Representative basins should be selected so that they contain features of the ground-water and surface-water régimes that typify certain areas, The introductory chapter of the Unesco guide on iesearch in representative and experimental basins (Unesco, 1970) includes a definition of representative basins.
As stated previously, the ground-water régime is formed under the complex interaction of a number of natural and artificial factors, and it is not always possible to establish the effect of any individual factor on the régime. Thus, when subdividing an area according to the conditions under which the ground-water régime was formed, the principal régime- forming factors must be identified. These factors may characterize both specific climatic and ground-water environments. The factors influencing the principal features of a ground-water régime include:
1. Excess of precipitation over evaporation determining the magnitude of the opportunity for ground-water recharge.
2. Radiation (thermal) balance, reflecting variations in seasonal and average annual air temperature, and therefore variations in such critical conditions as freezing in the zone of aeration.
3. Lithologic composition of the porous earth materials in a vertical sequence, beginning with the properties of the soil cover at the land surface and extending downward through the zone of aeration and through the beds of interest in the saturated zone.
4. Thickness of the zone of aeration.
5. Over-all topographic relief, which establishes the geometry between the ground-water 6. Geomorphological features of the terrain, including the micro-relief.
7. Régime of surface streams and water bodies.
8. Ground-water leakage between adjacent aquifers.
9. Character of vegetation cover.
The degree to which any one factor can be observed varies. Thus, variations in climatic conditions can be recorded only over large aieas, whereas changes in the thickness of the zone of aeration or in geomorphologic features may be observed over relatively small areas, even within a single representative basin. Therefore, when selecting such basins, the more c o m m o n factors controlling the ground-water régime, and the degree to which each can be observed, must be taken into account. The remaining factors that exercise control on the ground-water régime over comparatively smaller subareas may be consider- ed at a later stage when determining the distribution of an observation network.
intake and discharge areas.
Figure 4 shows features relating to some of these controlling factors.
Recharge areas for the ground-water reservoirs are commonly situated near the water
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Ground- water studies
FIG. 4. Features of interrelation between unconfined and confined ground waters (-
- -
= water table;
-
.-
.-
= piezometric level in upper confined aquifer;-
. .-
. . = piezome- tric level in lower confined aquifer).divides and discharges areas are located in the liver channels. The design of ground-water hydrological networks was discussed in a symposium held at Quebec in 1965 (WMO- IASH, 1965).
A ground-water province is the largest unit considered in selecting representative basins. Three such provinces characterized by markedly different factors affecting the formation of the unconfined ground-water régime will be discussed.
T h e first province is characterized by an extremely short summer period of ground- water recharge and by freezing of ground water in winter. Such a régime of unconfined ground water, with periodic transition from solid into liquid state, is characteristic of almost all Siberia, northern Canada and Alaska.
The second province is characterized by seasonal freezing of the zone of aeration and by spring and autumn recharge of ground water. The presence of a frozen zone of aeration during the winter months prevents any significant recharge of ground water during this period. Winter precipitation accumulating in the form of snow and ice infiltrates in the spring. This is the major type of ground-water recharge in this province. In a number of places there may be a shorter but more significant autumn stage of ground-water recharge associated with infiltration of autumn precipitation.
The third province is characterized by the absence of freezing and by annual ground- water recharge mainly in the winter (in subtropics, deserts and semi-desert regions) or in rainy seasons (in equatorial regions, tropics and savannahs).
The boundary between the first and second provinces coincides with the permafrost boundary, and the boundary between the second and the third provinces with the northern boundary of the zone characterized by the absence of seasonal freezing of the ground.
Each ground-water province can be further subdivided on the basis of natural recharge, identifying zones of abundant, moderate and poor recharge to the ground-water reservoir.
As an example, consider such subdivision in the second type of ground-water province previously discussed.
The zone of abundant ground-water recharge coincides with the tundra and forest environments in which there is an excess of precipitation over evaporation. In this zone the coefficient of water balance (Kostyakov, 1933), which is (l-Kn)P/Z, is greater than or equal to 1.3, where Kn is the coefficient of surface run-off-that is, the proportion of the average precipitation (P, in mm) which is discharged as eurface run-off-and Z is evaporation in m m .
Selection of areus Jbr detailed stridies
The occurience of ground water at shallow depths characterizes this zone; ground water is, therefore, closely associated with meteorological factors. The discharge from the unconfined ground-water reservoir in this zone exceeds the evaporation and the ground water is commonly fresh.
The zone of moderate ground-water recharge is located within the boundaries of the forest-steppe and steppe environments with intermittent recharge. The coefficient of water balance ranges from 0.5 to 1.3. Discharge from the unconfined ground-water reservoir in this zone and evaporation from the water table are commonly about equal, therefore both brackish and fresh ground-water may be encountered.
The zone of poor ground-water recharge is found within arid steppes, deserts and semi-deserts. The coefficient of water balance is less than 0.5. Evaporation is the dominant factor jn the ground-water balance foi this zone and thus brackish and saline waters commonly occur.
The zone of abundant recharge willinclude tropics and moist subtropics where. ground waters are recharged abundantly for a considerable part of the year and the waters are mostly fresh. The zone of moderate recharge includes savannah and subtropics; the zone of poor recharge includes deserts and semi-deserts where replenishment is not constant or occurs only occasionally, following droughts. In these latter zones ground water is commonly recharged at the expense of surface run-off, and infiltration takes place to depths from which evaporation is practically absent.
Aseas in which the unconfined ground-water reservoirs reflect different degrees of drainage may be defined. Areas of poor drainage m a y be characterized as ranging in elevation up to 250 m, and eroded by stream systems to depths of 50 m , that is, the relative difference in elevation between the watershed and the local drainage base level may be as much as 50 m. Well-drained areas ate characterized by elevations ranging from 200 up to 500 m and by local depths of erosion of as much as 150 m. Highly drained areas reflect elevations exceeding 500 m and local depths of erosion of more than 300 m .
The orographic criteria used in classifying ground-water drainage situations agree with certain geologic-tectonic environments. Thus, poorly drained areas tend to coincide with depressions, saddles, troughs and other synclinal structures, well-drained areas with elevated sections of shields and anticlinal structures, and highly drained with mountain- folded areas. Within each of these three broadly specified areal types, ground-water provinces may be defined.
Watersheds may be characterized by different degrees of hydraulic connexion between ground and surface waters and by different régimes of base-flow. C o m m o n patterns of interrelationship between ground and surface waters include :
1. Aquifers with permanent hydraulic connexion with a stream;
2. Aquifers with no hydraulic connexion with a stream;
3. Aquifers with an intermittent hydraulic connexion with a stream.
C o m m o n régimes of base-flow include: (a) a stream in which base-flow stems from dis- charge from one aquifer; (b) a stream draining several aquifers, each being characterized by different degrees of hydraulic connexion with the stream; (c) a stream with base-flow derived from both unconfmed and confined ground waters.
An important criterion used in classifying areas with respect to the occurrence of ground water is the lithologic composition of the water-bearing formations, as, for example, sandy, calcareous or sedimentary rocks. This has a considerable effect on the processes of recharge and water movement and is thus reflected to some degree in the character of the ground-water régime as displayed by water-level fluctuations.
Representative basins for detailed studies of the ground-water régime and balance should be chosen so that typical ground-water conditions can be investigated. In other words, such a basin should be selected with due regard for the manner in which it can be subdivided into areas typifying specific conditions of ground-water occurrence and specific types OF water balance. Within many countries almost all gradations of the
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Ground-water studies
previously described environmental conditions may be found, whereas in countries of small area only a limited range of conditions may be available for study.
The role of artesian basins in the formation of the ground-water régime, in the base- flow of streams, and in the determination of water balance, requires selection of Foine representative basins large enough to embrace areas of recharge to, movement through and discharge from the largest artesian basins.
W h e n selecting representative basins in mountain areas, altitude and its relationship with other factors affecting the ground-water régime, the water balance and the conditions under which base-flow occurs must be considered. W h e n selecting IHD representative basins the choice should favour those which have not been affected significantly by man’s activity and wheie the natural ground-water régime prevails. The detail with which the selected representative basins can be studied willvary, but it is desirable for the basins to include areas which are of practical significance. For studies of artificially modified conditions and h o w they influence the ground-water régime and water balance, special experimental basins are established which include such features as pumping stations and typical tracts of land under irrigation and drainage.
Representative basins selected to display a natural or undisturbed water régime are subdivided into two types, namely, type I requiring intensive water-balance investigations, and type II requiring only limited investigations.
Representative basins of type I are commonly small in area (not more than 300 km2), and are subjected to detailed study of all the components of water balance, including run-off, precipitation, snow-cover, evaporation from soil and water table, ground-water régime, soil moisture, depth of freezing and other elements of water and salt balances.
Repiesentative basins of type II are larger in area (up to 1,000 km2) and are studied primarily with respect to run-off, precipitation, and snow accumulation. Ground-water observations ale made at individual localities for estimating relative changes in the amount of base-flow. For all basins selected to represent certain ground-water conditions, a ground-water balance should be prepared as a component of the total water balance.
In addition to representative basins for detailed studies of the ground-water régime, zonal IHD stations may be selected. The design of observation networks and the methods of operation of such stations are also discussed in Representative and Experimental Basins
(Unesco, 1970). The choice of zonal stations is determined mainly by the nature of the climatic and ground-water conditions that typify the zone. The choice does not depend on the presence or absence of a drainage network (surface run-off); in other words, the total water balance and ground-water balance may be prepared for a tract of land or an interfluve characteristic of a certain climatic zone, and not for aD entire river basin.
The majo, scientific purposes to be served by IHD stations located in typical physico- geographic areas include :
1. Study of the general principles of formation of various ground-water régimes which are connected with rivers and surface-water bodies and which are of practical im- portance (water supply, irrigation, drainage).
2. Establishment of the genetic relationship between régime elements (water level, yield, water temperature and chemical composition) and principal régime-forming factors (meteorological elements, surface run-off, artificial reservoirs, vegetation, soils and lithology) leading to the forecasting of both natural and artificial changes in the ground-water régime.
3. Use of obseIvationa1 data derived from the ground-water régime to determine hydraulic parameters of the porous materials within the zones of aeration and saturation for making quantitative estimates of elements in the ground-water balance.
4. Quantitative estimation of seasonal, annual and long-term variations in ground-water resources.
5. Study of the inteirelationships between unconfined and confined ground water, and surface iun-off.
Selection of areas for detailed studies
In addition to the foregoing regional and scientific aspects of surface and ground-water studies, the data collected at IHD stations willserve many practical water-development purposes associated with irrigation, drainage and use of the water resources for public and industrial supply.
The general purpose of ground-water studies in experimental basins is special studies of specific problems that might include: defining the nature and rate of movement of ground-water pollutants; the change in amount of ground-water replenishment during exploitation or artificial recharge; the influence of irrigation or drainage practices on the ground-water régime and water balance; and the changes in ground-water quality which in turn relates to the degree of improvement of the land.
Successful resolution of the many purposes to be esrved by detailed ground-water studies in the selected areas requires basically the quantitative evaluation of the role of the ground-water régime in the over-all water balance of the basin. Such an evaluation inevitably entails the synthesis, analysis and tabulation of a substantial mass of geologic and hydrologic data (Konoplyantsev et al., 1963, 1964).
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Selection of areas foi detailed studies
Refer ences
KONOPLYANTSEV, A. A.; KOVALEVSKY, V. S. 1964. Principy razmescenij nabludateljnoi seti dlj izuceng estestvennogo rezimu podzemnyh vod (Metodiceskie ukazung) [The principles of locating an observation network for studying the natural ground-water régime]. M o s c o w , Izd. Vsegingeo.
_ _ -
; SEMENOV, S. M. 1963. Estestvennyi rezim podzemnyh vod i ego zukonomernosti [The natural ground-water régime and its features]. Moscow, Izd. GosgeoltekhiLdat.KOSTYAKOV, A. N. 1933. Osnovy melioratsii [Fundamentals of land reclamation]. M o s c o w , Selkhozizdat.
WMO-IASH. 1965. Symposium: Design of hydrological networks, vol. 2. (IASH publ. 68.) UNESCO. 1970. Representative and experimental basins. An international guide for research and
practice. Paris, Unesco. 348 p.
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