, and hence the longer the period
drainage systems i t is necessary to have several such records, especially where a portion of the drainage area is
mountainous. The U. S. Weather Bureau obtains such data at important population centers and much use is made of them as a basis for storm sewer and culvert design.
The originals [W. B. Forin 1017-hletlJ of such records twe sent to Washington, D. C., for filing, and it would
automatic rainfall recorder and carefl 19 study of the record
covered, the more va ff uable is the record. For large storm
JANUARY, 1933 MONTHLY WEATHER REVIEW 9
often be of convenience to engineers if photostat nega-
tives of these records were kept in the office of origin. In the more thinly populated areas there is very little data
available, and for design of highway and railway culverts, flood control reservoirs, reservoir spillways, etc., it is still necessary to use assumptions in the design of waterway
areas.
Character of precipitation.-In making water-supply estimates it is very important to have knowledge of the
character of precipitation, whether rain or snow, and if
the latter, the equivalent depth of water. Snow falling a t high altitudes melts slowly and is the equivalent of
reservoir storage in maintaining summer streamflow. It is very essential, however, to know the equivalent depth of water of the snow catch or the water content of snow
on the ground. I n the published records of precipitation falling as snow the catch is reduced to equivalent water, but there is no inforination regarding the water content
of snow on the ground. This would be a very useful addition to the extensire snowfall records regularly published by the Weather Bureau.
TEMPERATURE
The meteorological data next to precipitation in im- portance to the engineer is probably air temperature. He is interested both in average values and in extremes,
especially where minimum values go below freezing. Daily, monthly, annual, and 10-year annual averages, as
published by the Weather Bureau, are nll in demand. One of the important uses of such data is in the planning
and financing of irrigation systems. Temperature con- trols the length of the irrigation season, the character and
variety of crops which can be successfully grown, and the number of crops which can be raised each year.
Knowledge of these is essential for adequate preliminary investigation of irrigation projects and the financial suc-
cess of such projects frequently depends upon the availa- bility of complete temperature data.
The design of engineering structures and plant is fre-
quently influenced by temperature conditions. Provision for ice control in reservoirs and canals is necessary in a
region where freezing temperatures prevail. Highway
subgrades must also be protected from frost heave by
adequate drainage, and concrete structures which with- stand water pressure must be made of sufficient thickness
and density to prevent spalling off of the face by the freezing of water in the interstices of the material.
Provision for artificial heating in buildings and tenipera- ture control in steam power plants is also dependent upon knowledge of air temperature.
Large outdoor construction operations are vitally in- fluenced by temperature. Quarters for men engaged
upon the work must be provided which will mitigate as far as possible the extremes of heat and cold. Transpor-
tation must be planned which will not be disrupted by
cold weather. Provision for hot water as a fenture of concrete mixing plant must be made if air temperature
falls below freezing for any length of time, and also pro-
tection a t night for each days fresh pour. Temperature
conditions are also of importance in selecting locations for construction plants and camps as well as the more
permanent locations for industrial plants or communities. As an example of the latter, the choice of location for Boulder City, made by the engineers of the United States
Reclaiiiation Service, was largely based on temperature
conditions.
162368-33-2
Temperature is the most important factor controlling evaporation during an extended period of time, such as
a month or a year. Air temperature is frequent,ly studied by hydraulic engineers in the absence of direct observa-
tions to determine the approximate evaporation from proposed reservoir surfaces.
WIND
Information regarding wind movement is probably as
much sought after by engineers as temperature. Direc- tion, velocity, and pressure against high buildings are all desired. Wind direction has an important bearing on the layout of buildings and grounds, and especially
upon plant location where offensive odors, smoke, dust,
or fumes are produced. Sewage treatment, garbage
treatment, and T ~I ~O U S industrial plants such as smelters, cement plants, etc., must all be carefully located with respect to prevailing winds so as not to create nuiqance
in established communities. Floating solids on lakc B
and reservoirs and dust and flies in the air are all carried by the wind.
Knowledge of wind velocity is often of importance in
planning construction operations. For example, work on bodies of inland water which requires floating equip- ment, iiiay encounter considerable interruption by rea-
son of seasonal or periodic daily winds of velocity great enough to produce waves. Wind velocity is also of
iinportance in studying short tinie variations in evnpora-
tion rate from reservoirs and the fluctuations in level of lakes. The greatest use which engineers make of wind velocity data is in connection with building design.
Brnong the other forces which inay act upon isolated
buildings are those induced by wind. Partial failures of large modern buildings have occurred during heavy
winds, as at Miami, Flu ., during the hurricane of several years ago, and for buildings of lighter construction it is
necessary to provide against wind forces of much lesser degree. Wind velocities are observed a t all regular
Wheather Bureau stations and published as average hourly wind movement for each month. These are of
little value to the designing engineer, however, since wind movement is very irregular, and it is maximum
velocities which do the damage. The printed recording anemometer sheets are of much greater value. These must be consulted for considerable periods of time in order to determine the relative probability of maximum
winds of various intensities. If photostat negatives of these records were kept in local Weather Bureau offices it would increase the usefulness of such data.
There is great difference in wind velocity at differing heights above the surface of the ground. There is no standard height for Weather Bureau anemonieters and no
inforination published relative to surroundings or height at any station. It thus is necessary to study a t f i s t hand
the environment of equipnient a t wind velocity stations
in order to properly use the records. It would be of con-
siderable value to have such information in published forin, illurtrated by sketches or photographs. The es-
posure on the tops of tall buildings, which is quite
common, is not always productive of a true wind record, and it is to be hoped that it may sometime become possible to place anemometers on skeleton towers such as radio
towers, where they will be free from eddies and cross currents. Such a location would also be ideal for deter-
mining the effect of differing elevation upon wind velocity.
10 MONTHLY WEATHER REVIEW JANUARY, 1933
The conversion of wind velocities into wind pressures is a step which, although possibly not a meteorological
problem, is yet of greatest importance to the engineer and
one about which very little is known. The erection of large and costly structures in areas of high wind velocity is now causing concern to the designing engineer, and
there is great need of scientific investigation of this subject. Such pressures vary greatly, not only with wind velocity
but with shape and area of the exposed surface.
EVAPORATION
This is a meteorological process of greatest interest to the engineer as it represents an important loss of water
after the latter has been reduced to possession in a reser-
voir or canal system. All water supply estimates must
take it into consideration, and it frequently influences
the operation program of water systems. Evaporation rates from free water surfaces differ greatly depending upon the temperature, humidity, and movement of the
air, the temperature of the water, and the area of the water surface. The engineer usually desires to know the daily, monthly, and annual rate from a large water sur-
face such as a lake or reservoir. It has been found by
experiment that the evaporation from small pans set on or even in the ground, does not reproduce conditions on
the surface of a large water body. The nearest approach
to reproduction of actual conditions with a small pan is when submerged and floating upon the surface of a large
body of water, with water level a t the same elevation both inside and outside the pan, and side walls of the pan
not projecting far above the water level. Such a pan 3
feet square or circular, protected more or less from wave
splash, is frequently installed by engineers. For a land pan set into the soil it has been found that the diameter
must be a t least 12 feet before conditions closely approach
those of a large water surface. The standard Weather Bureau pan is 4 feet in diameter
and set on a platform slightly above the ground surface with free air circulation beneath. Observations from
such a pan have been of little value to engineers until very recently when intensive investigations were made
with pans of different sizes, shapes, and exposures, to de-
velop the general principles involved. Most of this
experimental work has been done by the Division of
of Agricultural Engineering, United States Department
of Agriculture, in the vicinity of Denver, Colo., although
it is understood that similar work is in progress in Cali-
fornia. The data thus far available indicate that a fact>or
of 0.70 applied to the evaporation as observed in standard
Weather Bureau pans, will indicate approximately the
loss from a large water surface exposed to similar at-
mospheric conditions. This factor, although possibly
subject to modification under conditions differing from
those a t Denver, as for example, relative day and night
temperatures, now enables the engineer to utilize Weather
Bureau evaporation data. Recent studies made by the
writer in the San Francisco Bay region, where there have
been si. records kept at various times with widely differ-
ing pan equipment and exposure and including one
Weather Bureau pan, showed that all observations could
be reduced to reasonable agreement when corrected for
differences of temperatures and pans.
OTHER DATA
The four classes of observational data made available by the Weather Bureau, as discussed above, are the
principal ones used by engineers. There is frequent use of other data, such as relative humidity, barometric pressure, sunlight hours, killing frosts, and especially the
current weather forecasts. The lat,ter are of frequent use in planning field work, construction operations,
t'ransportation of material, and the operation of reservoirs
and canals. Most hydraulic engineers make it a daily practice to examine the weather map and speculate as to
the probability of rain or snow within the next day or
two. The long continued series of dry years through which we have been passing may ha.ve had its influenc.e
in forming this habit, although the use of reports from
vessels on the Pacific Ocean in making forecasts has greatly stimulated it. All governmental bureaus must of necessity carry on
their work under definite financial appropriations or
budget allowances, and in actcmhce with fiscal and
departmental rules and regulat'ions. These are restric- tions which more or less limit any organization, whether
goveiiimental or private, but must be especially rigid in
the governmental service. The United States Weather Bureau, being one of the older scientific bureaus of our Government,, is possibly more burdened with these restric-
tions than some of the more re,cently organized services.
Regardless of such handicap, however, the work of the Weat,her Bureau is touching an ever-increasing number of people and serving a widening variety of interests.
The usual purpose for the creation of permanent
governmental agencies engaged in sc.ientific work is to meet some direct need either of the people or of govern-
ment acting in the intemst of the people. Once organized, with an established administ'rative routine, the original
purpose is sometimes lost sight of either by a failure to
keep pace with the chana@ng needs of the people for current information of a scientific character, by a lack of vision and energy in unde,rtaliing fundamenta.1 research,
or by inadequate facilities for making the data available. Although this tendenc.y has a t h i e s been noticeable in
governniental bureaus of the United States, it has never
reached such extreme.s as have be,en witnessed in certain older countries where the ideal of government being for
t,he benefit of the people is not so ge.nerally recognized. In the practice of his profe.ssion the writer has had
contact with the meteorological departments of foreign governnients as well as the United States, and has been
impressed by the difl'erenc.es in objective. Our We.ather Bureau is not administered as a background for the scien- t,ific interest,s of any official or organization, nor to meet
the specific requirements of some other branch of the Government servic,e to the exclusion of the needs of the
public. Its objective is to render promptly a wide variety of service to the estent w-hich funds and departmental rules and regulations will permit,, and in so doing it is
filling a real need. Many engineers would like to see broader scope or refinement in certain phases of its
meteorological work, but it should be remembered that in the final analysis the burden of improvement and
enlargement can not be carried by bureau officials alone.
It must be partic.ipated in by public spirited citizens working through the various channels which create and
make effective public opinion.