ABSTRACT

The hydrological cycle involves the movement of water in all its forms over, on and through the earth (Fig. 7.1). As such, the cycle can be visualized as beginning with the evaporation of water from the oceans. This is followed by the transport of the resultant water vapour by winds, with some water vapour condensing over land and falling to the surface of the earth as precipitation. To complete the cycle this precipitation then makes its way back to the oceans via rivers and underground flow. However, some precipitation may be evapotranspired and describe several subcycles before completing its journey. Groundwater forms an integral component of the hydrological cycle. Most groundwater recharge is brought about by the infiltration and percolation of precipitation into the ground. Less significant, though important locally, is the direct contribution from rivers and lakes. In fact, the hydrological cycle can be regarded as a series of storage components, with water moving slowly from one to another until one circuit has been completed. Table 7.1 shows the estimated amount of water available within the various storage components and the total quantity that would be available if it could all be released from storage. Only 0.5 per cent of the total water resources of the world is in the form of groundwater. Not all this is available for exploitation since about half is below 800m, and therefore is too deep for economic utilization. However, the capacity of the underground resource should not be underestimated in that about 98 per cent of the usable freshwater of the earth is stored underground. The principal source of groundwater is meteoric water, that is, precipitation (rain, sleet, snow

and hail). However, two other sources are very occasionally of some consequence, that is, juvenile water and connate water. The former is derived from magmatic sources whilst connate water was trapped in the pore spaces of sediments as they were formed. As can be inferred from above, precipitation is dispersed as run-off, infiltration/percolation

and evapotranspiration. Run-off is made up of two basic components, surface water run-off and groundwater discharge. The former is usually the more important and is responsible for the major variations in river flow. Run-off generally increases in magnitude as the time from the beginning of precipitation increases. Infiltration refers to the process whereby water penetrates the ground surface and starts

moving down through the zone of aeration. The subsequent gravitational movement of the water down to the zone of saturation is termed percolation, although there is no clearly defined point where infiltration becomes percolation. The amount of water that infiltrates into the ground depends upon how precipitation is dispersed, that is, on what proportions are assigned to immediate run-off and to evapotranspiration, the remainder constituting the proportion allotted to infiltration/percolation. The infiltration capacity is influenced by the rate at which rainfall occurs, the vegetation cover, the porosity of the soils and rocks concerned, their initial moisture

content and the position of the zone of saturation. Accordingly, the rate at which groundwater is replenished is dependent basically upon the quantity of precipitation falling on the recharge area of an aquifer, although rainfall intensity also is very important. Frequent rainfall of moderate intensity is more effective in recharging groundwater resources than short concentrated periods of high intensity. This is because the rate at which the ground can absorb water is limited, any surplus water tending to become run-off. As noted, some precipitation is lost through evaporation and transpiration. The rate at which water can be lost from the ground surface through evapotranspiration is dependent to some extent upon the amount of water that is present in the soil. If lower strata are less permeable than the surface layer, the infiltration capacity is reduced

so that some of the water that has penetrated the surface moves parallel to the water table and is called interflow. The water that becomes interflow normally is discharged to a river at some point and forms part of the baseflow component of the river. The remaining water may continue down through the zone of aeration until it reaches the water table and becomes groundwater recharge. This can be a slow process (typically about 1myear−1), since the percolating water may become temporarily suspended in the zone of aeration as a result of the various dynamic forces that operate in the zone. Although infiltration may be high in a dry soil, the fact that the soil is dry means that water

is more likely to be held in the surface layers of the soil and either evaporated or transpired by plants, and therefore less likely to reach the water table. Consequently, most groundwater recharge occurs when the ground is comparatively wet and evapotranspiration is relatively insignificant. When evapotranspiration exceeds precipitation and vegetation has to draw on reserves of water in the soil to satisfy transpiration requirements, then soil moisture deficits occur. The soil moisture deficit (SMD) at any time is the difference between the moisture remaining and the field capacity of the soil, which is the amount of water retained in the soil by capillary forces after excess water has been drained from it. Before recharge can happen, any soil moisture deficits must be made up so that the soil is returned to its field capacity. Once this has been achieved, then any additional or excess water becomes groundwater recharge. Plants extract water from the soil until, as the soil dries out, it becomes impossible for them to continue doing so. At that point plants wilt and die if the soil is not rewetted. The point at which permanent wilting starts is referred to as the permanent wilting point. The field capacity and the permanent wilting point can be defined in terms of soil suction, their respective pF values being 2 and 4.2. The retention of water in a soil depends upon the capillary force and the molecular attraction

of the particles. As the pores in a soil become thoroughly wetted, the capillary force declines so that gravity becomes more effective. In this way downward percolation can continue after infiltration has ceased but as the soil dries, so capillarity increases in importance. No further percolation occurs after the capillary and gravity forces are balanced. Thus, water percolates into the zone of saturation when the retention capacity is satisfied. This means that the rains that occur after the deficiency of soil moisture has been catered for are those that count as far as supplementing groundwater is concerned. The pores within the zone of saturation are filled with water, generally referred to as phreatic

water. The upper surface of this zone therefore is known as the phreatic surface but more commonly is termed the water table. A perched water table is one that forms above a discontinuous impermeable layer such as a lens of clay in a formation of sand, the clay impounding a water mound. The zone of aeration, in which both air and water occupy the pores, occurs above the zone of saturation. The water in the zone of aeration is referred to as vadose water. Meinzer (1942) divided this zone into three belts, those of soil water, the intermediate belt

and the capillary fringe (Fig. 7.2). The uppermost or soil water belt discharges water into the atmosphere in perceptible quantities by evapotranspiration. In the capillary fringe, which occurs immediately above the water table, water is held in the pores by capillary action. An intermediate belt occurs when the water table is far enough below the surface for the soil water belt not to extend down to the capillary fringe. The geological factors that influence percolation not only vary from one soil or rock outcrop

to another but may do so within the same one. This, together with the fact that rain does not fall evenly over a given area, means that the contribution to the zone of saturation is variable, which influences the position of the water table, as do the points of discharge. A rise in the water table as a response to percolation is controlled partly by the rate at which water can drain from the area of recharge. Mounds and ridges form in the water table under the areas of greatest recharge. Superimpose upon this the influence of water that may drain from lakes and streams, and it can be appreciated that a water table is continually adjusting towards equilibrium. Because of the low flow rates in most rock masses this equilibrium is rarely, if ever, attained before another disturbance occurs. As noted, the water table fluctuates in position, particularly in those climates where there

are marked seasonal changes in rainfall. Thus, permanent and intermittent water tables can be distinguished, the former marking the level beneath which the water table does not sink, whilst the latter is an expression of the fluctuation. Usually, water tables fluctuate within the lower and upper limits rather than between them, especially in humid regions, since the periods between successive recharges are small. The position at which the water table intersects the surface is termed the spring line. Similarly, intermittent and permanent springs can be distinguished. Dykes often act as barriers to groundwater flow so that the water table on one side may

be higher than on the other. Fault planes occupied by clay gouge may have a similar effect. Conversely, faults may act as conduits where the fault planes are not sealed. The movement of water across a permeable boundary that separates aquifers of different permeabilities leads to deflection of flow, the bigger the difference the larger the deflection. When groundwater meets an impermeable boundary it flows along it and, as noted previously, in some situations, such as the occurrence of a dyke, may be impounded. The nature of a rock mass also influences

whether flow is steady or unsteady. Generally, it is unsteady since it usually is due to discharge from storage.