S. Geological Survey Water Supply Paper 1103, 1950

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decrease in discharge of ground water to effluent stream flow and ground water, and the vital problem streams.

This

topic will certainly warrant further of climatic trends as they affect the functioning of

the

exploration for within its complexities lie hidden plans we n o w make, based on past data, for

the

utiliza- answers to some of the problems of the relation between tion of our water resources.

B I B L I O G R A P H Y

1. BURNS, J. I. Small scale topographic efects on precipita- tion distribution in San Dimas Experimental Forest, Calì- fornia. Stanford University, Engineer’s thesis, April

1952.

2. COLMAN, E. A. “The place of electrical coil-moisture meters in hydrologic research”’, Transactions American

Geophysical Union, vol. 27, Dec. 194(6, pp. 847-53.

CROFT,

A.

R.

“A

water cost of runoff control”, Journal ofsoil and Water Conservation, vol, 5, Jan. 1950, pp. 13- 3.

’ 15.

I48

4. DUNFORD,

E.

G. and FLETCHER, P. W. “Effect of removal o€ streambank vegetation upon water yields”, Transac- tions American Geophysical Union,vol. 28,194g7,pp. 105-10.

5. GATEWOOD, J. S., ROBINSON, T. W., COLEY, E. R., HEM, J. D. and HALPENNY, L. C. “Use of water by bottom- land vegetation in the lower Sall‘ord Valley, Arizona”,

U.

S. Geological Survey Water Supply Paper 1103, 1950.

HOOVER, M. D. “Effect of removal of forest vegetation upon water yields”, Transactions American Geophysicul Union, vol. 25, 1944, pp. 969-77,

6.

7.

HUNT, G. W.

“Description and results of operation of the Santa Clara Water Conservation District’s Projects”, Transactions American Geophysical Union, vol. 21, 194m0, JACOB, C.

E.

“Correlation of ground-water levels and precipitation on Long Island,

New

York”, Transactions American Geophysical Union, vol. 24, 1943, pp. 564-73, and vol. 25,1944, pp. 928-39.

KOHLER,

MAX

A. and LINSLEY, R. I<. “Recent develop- ments in water supply forecasting from precipitation”, Transactions American Geophysical Union, vol. 30, June

PETERSON, H.

V. “The effect of stock ponds on stream flow’’ (paper presented to the South Pacific Regional Meeting), Transactions American Geophysical Union, Feb. 1952. range vegctation”, Proceedings American Society of Civil Engineers, vol. 77, Oct. 1951.

SPREEN,

W.

C.

“A

determination of the cffect of topo- graphy upon precipitation”, Transactions American Geo- physical Union, vol. 28, April 1947, pp. 285-90.

THOMAS, €1.

E.

Conservation ofground water. N e w York, McGraw-Hill Book Co., 1951.

TOLMAN, C. F. and POLAND, J. I?., “Ground water, salt- water infiltration, and ground-surface recession in Santa Clara Valley, California”, Transactions American Geophy- sical Union, vol. 21, 1940,’pp. 723-34.

WILM,

H.

G.

and DUNFORD,

E.

C., “Effect of timber cutting on water available for stream fIow from a lodge- pole pine forest”,

U.S.

Dept. Agriculture Technical Bulls- tin 968, 1948. points raised in Mr. Linsley’s very interesting paper, namely, how necessary and difficult it is to measure precipitations accu- rately.

As

fi. Linsley has stressed, this measurement is of funda- mental importance in any assessment of hydrological resources and it is seldom accurate enough, owing to the distance be- tween rain gauges.

It

is thus possible to make mistakes which m a y prove important.

Calculations m a y be even wider of the mark when dealing, not with average rainfalls over fairly long periods, but with rain lasting only a short time. By making a systematic study of a dense network of rain gauges spread over a small area, w e discovered that the discrepancy between the figures register- ed by each rain-gauge was not more than 3 per cent for annual precipitations, whereas it was 10 per cent for monthly figures and 20 to 40 per cent for a single storm.

As

underground water-tables in arid zones are fed by rain- fall which is often local but is above all very sporadic, it would appear necessary, if reliable pluviometric data are to be obtained in this case, to analyse the actual pattern of precipitations by means of long-term recording apparatus.

In conclusion, if a dense network of pluviometers makes it possible to reduce inaccuracies due to the distribution of rainfall in space, it should also be possible to reduce inac- curacies due to distribution in time by means of continuous recordings over a long period.

PROFESSOR LINSLEY.

Mi. Nizery has stressed a very import- ant aspect of the problem of precipitation measurement.

However large the errors in an estimate of mean annual rainfall, one m a y be sure that still larger errors are possible in the estimates of rainfall for a specific year, month, or storm. Moreover, as the mean annual rainfall is decreased, the time distribution of this rzinfall becomes increasingly important in the determination of recharge to ground water.

The apparent simplicity of rainfall measurement should not lead anyone to think that this aspect of the hydrologic balance

is adequately understood. I

DR.

NAZIR AHMAD. I

a m much interested to learn of an elec- trical device for moisture estimation in the field, The oldest

.

method and one still used extensively is to collect samples in air-sealed containers and to estimate the moisture content by weighing before and aiter drying to a constant weight, but several attempts have been made to use devices which include the tensometer, based upon the principle of the porous pot connected to a manometer, variations of dielectric constant of a capacity, heat conducting properties of a given soil, etc.

In

Pakistan

I

too have tried to make use of the principle of electrical resistance to estimate moisture content by a single and double electrode cell, and m y experience is that this method has two fundamental sources of error: the first being the presence of soluble salts which makes the estimation of resistance ,quite unreliable, and the second is the polarization defect.

I

would ask the author to give m e more details about the cell as regards the metal of electrodes, their distance apart, use of current and the order of resistance charges measured.

The

enclosing material of the cell would also be of interest.

I

would like information on the author’s experience about the accuracy of the cell records.

PROFESSOR LINSLEY.

Almost all methods of electrical soil moisture measurement use high frequency alternating current to avoid polarization. The Colman element consists of monel wire screens embedded in &re glass and encased in a stain- less steel case. Other elements have utilized gypsum, nylon fabric and other material. Soil salts are not reported to have a serious effect on the measurements until very high concentra- tion. Resistances in the usual soils range from lo2 to lo6 ohms.

If

temperature corrections are applied, the data seem to be quite accurate. .

DR. BOYKO.

Professor Linsley quoted a number of veryinterest- ing examples of water loss by vegetation.

I

should like to stress the excellent work which has been done in this line of research, particularly in California. M a y

I

ask if there exists a new summarizing paper on this subject since the last one of, 194.2 (Use of water by natural vegetation, Sacramento, 1942, 160 pp. Bull. no 50, Dept. of Public Works, State of Cali- fornia)?

In this connexion

I

should like to draw your attention to

c

The hydro7ogical balance and the influence of utilization of underground water the possibility of calculating the minimum of yearly influx

of water into a subsuriace water reservoir, often in desert regions, by the investigation of the vegetation.

I

have in mind such semi-surface water reservoirs as occur for instance in Wadi Araba.

In

Gin Ghadian, for example, there are about 30 sq. km. covered by a specific vegetation, which is distinctly zonated according to the depth to the ground- water table.

The water consumption of each plant community and zone can be determined within relatively narrow limits of error and the zones can be mapped, and the m a p confirmed by air photos.

If

w e leave out the outer zones of acacias, which are not in direct contact with the ground water, w e are able to arrive at reliable figures for the minimum amount of water which is naturally recharged yearly to this standing subsurface water reservoir, which w e can use for economic purposes without clanger of overcharging.

A

second very important remark of Professor Linsley was on the reliability or, better, non-reliability of a sparse network of rain gauges in arid areas. But on the possibilities of improv- ing this by plant ecological methods

I

shall speak in my paper tomorrow.

PROFESSOR LINSLEY. I

do not believe there has been any sup- plement to the bulletin mentioned by Dr. Boyko but there have been many papers from other sources on this subject.

These will be listed in the forthcoming “Bibliography of Hydro- logy, U.S. and Canada”, to bereleased by the American Geo- physical Union late this year.

Ecological indications of precipitation m a y well prove help- ful in improving our knowledge of precipitation distribution.

W e need to employ every possible aid in this direction.

PROFESSOR HILLS.

Figures for precipitation, measured from rain or gauges situated in the open spaces of forest country, do not take account of fog-drip. Figures obtained by research- ers in the Botany School of the Udiversity of Melbourne indicate that in our forests, fog-drip m a y yield up to 8 in. of precipitation per year. Experiments on_individual trees in the city of Melbourne yield comparable figures. Euca- lyptus are very effective in giving fog-precipitation.

In

grasslands, dew supplies a considerable amount of moisture.

PROFESSOR LINSLEY.

Fog-drip and dew arc important sources of moisture for vegetal growth in many areas. More com- plete data on the exact quantities of water so provided should be included in studies of the hydrologic balance.

It

is not likely that these forms of precipitation contribute water to the ground water except under very special geologic conditions.

MR. ROBAUX. I

a m in favour of a detailed m a p of precipita- tion which is absolutely necessary for any serious hydrolo- gical redearch.

I

would ask for supplementary information on the percentage of infiltration as a function of the rainfall and

I

would remark that in general the water-bearing horizons are fed by Piedmont strata.

I

should like to know how underground water recharge is effected by reservoirs coaecting flood water. Finally

I

would stress the importance of determining the time-lags between precipitation and entry to ground water (estimated as 25 years). There are certainly shorter periods, depending on the rock in which the ground water is stored and the dis- tance traversed.

PROFESSOR LINSLEY. I

know of no specific studies of the rela- tion betw’een rainfall and infiltration in thc Canta Clara Valley.

An

approximate relation between ground-water recharge and average rainfall over the valley and its tributary arca might be obtained by assuming that the average rainfall is about 1.5 times the San Jose rainfall appearing on Fig. 9.

With

a total area of approximately 1,000 square miles it then appears that about

5

per cent of the rainfall actually reached the ground water under natural conditions and that this has been increased to about 8 per cent by recharge operations. On an average, roughly 80 per cent of the rainfall actually infil- trates into the surface soil, but because of the great thickness of soil above the ground water and because of the low pcrmea- bility of this soil, very little of this water reaches the ground- water table. Actually recharge to the ground water is accom- plished by the percolation of a portion of the surface stream flow through the permeable gravels of the alluvial cones.

These permeable gravels bave only a limited capacity to permit percolation and much of the winter flood flow is lost to the sea because it cannot penetrate the gravels. The reservoirs serve merely to store these flood flows and to release the water at moderate rates so that a greater percentage of the stream flow m a y percolate through the alluyial gravels. Some of this stored water is also diverted to spreading areas away from the stream channel where suitable gravel outcrops occur.

M y reference to

Dr.

Jacobs’ calculation that ground-water levels in Long Island are a fxmction of precipitation over a period of 25 years should not be construed to indicate a lag of 25 years between the occurrence of precipitation and its e a trance to the ground water. What is meznt is that the ground- water lcvcl at any time reflects the effect of precipitation over a 25-year period.

An

analogy might be seen in a large surface reservoir with a capacity equal to 15 times the mean annual stream flow of the streams tributary to it.

With

no with- drawals at all the volume of water in this reservoir might easily reflect precipitation over 15 years or more. With with- drawal of water the volume in storage at any time might well be dependent in part on the rainfall 25 years before.

Likewise the quantity of water in a large ground-water reservoir m a y be influenced by what has happened many years earlier.

MR. CLARK.

Because of the pressure of time no comment was offered during the fiession. However,

I

should like to make one small point with regard to Professor Linsley’s paper.

We remarked that drainage of swamps and ponds containing aquatic vegetation would not only decrease evaporation and transpiration losses, but might bave public health benefits as well. H e was alluding, no doubt, to the reduction in mosquito breeding which would follow the drainage of such areas.

brecding of malaria mosquitoes in such surroundings usually depends on two factors, the existence of emergent aquatic Vegetation, and warm temperatures. It is possible that in some situations, where thc rainy season coincided with the colder season, there would be no breeding during the period of maximum precipitation.

It

would be sufficient, therefore,

if

the swamps or ponds were drained during the warmer season only. . In some situations, also, the arca need not be drained rapidly upon the onset of the warm season, since a gradual recession, which progressively exposes water margins, can be an effective mosquito control measure. From Lhesc conbidera- tions, it is evident that in regions where rainfall coincides with a cold season, a seasonal fluctuation of exposed water surfaces is not inconsistent with good malaria control.

The

.

150

\

Directress of the Institute of Geography of the Autonomous National University of Mexico

\

T h e Republic of Mexico lies between parallels 140

and

320 and that part of its territory

in

the northern sub-tropical zone is broadest from east to west. A s a result of these two circumstances and of the country’s general relief, the climate is

dry

over extensive areas a.nd this condition seems to

be

becoming increasingly aggravated. It has already been a serious problem for some years, and for that reason special attention has been paid, in Mexico, to the study of hydraulic resources.

Another phenomenon threatening Mexico’s well- being is the destruction of the soil., Though water erosion is accelerated

by

excessive deforestation, this practice is not the sole reason for its intensity

in

Mexico.

Here it is primarily a consequence of factors which are changing the physiography of the whole country and

€or that reason the steps so far taken to conserve the soil have proved ineffective.

These two problems, of increasing aridity and soil destruction, have been constantly in

my

thoughts and a possible explanation of both has been suggested to m e ,

by

a study of the physiography of Mexico, through certain phenomena which

I

think

I

have detected

in

the evolution of the relief.

For a proper grasp of the question, a brief preliminary outline of Mexico’s physical characteristics

w i l l

be necessary.

South of parallel 300 north, the North American land mass narrows considerably as a result of the marine transgression forming the

Gulf

of ‘Mexico.

A t parallel 200, the narrowing of the land mass is still more marked as a result of a change

in

the axis of the relief and with the Isthmus of Tehuantepec there begins the seiies of inter-oceanic depressions which is one of the mosttypicall features of the relief of Central America.

North of parallel 200, the most notable morphologi- cal feature is a plateau, the Altiplanicie Mejicana,

with

a fall from

SE.

to

NW.

and Banked

by

two lofty mountain chains; to the east, the Sierra Madre Oriental with its seaward slopes running d o w n to the

Gulf

of

Mexico, and to the west, the Sierra Madre Occidental falling to the Pacific. T h e southern boundary of the plateau is a line of volcanic cones,

known

as the Vol- canic Axis, running from east to west along parallel 190, while another volcanic chain, the Sierras de Zaca- tecas, starts from the Sierra Madre Occidental at latitude 240 North and runs

NW.-SE.

across the Alti- planicie to merge with the Sierra Madre Oriental and the Volcanic Axis

in

the south-east.

Below parallel 200, the run of the relief changes from

NW.-SE.

to

E.-W.

Near the Pacific littoral and parallel to it is another range, the Sierra Madre del Sur, bordering an extremely clislocated massif against which the orogenic pressure waves which formed this range and the southern part of the Sierra Madre Oriental (Sierra de Zongolica) were checked. T h e presence of this massif, to be called hereafter the W x t e c shield, as it comprises the high Mixtec region, is the explana- tion of the convex line

of

the Pacific littoral between parallel 200 and the isthmus of Tehuantepec.

T h e meeting point of the pressures which respec- tively form the Sierra Madre del

Sur

and the Sierra Madre Oiiental, i.e. the region of the Mixtec shield, k n o w n traditionally but most misleadingly as the Zempoalte- pet1 “knot”, exhibits an extremely complex relief, with mountains falling very steeply towards the+

Tehuantepec isthmus.

Beyond the isthmus, the run of the relief is still east to west and the country falls, from south to north, into five parallel regions-the Sierra Madre de Chiapas, the central depression, the central plateaux, the north- ern sierras and the Tabasco plains.

In

the peninsula of Yucatan, a n almost flat limestone shield, the fall is from south to north; it is bounded to the north and the west

by

the

Gdf

of Meyico and to he east

by

the Caribbean.

T h e long, narrow peninsula of Lower California lies

1 See plates between pages 144-145 and 160-161.

Dans le document HYDROLOGY SYMPOSIUM (Page 145-148)