The term ground-water régime is used in the following discussion in its broader sense to refer to the general pattern of occurrence of ground water. Thus, it must necessarily include reference to the physical framework in which ground water occurs as well as to the hydrologic balance that results from natural and imposed recharge and discharge on a ground-water system.
Logically, a discussion of the ground-water régime should begin with a brief résumé of the origin of ground water so as to clarify the relationship of this resource to other modes of water occurrence.
1.1.1
Origin of ground water
According to Meinzer (I 923), ground water may be classified as being of either internal or external origin. Internal water (juvenile water) is derived from the interior of the earth as a new resource. External water is derived from atmospheric or surface water and m a y be trapped in rocks at the time the constituent material was deposited (connate water), or it may be absorbed into interstices some time after deposition (absorbed water).
For practical purposes, juvenile and connate water can be regarded as being largely academic curiosities. In rare instances they may contribute small quantities of minerals that might affect the local quality of the ground-water resource, but the volumes of water yielded by these sources are inconsequential relative to the volumes of water absorbed at the land surface. For that reason, the following discussion of the ground-water régime is based on the tentative assumption that the existing ground-water resource was initially absorbed from atmospheric or surface-water supplies (meteoric water) and that this resource is currently being recharged wholly from these sources. All other terrestrial and extraterrestrial sources of water can be safely ignored.
1.1.2
Hydrological cycle
Virtually all waters comprising a part of man’s environment are continuously being recycled in response to the forces exerted primarily by the sun’s energy and by the earth’s gravity. Water at or near the surface of the lithosphere tends to move upward into the atmosphere through processes of evaporation and transpiration. This moisture eventually falls back to the earth’s surface through processes of condensation and precipitation. This continuous recirculation of water is referred to as the hydrological cycle. Figure 1.1.2 is a
Growid-water studies
schematic representation after Parker et al. (1955). Detailed descriptions of the processes involved are presented by Meinzer (1923, 1942), Savarensky (1933), Lange (1950), Wundt (1953), Ovchinnikov (1955), Linsley et al. (1958), Todd (1959), Leopold and Langbein (1960), Keller (1961) and Schoeller (1962).
FIG. 1.1.2. Schematic representation of theLhydrologica1-cycle.
In this continuous cycle of water movement, the oceans provide the principal source of water, the atmosphere functions as the mover, and the land receives the primary benefit. Of the water that falls on the land, a part immediately returns to the atmosphere through evaporation, a part remains on the land surface, and a part enters the ground.
Water remaining on the surface contributes directly to the local surface-water régime.
Water entering the ground initially must generally satisfy the moisture-holding capacity of materials in the unsaturated zone before any increment is added to the ground-water régime. Thus, two gciieral modes of occurrence of water on and within the lithosphere are directly attributable to the circulation of water in the hydrological cycle. They are surface water and subsurface water. The latter includes water in both the unsaturated and saturated zones.
1.1 pugc 2
Principal features of the ground-wuter régime
1.1.3
Surfuce wuter contrasted with ground wuter
M a n is understandably most familiar with the surface-water régime as this resource is everywhere exposed to his direct observation. Even the untrained observer is generally aware of temporal and spatial variations in both quantity and quality of surfacewater supplies and commonly relates the origin of this resource to precipitation. Similarly, man’s use of surface water, the problems associated with its development and conservation, and the legal considerations that arise therefrom generally can be appreciated, if not fully understood, by the layman.
In contrast, water entering the ground passes out of direct contact with man’s senses and immediately assumes an aura of mystery to the non-technician. He may retain some intuitive comprehension of the occurrence of moisture in the soil‘ veneer and its relation- ship to precipitation or irrigation, but he has virtually no innate understanding of the movement of water through the zone of aeration and into the saturated zone. Consequent- ly, he cannot appreciate the general conditions of occurrence of this potential resource, nor the technical and legal problems associated with its location and development.
Partly because of man’s tendency to specialize and partly because of the marked contrast between the surface-water and subsurface-water environments, most workers in the past have concentrated their attention on one of these modes of water occurrence with only cursory reference to the other. As a result, the surface-water and subsurface- water régimes have commonly been treated as being separate and distinct. It should be stressed, therefore, that the two régimes are hydrologically interconnected and that they function together to form an integrated dynamic system in which water commonly moves alternately from one environment to the other on its way back to the oceans.
It is true that the two régimes are dissimilar in many respects, but because of their hydro- logic interdependence, it is technologically incorrect to treat them separately, except for purposes of classification, when evaluating the water resources of an area. This point is crucial to an understanding of the ground-water régime and willbe emphasized repeatedly in the following pages.
1.1 page 3
Principal features of the groicnd-water régime
1.2
Concept of the ground-water régime
In visualizing the ground-water régime the reader must be generally familiar with the divisions of subsurface water (Fig. 1.2a), and the principal modes of its occurrence (Fig.
1.2b). H e should also understand the definitions of certain basic terms which have been included in the Appendixes. To strengthen the preceding requisites, however, the reader must understand the geologic framework in which ground-water occurs and the hydraulics of ground-water movement. Discussions of these latter subjects are given in Chapters 2 aiid 3.
The concept of the ground-water régime is based on the fact that the local occurrence of ground water is not merely a product of chance, but the consequence of a finite com- bination of climatic, hydrologic, geologic, topographic, ecologic and soil-forming factors that together form an integrated dynamic system. These factors are interrelated in such a way that each provides some insight into the functioning of the total system and thus serves as an indicator of local conditions of ground-water occurrence. It is possible, therefore, to evaluate the general potential of an area for ground-water development by appraising as many of the factors listed above as practical and then by interpreting the
Land surface
FIG. 1.2a. Diagram showing-divisions of subsurface water (after Meinzer, 1923).
I .2 page I
Gr6iir~E- water stirdies
local régime on the basis of known relationships among the factors and their eKect on the régime. The fundamental theory and field techniques for accomplishing this are discussed primarily in Chapters 5 and 6.
Most important among the preceding relationships are the physical characteristics of the framework in which the ground-water system occurs, the balance between ground- water recharge and discharge and the consequent hydrologic and lithologic implications that m a y be drawn, and the relationship among factors affecting the movement of ground water from the point(s) of recharge to the point@) of discharge. The manner in which water balances are drawn for ground-water systems is discussed primarily in Chapters 7 and 8.
Transpiration (xerophytes)
Transpiration (phreatophytes)
, - . . . .
-
I . . ,I ---*
. Unconfined aquifer . . .
. .
. . .
.
-. ' C ' O N F I N E D A Q i I F E R , *
.
-.* .
. .
.
b . , . . G - R O U N D . W A T E R
. . . . .
FIG. 1.2b. Schematic cross-section showing occurrence of bround water.
1.2 pugc 2
Principal feutures of the ground-water régime
Obviously the ground-water régime must be viewed as a dynamic system in which water is absorbed at the land surface and eventually recycled back to that surface.
Water movement is through interconnected openings in the rocks that mantle the earth.
Thus, the framework in which ground water occurs is as varied as those rocks and as intricate as their deformation, which has progressed through geologic time. The possible combinations of variety and intricacy are virtually infinite and embrace most geologic disciplines, leading to the unavoidable conclusion that ground-water investigations at a given site almost always exhibit a certain uniqueness not completely amenable to wholly generalized prescriptions for their execution.
Ground water may be visualized as occurring in a subsurface reservoir, the boundaries of which are formed by adjacent less permeable or impermeable rocks. The reservoir may be open everywhere to the land surface (unconfined), or it may be capped in large part by impermeable 01 relatively impermeable rocks (confined). It may cover an extensive area or represent only an elongate ribbon of sand deposited in an ancient stream channel.
Matet ials forming the reservoir may be composed largely of unconsolidated sediments or of bed-rock; they may be uniformly permeable or vary widely in permeability both horizontally and vertically.
1.2.1
In developing an estimate of balance between recharge to and discharge from a ground- water régime the general manner in which that régime functions must be identified. The potential for recharge to the ground-water iégime in an area depends on the amount and pattern of annual precipitation in relation to the potential for evaporation and to the occurrence of any surface or subsurface inflow from adjacent areas. Most of this potential recharge is commonly intercepted by the soil veneer and eventually returned to the atmosphere through processes of evapotranspiration or dissipated though surface run-off. The amount that actually contributes to gi ound-water recharge varies seasonally and from year to year. Although perhaps difficult to quantify, it generally represents a comparatively small part of the total potential for recharge. Similarly, ground-water discharge may be difficult to quantify because of temporal variations, especially if it occurs at a number of scattered locations, either at the land surface in the form of springs, gaining streams, lakes, ponds, marshes or growths of phreatophytes, or at depth through permeable formations.
Thus the relationship between recharge and discharge under natural conditions is often obscured and not readily apparent from field observations. However, useful guide-lines that are generally applicable to specific problem areas can be drawn from the practical experience accumulated through &ld study of ground-water régimes in a variety of natural environments. These guide-lines may be sufnmarized as follows:
1. In relatively undeveloped areas long-term average discharge from a ground-water reservoir can be presumed to be in equilibrium with long-term average recharge.
2. As a consequence of 1, it follows that a large volume of ground-water discharge at the land surface is proof of corresponding high recharge to the system.
3. The potential for recharge in an area as determined, for example, by observing pre- cipitation, should not be confused with actual recharge. The two are telated only in that actual recharge cannot exceed the potential for recharge. Thus, desert areas characteristically receive low recharge because of low potential for recharge, but humid areas do not necessarily receive high recharge unless rocks underlying the land surface are highly permeable.
4. Surface-water discharge represents both run-off and ground-water discharge. The latter can be approximately equated with the low-flow natural discharge less inflow of streams not originating in the area.