General concepts
3.2.2 Climatic factors
3.2.1
Dynamic factors influencing ground-water régime
General concepts
The concept ‘ground-water régime’ is defined in terms of the nature of the basic conditioris of and the changes in the quality, quantity and physical properties of ground waters in space and time. Study of the ground-water régime leads to an understanding of the origin and occurrence of ground water and its movement under natural or disturbed conditions. Within a given porous medium, ground water and its dissolved solid and gaseous constituents are influenced by a complex of interacting régime-forming factors.
Chapter 2, on ground-water fundamentals, summarized the characteristics of the rocks-the lithosphere-which constitute the framework in which ground water occurs and through which it moves, and the physical laws or principles governing that occurrence and movement. The framework does change with time, as is shown by geologic history, but the change is so slow that for practical purposes it is regarded as a static factor in the ground-water régime.
The fresh water on the land masses of the earth is replenished by precipitation. The water from precipitation at any point may accumulate on the land surface, infiltrate into the ground, or run off overland. Water may accumulate on the surface as snow or ice, or in ponds, lakes, marshes, etc.; it is vulnerable to evaporation and may return to the atmosphere, or it may disappear by infiltration into the soil. Soil moisture may exist in liquid, vapour and solid forms, and may be depleted by transpiration and evaporation;
where there is more water than the soil can retain, the excess percolates downward to be retained by capillarity or to become ground water. Ground water may be recharged in this way and also in some places from streams, lakes, ice fields and other surface accu- mulations, including artificial reservoirs and devices. The ground water accumulates, moves in the direction of decreasing head, and may ultimately be discharged into lakes or stream channels or oceans, or at the land surface by springs or seeps, or by evaporation and by the action of phreatophytes.
The ground-water régime-in the zone of saturation-is seen to be influenced by factors falling under several basic categories : climatic (relating to the atmosphere and particularly to the water crossing the interface between atmosphere and lithosphere)
;
surface-water hydrologic (relating to streams and lakes); biologic-soil (relating to water in the zone of aeration, inluding soil moisture); geologic (relating to changes in the lithosphere) ; and artificial (relating to activities of man).The period over which these five factors operate varies and this can be used as a means of further classification : episodic (short period influence, as for example earthquakes) ; day- or hour-long (changes in atmospheric pressure and air temperature, and tidal influences); long-term (atmospheric circulation and solar activity) ; and secular (changes due to tectonic influences and to long-term meteorological factors).
3.2.2
Climatic factors
Climate is a dominant factor determining the relative abundance or scarcity of fresh water on the land masses of the earth. The contrasts in climates of various regions have long been recognized-tropic, temperate or arctic, oceanic or continental, arid or humid -and several climatologists (Thornthwaite, 1948) have devised methods for quantifying the climate factor and classifying climates accordingly.
Climate is the synthesis of weather, which at any locality varies from hour to hour, day to day, season to season and year to year, as shown by meteorologic elements that
3.2 page I
Groiind- water stiirlies
are customarily measured : precipitation, temperature, atmospheric humidity and pressure, wind velocity, and pan evaporation. The World Meteorological Organization has defined the term ‘normal’ as the annual, seasonal, monthly or other periodic averages computed for a period comprising at least three consecutive ten-year periods. With this statistical base, meteorological observations may be reported as measured and also as departures from the ‘normal’ for the locality.
The concept of ‘water balance’, determined by the difference between the average annual precipitation and the average amount of water needed for evaporation and transpiration (the ‘potential evapotranspiration’) is used in several classifications of climate (Thornthwaite and Mather, 1955). Potential evapotranspiration is not measured, but, because it is a function of solar energy, numerous empirical techniques have been developed to estimate it, and these give a wide range of answers. Water-balance maps, even though rough approximations, indicate the climatic norm as set by such relatively permanent factors as solar radiation, distribution of land and water masses, land relief and ocean currents. The balance is negative in areas of perennial water deficiency, and in such areas the natural vegetation, wildlife, and human activities have developed in response to a water-scarce environment. Such development relies wherever possible upon inflows from areas of perennial surplus, regulation and use of seasonal or occasional local surpluses, and depletion of ground-water storage for temporary benefit. A positive water balance yields a perennial surplus and human activities are adapted to the environ- ment of water abundance, counting on the surpluses for various uses and fol carrying away wastes, and managing them to overcome occasional deficiencies and to protect against damaging floods.
The ground-water régime is actively influenced by change with time of the following meteorological factors: precipitation, evaporation, air temperature and atmospheric pressure.
3.2.2.1 Precipitation
Precipitation is, obviously, one of the principal factors influencing the ground-water régime. Infiltration of precipitation increases soil moisture and may subsequently cause a rise of the ground-water level and some changes in ground-water chemical composition and temperature. The time required for infiltrating precipitation to reach the water table increases with increasing depth. The rate of evaporation from the water table decreases with increasing depth. The graphs given in Figure 3.2.2.1 demonstrate the dependence of changes in infiltration and evaporation on the thickness of the zone of aeration in a typical ground-water situation. The character of the curves can change considerably, depending upon the lithology of the zone of aeration and the climatic zone, but the general feature of increase in lag time for water reaching the water table and decrease in evaporation with depth, remains valid. The amounts of precipitation that infiltrate to the principal ground-water body depend also on a number of other factors, including morphological structure, which affect the proportion of surface run-off and character of the surface vegetation (forest, grass, cultivated crops, etc.), as well as intensity of precipitation.
Experimental and theoretical investigations carried out by Bindeman (1957, 1960) led to the establishment of a general expression relating the dependence of rates of infiltration from precipitation to the rain intensity, the degree to which the pore spaces are not saturated and the filtration coefficient of the rock materials in the zone of aeration.
This expression has the form :
1 a
w = i I , -I- ,
3.2(1)3.2 pugc 2
Defining the ground-water régime
where :
W is rate of infiltration or percolation;
i is intensity of precipitation or application of water to land surface;
O is porosity of the material in the zone of aeration;
Vo is specific retention: water remaining in the soil which does not drain under the influence of gravity;
K is hydraulic conductivity of the porous rocks in the zone of aeration;
go is initial moisture content of the soil;
CL is an empirical coefficient defined through laboratory experiment and related to the extent of surface coverage with a layer of water and to the size of raindrops and of soil pores (O 4 a 4 1).
FIG. 3.2.2.1. Relationship between recharge to and discharge (by evaporation) from unconfined ground water and the depth to water.
?.2 page 3
Ground- wuter studies
The longer the period of rainfall the deeper the water percolates into the zone of aeration. The end of a period of rainfall may result in disturbance in the sense of a slowing down or cessation of the moisture percolation process. Therefore, the best conditions for ground-water recharge ai-* those of shallow depths to the water table and prolonged rainfall periods. Such conditions usually exist in spring at the time of snowmelt, and after prolonged precipitation. Amounts of precipitation and their distribution within a year are closely related to the diversity of climatic conditions over the area.
An analysis of observation data on the ground-water régime shows that the most intensive percolation occurs as a consequence of winter precipitation which accumulated on the land during the winter period (in areas where the zone of aeration freezes) and percolates downward mainly during the spring thaws forming the spring peak of ground- water levels. In areas where the zone of aeration does not freeze, infiltration takes place more often during the winter period or the rainy season. In many areas some part of the winter precipitation infiltrates into the soil, but not all this amount reaches the water table inasmuch as some moisture may be needed to satisfy any soil-moisture deficiencies, some may be lost by evapotranspiration, and some may be discharged as intermittent seepages from temporary perched zones of satmation above the water table.
Precipitation may influence the chemical composition of ground water. Precipitation in humid regions (temperate zone and tropics) is generally less mineralized (10-25 mg/l) than in arid regions (up to 100 mg/l and more). In most cases, ground waters are diluted by infiltrating precipitation and their dissolved-solids content decreases during periods of natural recharge.
3.2.2.2 Evaporation
Moisture evaporates both from the land surface and from the zone of aeration (including evaporation from the capillary fringe). The magnitude of evaporation depends on a number of factors (air humidity, wind velocity, air temperature, solar radiation, roughness and colour of the land surface, plant associations, etc.).
Unfortunately there is no accurate method for determining evapotranspiration which takes into account evaporation directly from the water table, evaporation of precipitation detained by vegetation, and transpiration of moisture from the zone of aeration and from the water table. Estimates of evapotranspiration obtained with the help of soil tanks or lysimeters are commonly low. Determination of evapotranspiration by means of finite-difference equations is dependent upon the accuracy of the permeability coeffi- cients found for the rocks of the plot under investigation. Various scientists have devel- oped equations for approximating evapotranspiration.
Along with precipitation, evaporation is a factor that may strongly influence locally the chemical composition of the ground-water régime, Water-quality observations have shown that changes in water composition are closely related to the behaviour of the water table under certain circumstances : the maximum total dissolved-solids content of ground water in most cases is observed at a period of the lowest position of the water table. The annual range of the total dissolved-solids content of ground watexs in the U.S.S.R. increases in a southerly direction from about 100 to 150 mg/l in the north to about 3,000 to 42,000 mg/l in the south. The régime for individual components of chemical composition of ground waters changes conformably. Fluctuations in total dissolved-solids content of ground water in the northern regions of the U.S.S.R. are predominantly due to changes in the ion content of bicarbonate, calcium and magnesium.
In the southern regions they are due to changes in ion content of chloride, sulphate, sodium and calcium. Magnitude of the seasonal fluctuations in the total dissolved-solids content is directly proportional to the values of the total dissolved-solids content.
Seasonal changes in the chemical composition of ground water also attenuate with increasing depth below the earth’s surface. In noith-west Europe and the humid tropics seasonal changes in the chemical composition of ground water aie commonly insignificant.
Defining the ground-water rkgime
3.2.2.3 Air temperature
Changes in air temperature may lead to changes in ground-water temperature. Also, air temperature may affect the conditions of ground water recharge. In winter, when air temperature is below zero, the recharge of ground water does not occur at the expense of infiltration of precipitation. In regions of permafrost where ground water freezes in winter, thawing of the zone of aeration can cause intensive recharge to the ground-water system; even short-term periods of temperature rise above O” C are accompanied by a rising water table. The influence of thaws on the ground-water régime is clearly observed in highly permeable limestones and sands, where changes in air temperature, causing temperature fluctuations in ground water, bring about changes in chemical composition, total dissolved solids content and physical properties. Changes in air temperature do not always affect ground water directly. The effect m a y be exerted through the rocks of the zone of aeration and by the moisture moving through this zone. A gradual decrease in the annual amplitude of temperature fluctuations with depth, down to the so-called belt of ‘constant’ temperatures (revealed by observations of temperature changes of ground water and rocks in the zone of aeration), testifies to the attenuation of the conduction factor from the ambient temperature régime into the lithosphere.
The depth to the belt of ‘constant’ temperature depends on the lithological composition of rocks in the zone of aeration, their moisture content, composition of the water- bearing rocks, depth to ground water, geographic latitude of the locality, temperature fluctuations at the land surface and intensity of ground-water movement. In middle latitudes and especially in the areas of continental climate, depth to the ‘constant’
temperature belt reaches 30 m . The relationship between the decrease in amplitude of temperature fluctuation and the depth to the belt of constant temperatures is given by Ogil’vi (1932) as:
Pz = Po
e-%
4 -&
3.2(2)where :
Pa is amplitude of ambient air-temperature fluctuations ; Pz is amplitude of temperature fluctuations at depth x ; r
kt is coefficient of thermal conductivity of rocks.
is length of the fluctuation period;
Within the belt of ‘constant’ temperatures the ground-water régime may be affected indirectly by air temperature that can cause a more intensive movement of ground water.
If the ground water is descending, then temperature fluctuations can be observed at much greater depths, depending on the rates of movement and the amplitudes of tem- perature fluctuation in ground water in the recharge area. Conversely, because of these temperature anomalies of ground water, shallow aquifers in some localities have been detected by anomalies in soil temperature (Cartwright, 1968).
Downward transmission of annual temperature fluctuations occurs without changing the period of fluctuation at different depths, i.e. the period of time between the maximum and minimum temperature remains approximately the same at different depths. However, there is a gradual lag of temperature changes with depth as compared to the oiiginating air temperatures. For instance, at depths down to 2 or 3 m water temperature may be directly related to the air temperature of the current month. At depths of about 10 to 12 m the lag may be two months; at depths of about 25 to 27 m the lag may be six months, and so on (Konoplyantsev et al., 1963).
3.2.2.4 Atmospheric pressure
The influence of atmospheric pressure upon the ground-water iégime is generally as follows : with an increase in atmospheric pressure, water levels in wells tapping confined
3.2 page 5
Ground-woter studies
aquifers decline and the yields of artesian springs reduce and vice versa. W h e n atmospheric pressure changes are expressed in terms of a column of water, the ratio of water-level change to pressure change expresses the barometric efficiency of an aquifer. Most obser- vations indicate efficiencies in a range of 20 to 75 per cent. Barometric efficiency may be considered as a parameter of an aquifer characterizing its elastic properties and the degree of its confinement or isolation from the atmosphere.
Just as atmospheric pressure changes cause variations in water levels in wells tapping confined aquifers, so do tidal fluctuations cause changes in pressure in aquifers extending under the ocean floor. But the effect of tidal fluctuations is direct rather than inverse, so that when sea level rises, ground-water level rises also. The tidal efficiency is thus a measure of the incompetence of overlying confining beds to resist pressurz changes.
A s pointed out by Jacob (1940), the values of the barometric efficiency and the tidal efficiency in the same well are related, in that their sum is equal to 1.
Proceeding from the uniform transmission of pressure over the whole aquifer, Jacob (1940) obtained an expression which related barometric efficiency, Bb, to properties of the aquifer and water. Hubbert showed however, that Jacob's equation can be reduced to the simple form (Hubbert and Rubey, 1959, p. 129-37):