Some aspects of watershed management research in the United States
By Bernard Frank
Assistant Director, Division of Watershed Management Research, Forest Service, United States Department of Agriculture, Washington
The objectives and emphases of forest influences and watershed research in the Uni- ted States are necessarily affected by the great variations in physical and cultural condi- tions which prevail here. These conditions are reflected in regionally distinctive inter- relations among soil, native vegetation, and water behavior. These relationships - or ra- ther our ignorance or neglect of them - have directly affected the nature, scope, and severity of the problems we have inherited from more than a century of land and water m1suse.
In an effort to help solve these problems, the Forest Service of the U. S. Depart- ment of Agriculture has been conducting for over 25 years a program of comprehen- sive research into the influences of forest and range vegetation upon runoff, siltation, and streamflow, and the principles and methods of repairing and managing watersheds in such cover. The major objectives are: (1) A sounder understanding of the inter- actions of vegetation types with their natural and cultural environments and their effect upon the operation of the water cycle, including erosional as well as hydrologic proces- ses; (2) the development of more effective methods for stabilizing and improving water flow on damaged forest and related rangelands; ( 3) methods of detecting incipient in- stability and of protecting, utilizing, and managing such lands to insure their proper hydrologic functioning, and to reduce the adverse effects of utilization upon the de- pendent downstream values; and ( 4) techniques to increase or otherwise improve the usefulness of water supplies.
Such objectives dovetail with the mature thinking of professional foresters, who in a nationwide referendum in December 1947 approved a policy on multiple use. This po- licy reads as follows:
Policy on Multiple Use
The Society of American Foresters subscribes to ihe principle of multiple use of fo.
rest and other wild lands, meaning by that a conscious effort to manage each unit of land for its highest sustained productivity. In some cases, this may mean utilization of an identical area for several purposes at the same time; in others, utilization of different parts of an administrative unit for different single or limited purposes. In all cases, the development of management policies and plans requires adequate recog- nition of all resources and benefits, with due consideration of the relative social and
«Mitteilungen der Schweizerischen Anetalt für das forstliche Versuchswesen, Bd. 35, Heft l»
economic values of each resource present and of the effects of utilizing one resource upon the stability, value, and appreciation of the others. More specifically:
1. Satisfactory conditions of soil, cover, and water flow are basic, and all forest lands, regardless of type or purpose of ownership, should be so managed and protec- ted as to maintain fully those conditions;
2. Management policies, plans, and practices governing the use of a principal re- source, such as timber, should take into account and make specific provisions for all the resources capable of use or appreciation, not just the principal resource alone;
3. Methods or intensities of practice, as in timber harvesting, livestock and big game grazing, recreation, and road and trail construction, which may bring about soil instability or adversely affect runoff and streamflow conditions, should be modi- fied or avoided;
4. Knowledge and skill in multiple use management are as yet inadequately deve- loped. Research on different phases of wild land use should be more effectively coor- dinated on the same experimental areas, and the study of multiple use problems should be made an integral part of such research;
5. Foresters, as managers of wild lands, have a professional obligation to keep ab- reast of scientific findings on the characteristics and interrelations of forest resources, and of the effects of devcloping any one of them upon the others.
This policy gives considerable weight to the kinds of scientific knowledge that forest influences and watershed research is capable of furnishing.
Character and Extent of W atershed lmpairment
Much progress has been made in improving our forest and range practices. Never- theless, the effects of destructive practices in the past, when the benefits of conservation were not so weil understood and the application of sound measures was quite limited, are evident in the forest conditions that are still found today. These effects can heller be appreciated when we consider that how the cover is handled on at least 470 million acres, or three-fourths of the total commercial and noncommercial forest land, is crucial to the control of floods and sedimentation, maintenance of desirable water supplies and optimum ground-water recharge. In general, the removal or deterioration of any appre- ciable portion of the cover in most parts of our nation can initiate or step up soil move- ment and erratic water flows, and lower the quality, quantity and utility of water supplies.
Interest in watershed management is so recent, and so few people are trained in this field that we still Jack an adequate nationwide watershed appraisal. However, informa- tion from surveys of numerous individual drainage basins over the past 20 years does indicate the nature and extent of the present situation as a whole. Thus, of the 4,70 mil- lion acres having crucial watershed importance, some 365 million acres are considered to be below par for desirable water production, downstream flood and sediment reduc-
tion, or both. (Many of the conditions on commercial forest lands are also responsible for deteriorated sites, hence for slower timber growth and less healthful stands than would otherwise prevail.)
Regional Variations
Regional conditions naturally affect the influence of forests upon local climate, streamflow, etc. Such conditions may be described in terms of broad climatic features.
Broadly speaking, we can designate the cool humid, warm humid, and arid to semiarid regions.
Cool Humid Regions
The cool humid regions include the northern part of the United States east of the Great Plains, and the higher mountain masses in the Southeast and in the West. In the eastern part, precipitation is fairly well distributed throughout the year, largely as rain.
In the more northerly latitudes, snowmelt produces more water flow per unit of water content than does rain. In the West, most of the precipitation occurs as snow, whereas the summers experience relatively little rain. Frost is common in winter and early spring, especially at times or in places where the snow cover is relatively thin.
Spring floods frequently result from rapid snowmelt. Flood runoff in some areas is often accelerated by the occurrence of hard frozen ground. Excessive rates of melt may also result where the forest cover has been dissipated, exposing the surface soil and its organic layers to wind and sun.
Our watershed investigations and plot observations show marked differences in the occurrence of frost between open and forested areas, between coniferous and deciduous stands, between well stQcked and poorly stocked stands, and between forest and brush cover. In open fields, and in burned, grazed or heavily cut forests, the frost is deeper, firmer, ( «concrete») and more extensive than in relatively undisturbed forest. (Such contrasts tend to disappear as the snow cover attains and maintains depths of 24 inches or more.) Such firm frost presents a distinct barrier to the absorption of rain or snow- melt water by the soil. Consequently, the water rushes rapidly off the surface directly into watercourses, lakes, ponds or swamps. Although frost also enters the soils of well- protected forests, it is commonly shallow, discontinuous, and porous ( «honeycomb» or granular) . The ability of the soil to absorb water and to drain freely, even at near-free- zing temperatures, remains unimpaired, and no surface runoff results from this cause.
Warm Humid Regions
The southeastern and south central regions are characterized by relatively mild win- ters and hot summers and fairly well-distributed rainfall. Inland from the flat coastal plain and its coarse-textured sandy soils, silty or clayey soils predominate. Once depri-
ved of their protective cover, these soils readily erode under the impact of heavy fall rains or the frequent intense summer thunderstorms. Alternate freezing and thawing during winter also dislodge much unprotected soil, especially along streambanks and gully walls. Extensive areas have been severely gullied, as in the Piedmont Plateau and in the mountain, plains, and lowland sections.
Flood damages have become greater and more frequent, sedimentation rates have increased, surface waters have depreciated in quality, groundwater replenishment rates have fallen, and the costs of providing satisfactory drinking and industrial water have risen considerably over the past 25 years. One of many illustrations is the Potomac Ri- ver which flows past the capital of the United States. Siltation is reported to have mate- rially increased since 1853, requiring higher expenditures for water treatment. The an- nual load of sediment which reaches Washington, D. C., has been estimated at approxi- mately 1.7 million tons, equivalent to a loss of 0.11 acre foot per square mile from the crop, pasture and forest lands and the rural roads a_nd highways of the drainage basin.
Much of the fine sediment comes from that portion of the basin which drains the igneous-derived loam soils of the Piedmont region, although the limestone soils of the Shenandoah River Valley also contribute suspended matter to Lhe main tributary.
Sedimentation surveys in South Carolina - a Piedmonl Stale - for example, indicate that during the past 150 years some 3 inches of upland soil have been deposited onto the floodplains. Actually, the loss of soil has amounted to nearer 6 inches, but not all of this has reached the valley floor. Frequent muddy floods have damaged 80,000 acres of corn land and 40,000 acres of pasture. In addition, the value of 180,000 acres of bottomland has steadily depreciated as sterile sandy washes have covered the fertile soils beneath. The average annual rate of sediment accumulation over the past 150 years is estimated at 1.1 acre-feet per square mile. Currently, however, it is closer to 2.2 acre- feet, equaling or exceeding the estimates for large parts of the semiarid regions, e\'en in such areas as the Colorado River and Rio Grande Basins, whe~e the deposilion of sedi- ment is considered a serious menace to costly storage reservoirs and irrigated farm lands.
Arid and Semiarid Regions
Much of the vast terrilory from the Rocky Mountains to the Pacific Ocean receives little rainfall and suHers high evaporation losses. The ratios of precipitation to evapora- tion (from free water surfaces) vary from 4.0 percent to as little as 20 percent or less, in contrast with ratios of 80 to 200 percent in the humid regions.
Rainfall usually comes as intense torrenlial downpours upon more or less poorly ve- getated and often highly erosive soils. Man and nature have conspired to produce formi- dable sedimentation problems as in the Colorado, lntermountain, Rio Grande, and Co- lumbia River basins.
Relatively little is known of the full extent and severity of sedimentation, but investi- gations thus far indicate tremendous variations in sediment discharge. On the main Co- lorado River, for example, rates vary from 0.6 to 0.9 acre-feet per square mile; on the
!arger tributaries, from 0.2 to 1.1 acre-feet; on minor streams as in Arizona and New
Mexico, from less than 0.1 acre-foot to as much as 2.5 acre-feet, andin the Intermoun- tain basin of Utah and Nevada, the range is from practically nil to 6.2 acre-feet per square mile.
Preliminary studies reveal strong differences in sediment production as between the lower and upper reaches of these streams. Thus the discharge of the Colorado River at Lee's Ferry, Arizona, amounts to 1,724 tons per square mile, as against a discharge of only 108 tons from the Upper Colorado at Kremmling, Colorado. Expressed in another way, the Upper Colorado contributes 72 percent of the total runoff at Grand Canyon, but only 22 percent of the sediment, whereas the lower portion above Grand Canyon contributes only 2 percent of the runoff, and 15.6 percent of the total sediment dis- charge.
Striking differences are found even within different parts of individual watersheds, depending on the nature of the soils. Thus the soils from the Chinle formation contri- bute on the average 1.6 acre-feet of sediment per square mile, whereas the rates from the De Chelly sandstone areas amount to only 0.2 acre-foot, or one-eighth as much.
These investigations are narrowing down the sources of the destructive movements of soil and rock material, thus facilitating further study of the basic causes.
Soil and climate are not the only factors involved in sedimentation, however. Re- search by the Forest Service indicates that density of the plant cover, and the manner in which the land is managed and protected also influence sediment production. Locali- zed tests on pinon-juniper and sagebrush lands in western Colorado have shown that
under moderate grazing use erosion rates amounted to an equivalent of 0.3 ton per acre per inch of surface runoff. On overgrazed lands, by contrast, soil losses were equi- valent to 9 tons per acre per inch of runoff. Similar results have been obtained from experiments in other areas. These findings indicate the importance of considering the condition of the plant cover in determining sediment production rates.
lt is highly significant that in some semiarid areas, as in southern ldaho, soils as deep as 10 feet have developed over the past thousands of years on slopes up to 173 per- cent ( 60 degrees). These, stable soils are well vegetated. The potentialities of erosion from such areas, once the protective cover is removed, are literally enormous. Yet, so long as dense vegetation is maintained, the soils will remain in place.
In Utah, desert communities depend for water upon mountain watersheds covered with brush, grass, or forest. These humid mountain slopes may on occasion also become the source of disastrous flood and debris flows. Depletion of the plant growth, even on small areas, has been followed by serious floods and mud-rock flows which have killed people, destroyed homes, and ruined highly productive farms. The mountains are subject to in- tense summer rains falling at rates of as much as 7 inches per hour for 5-minute periods.
Where the vegetation is unduly disturbed, and the soil compacted, as by livestock over- grazing, the water from such rains is unable to enter the surface. Surface runoff then cuts great gashes in the slopes and concentrates rapidly in the steep mountain channels, uprooting trees and huge boulders in its path. On the other hand, no damaging flows and only clear water have come from adjacent watersheds which have been lightly gra- zed and well protected from fire.
W atershed protective and management measures alone cannot stop sedimentation altogether, especially in the more arid regions. Considerable erosion and its by-products must always be expected where the climate is too dry to support a fully effective vege- tative cover and where such precipitation as does occur frequently comes as high inten- sity storms. In addition, we must expect large quantities of sediment in localities where
man-induced erosion, especially mass movement, has so far advanced that the stabiliza- tion of the slopes by vegetative means alone is practically impossible. But, on the great bulk of the forest and range lands, our investigations and experiments provide abun- dant evidence that upstream control can keep much silt out of reservoirs.
Effects of Timber Harvesting
Timber harvesting, among all forestry practices, probably exerts the greatest impact on the functions and interrelations of forested watersheds. Current harvest methods dif- fer radically in many respects from those that prevailed only a few years ago. Today massive tractors and road building machines make formerly difficult lerrain readily accessible, and many rough, steep areas are thus no longer immune to the unstabilizing influences of deep cuts and fills, drastically altered drainage, etc. Tractor Jogging, for example, bares a considerable portion of the Jogging chance. Skid roads occupy 25 per- cent of some unils. W ater percolation into the soil on these roads may virtually be eli- minated. Compaction favors surface runoff, even from light rains or moderate snow- melt, and the gullies that form may not heal for many years.
In a number of cases soils have washed to bedrock following lractor operations, thus permanently changing the hydrological regime. In one area, steepening the grades of short skid roads from 0-20
%
to 21-40%
increased soil movement by 75 percent.These disturbances greatly increased silt rates and stream turbidity, thereby raising the costs of lreating municipal and industrial water supplies.
Because roads can now be built so easily, a tendency exists to build them without reference to an overall plan. An excess mileage of poorly located roads may result, thus accentuating watershed disturbances. A well planned road network on steep Oregon terrain reduced mileage by 11
%
and decreased the proportion of steep grades by 73%-
Moreover, it tripled the area within optimum skidding distance.
Harvesting may be followed by control fires to clean up slash, prepare more favo- rable seedbeds, etc. Although such burns - along with wildfires - are often responsible for watershed damages, recent studies show that where effectively applied, fire is not nearly so serious as was once believed. This statement refers to single burns at times when the forest floor is moist and a reserve of humus is left after the fire.
lt is fortunate that much of the restorative work on commercial forest lands is also helping regain their timber producing capacity. Improved fire control, more effective control of grazing by game and domestic livestock, tree planting or seeding, and such supplemental measures as grass seeding, streambank or channel stabilization and flood- water retarding basins are all helping erase the results of past practices or neglect. Ne-
vertheless, in a number of instances maintenance of the improvements has been a con- tinuing burden because the original disturbances initiated a train of mass movements that can no longer easily or economically be controlled. Especially aggravating are the recurrent landslides and flash floods associated with road and highway construction.
These occurrences have assumed major proportions on mountain and hill terrain throughout the United States. Consequently we can expect heavier damage repair costs in the years ahead.
Research in W atershed Damage Prevention
lt is logical that the second major element in watershed management should be the ,development and application of practices to reduce unnecessary disturbances, thus avoi-
ding future losses and repair costs. Many situations occur where water values are so out- .standing and preventive measures are so obviously called for that the types of precau- tions to be taken are self-evident. Although logging practices have commonly caused
damages, in more recent years our forest industry has repeatedly demonstrated its capa- city to meet strict watershed protection requirements as in the case of harvesting timber -0n municipal water sources.
Examples include the watersheds of the cities of Bremerton, Seattle and Tacoma in the State of Washington, Corvallis in Oregon, Newark in New Jersey, and Waynesville in North Carolina. These operations provide for carefully located and maintained roads and skidways, cutting and skidding away from the streams, carefully chosen stream crossings, some uphill skidding by cable, and the adequate drainage of roads and skid- ways to prevent rapid surface runoff and gullying.
On several Forest Service experimental watersheds we are developing guidelines for timber operators and managers of both public and private holdings. On the H. J. An- drews unit in the Cascade Mountains of Oregon we are seeking efficient methods of managing old growth in mountain watersheds where clean and well-regulated flows need to be maintained. This unit - like others elsewhere - is equipped to measure precipitation, flows, and sediment, among other factors, on both treated and untreated drainages before and after treatments are applied. Similar research adapted to local conditions is underway in the Northwestern and Southeastern States, the Rocky Moun- tains in Colorado and Arizona, White Mountains in New Hampshire, Allegheny Plateau in West Virginia, and the Sierra Nevada in California.
What we have learned so far indicates that it is not usually necessary to close water supply sources in order to assure well distributed and good quality flows. lt is fortunate that the practices that assure the health and stability of the soil commonly maintain the productivity of the site for timber. Thus direct benefits accrue to the timber owner and the wood using industry as well as to downstream water users.
Problems in lmproving W ater Yield
The third major element of watershed management - water yield improvement - is still in its early development stages. lt represents a complex phase since it involves the deliberate planning of cutting or planting or cover conversions to increase or otherw~se improve the supply of u s e f u l water. Our public lands - especially the national forests - are today under the strongest direct pressure for this kind of management, but private lands cannot very well be overlooked because most of the forested water sources - which as a whole receive more than half the nation's precipilation and deliver more than half the nation's runoff - are privately owned.
The effective management of watersheds for improved water supplies requires a knowledge of forest influences and of streamflow responses to precipitation, and the relative stability of given soils and their infiltration, water absorption and storage capa- cities. These factors must be related to each of the major cover types and conditions on the watershed. This information will help the manager appraise the relative values of different portions of each drainage under his control for water production and the pro- spects of changing streamflow yields by the usual or by special practices. To do so, how- ever, the manager should understand the processes of the water cycle.
Major phases of the waler cycle include evaporation and transpiration. Evaporation rates vary with temperalure, relative humidily, wind, and Lhe amount of water on the surface and in the soil Lhat is available for evaporalion. On well-drained sites, coarse- textured soils like sand will lose water by evaporalion Lo much greater depths than finer-textured soils like clays or loams. (Exceplions occur in cases where !arge cracks occur during prolonged dry periods in certain clay soils.) Transpiration rales also vary with local climatic factors but, unlike evaporalion, depcnd on the ability of plants to extract soil moisture. Excluding slreamborder or swamp vegetation, the greater the depths to which roots can penetrate the soil, the grealer Lhe depths from which water available for plant use can be extracted. Thus, upland planls whose rools extend only 2 feet down can draw on available moisture only to that depth, whereas planls with roots 6 feet deep have four additional feet of soil from which to pump moislure.
Forest Service experiments demonstrate that transpiration can be reduccd by felling trees growing on deep soil where the roots extend below the zone of evaporation. By contrast, felling trees on shallow soil - where Lhe roots occur entirely within the eva- poration zone - would have little effect because the gains achieved by reducing trans- piration would be offset by the increase in evaporation. ( Conversely, tree planting on watersheds in central New York State with such shallow soils failed to reduce annual streamflow yields.)
The principles that apply to rainfall zones also apply to snowfall zones. Here, too, research indicates promise both for increasing annual flows and spreading the flows over longer periods, to the advantage of water users, especially in the dry western plains or valleys. Altogether, we have been able to add as much as 30 percent to the previous annual surface water yields on our experimental watersheds in both rainfall and snow- fall climates.
These results by no means permit us to recommend particular types of cutting in given areas for water yield purposes. On the contrary, we have much to learn about the shapes, sizes and patterns of cutting, planting, or type conversion in relation to given facets of local climate, topography, slope, elevation, soil, etc. Research in the heavy snowfall zone of the high Sierra, Rocky Mountains of Colorado, Arizona and New Me- xico and in New Hampshire and the Lake States promises to provide fairly definite guides to such practices in the not•too-distant future.
Applications Not Simple
Manipulating vegetation has its complications. lt is one thing to remove timber or to obtain a given type of cover. lt is quite another thing to maintain it. For example, in some situations water yield considerations alone might indicate the desirability of con•
verting trees entirely to grass. Yet maintaining the grass might be very difficult because of the climate, soil and other factors. Thus, herbs and woody plants - if not the original cover type itself - might return and then water use would rise again, and streamflow decrease accordingly.
Another aspect of a cover like grass is that we will usually need to graze it in order to maintain it. Unless such grazing is carefully controlled the compaction and soil pudd- ling that can otherwise result may lead to highly undesirable conditions of surface runoff, flash flooding and siltation, and also decrease site productivity.
Examples of Current Research
W atershed management research and closely allied soil moisture studies are being conducted by all the nine Forest and Range Experiment Stations in the continental Uni- ted States and by our Research Center in the State of Alaska. Quite representative of the activities under the national program is the research by our Rocky Mountain Station.
Current studies are being conducted on forest, brush, range and alpine lands within the States of Arizona, New Mexico, Colorado, Wyoming, and South Dakota. The pro- gram consists of 14 major projects and 51 studies within these projects.
Virtually all of the projects and studies are interrelated, a consequence of the nature of environmental research. Basic studies include measurement and analyses of precipi- tation, snow behavior, soil moisture, and vegetation characteristics, especially water use and rooting habits. The phreatophyte and alpine snow control studies involved largely research of an applied nature, yet here too basic investigations have opened up in order to pave the way for development of the more practical phases of the work. Studies of watershed instrumentation include such phases as the design or adaptation of field apparatus for estimating evapotranspiration losses of individual trees in s i tu via the infrared gas analyzer ( a modification of the C02 analyzer designed for use on sub- marines), design and testing of a modified Venturi flume for measuring flows of flashy,
sediment-laden streams, and the development of improved techniques for determining the rooting characteristics of woody and herbaceous species.
The Station's program covers the major vegetative types within its region from the alpine to the semiarid zones. Some of the major types, and especially the species that comprise them, need further field examination in order to permit more refined classi- ficatioi:J. for analytical purposes, and ultimately for watershed management application.
For example, in Arizona, spruce is very limited in distribution. The prevailing stands are comprised of ponderosa pine, Douglas-fir, and white fir.
The pinon-juniper type varies sufficiently to warrant distinctions according to clima- tic characteristics, elevation differences, species differences, and plant associations.
Similarly, the chaparral type exhibits considerable variation throughout the region.
The phreatophytes extend across all major types from the semidesert plains to the higher elevations, although presently the water-loving growths at the lower elevations in Arizona are causing the most concern to water users and are therefore receiving the most attention.
The alpine snowfield research is presently located along the Continental Divide near Loveland Pass in Colorado, at elevation 11,500 feet, and on the slope of Mt. Evans, also in Colorado, at 12,500 feet elevation. The latter area is accessible only in summer.
Heavy winds and snow make the installation of fences for controlling snowdrift, or at least the maintenance of such fences, very difficult.
Role of W atershed Research in Multiple Forest Use
As the science and art of manipulating watershed cover for improved water supplies progresses, the tendency may be to overemphasize this aspect of forestry at the expense of others. A potential field for such specialized management grows out of the expansion of crop and pasture irrigation in the humid parts of the United States. Landowners interested solely or primarily in the returns from such lands may be inclined to sacrifice any timber values their woodlands may possess if in their judgment they can earn more net income from their enterprise by clearing the woods completely or otherwise drasti- cally modifying their character to promote the maximum runoH into their irrigation works. Such tendencies will need tobe watched carefully, in order that the high produc- tivity of timber lands on the richer and deeper soils may not unnecessarily be lost.
This will call for careful step-by-step development of research findings and equally thorough analysis of the economic and social relationships among the several uses to which forest land is susceptible.
lt cannot be too strongly emphasized that behind the specific measures for repairing or preventing damages or improving water supplies must be basic research. This phase calls for soundly thought out programs of observations, measurements and tests on plots and watersheds and in specialized laboratories. W e will also want to manipulate the plant cover in different ways to obtain more refined data. Manipulations will involve such treatments as the deadening of shrubs under a timber stand, the complete felling
of forest growth where the water table is close to the surface, as along streams, strip clear- cutting or selective cutting on slopes, and other ways of testing the streamflow impacts of given measures.
After we have determined what practices are likely to yield the maximum water ef- fects, we will be in a strong position to find out how such practices harmonize with those designed solely or primarily for timber production. ( Of course, the effects on fish, wildlife and recreation will have to come into the picture to an increasing extent.)
Here then are goals of multiple forest use which when fully recognized will provide strengthened incentives for research - both theoretical and empirical - as weil as for modern forest practice. In view of present trends towards more intensive forestry and the corresponding need to resolve or forestall serious conflicts among alternative uses, we can be confident that the needed research will be forthcoming. W atershed research is pointing the way by throwing light on the soil and water aspects of different prac- tices, thus permitting closer evaluations of their effects and interrelationships.
Einige Aspekte der Forschung über die Bewirtschaftung von Einzugsgebieten in den Vereinigten Staaten
(Zusammenfassung)
Der Autor betont die Notwendigkeit einer eingehenderen Erforschung aller Fak- toren, welche den Wasserhaushalt eines Einzugsgebietes beeinflussen, insbesondere der Wechselbeziehungen zwischen Boden, Vegetation und Wasserkreislauf unter verschie- denen Bewirtschaftungsmethoden (Intensität der Hiebführung, Transportmethoden etc.). Derartige Untersuchungen erhalten ständig größere Bedeutung, einerseits infolge der wachsenden Beanspruchung bewaldeter Gebiete für verschiedene Zwecke ( H olzpro- duktion, Wasserproduktion, Erholung etc.), anderseits weil die Korrektur früherer Miß- wirtschaft immer schwieriger wird, sich aber wegen der unregelmäßigen W asserfüh- rung, Überschwemmungsgefahr und Geschiebeführung vieler Wasserläufe mehr und mehr aufdrängt. Von steigendem Interesse sind besonders die Untersuchungen, welche abklären sollen, wie der Wasserertrag eines Einzugsgebietes erhöht werden kann.
Quelques aspects des etudes faites aux Etats-Unis
concernant le mode de traitement forestier
a
adopter dans les bassins de reception des cours d'eau(Resume}
L' auteur souligne la necessite d' etudier tous les facteurs qui ont un ef Jet sur le regime des eaux d'un bassin de reception, en particulier les correlations existant entre le sol, la vegetation et la circulation de l' eau lors de l' application de traitements dif f erents ( In-
tensite des exploitations, methodes de transport, etc.). De telles recherches revetent une importance croissante d'une part a cause des nombreux services que l'on attend des forets ( production du bois, approvisionnement en eau, lieu d' agrement, etc.), d' autre part a cause des dif ficultes inherentes a /,a reparation d' erreurs passes ou il faut chercher generalement la cause des crues extraordinaires et des inondations. On s'interessera particulierement aux etudes tendant a preciser comment le produit qui peut etre retire de l'utilisation des eaux d'un bassin de receptwn peut etre augmente.
Alcuni aspetti delle ricerche sulla sistemazione di bacini imbriferi negli Stati Uniti
(Riassunto)
L' Autore sottolinea /,a necessita di una maggiore indagine di tutti i fattori ehe con- dizionano il regime idrico di un bacino imbrifero e specialmente le interdipendenze tra terreno, vegetazione e circo/,azione idrica con diversi sistemi di governo ( intensita dell'esecuzione dei tagli, nuitodi di trasporto, ecc.). L'importanza di queste ricerche au- menta continuamente, sia a causa del crescente sfruttamento, a scopo diverso, delle su- perficie ricoperte di bosco ( produzione di legna, di acqua, turismo, ecc.), sia a causa delle difficolta sempre crescenti ehe si incontrano nel/,a correzione di precedenti errati sistemi di governo, resa sempre piu impellente, per l'irregolare conduzione delle acque e il pericolo da alluvioni e il materiale ehe molti corsi d' acqua possono convogliare. Di interesse sempre maggiore sono specialmente le ricerche per chiarire il modo di accre- scere la produzione di acqua di un bacino imbrifero.
