Water storage is need for the humans.
Groundwater is water that, after infiltrating and percolating through surface soils, flows into an aquifer, an underground water-bearing layer of porous rock. Aquifers are a very important sources of freshwater. For example, about one-third of the UK's drinking water is drawn from aquifers.
To permit economic development, an aquifer must be able to transmit large quantities of water from one point to another and therefore it must have a high permeability. The groundwater contained in aquifers is released from springs and can be responsible for the bulk of river flows.
Usually, aquifers are alluvial sands (sands which have been carried in suspension by rivers or floods) and gravels, the coarser sedimentary rocks such as sandstone, and rocks such as limestone in which chemical action has increased the water-bearing capacity.
The strata of relatively impermeable rocks that lie either above or below confined aquifers (aquifers trapped between two impervious layers of rock) are called aquicludes.
The flow of groundwater takes place through the layer that is completely saturated. This layer, not surprisingly, is called the zone of saturation.
In a given fixed space at any phase of the hydrological cycle, there is an inflow and an outflow of water, the rates of which vary with time. The total cumulative difference between inflow and outflow is the storage. So within that space there is a body of water whose mass is not directly controlled by instantaneous values of inflow and outflow.
For example, in river flow the movement of the whole body of water in the channel is generally downstream, yet a given reach contains a volume whose size may not change very much over a period of settled, fairly dry weather. Also, water may be abstracted from this reach at a greater rate than the inflow.
The storage element is most stable when it is large in relation to input and output quantities. This implies that the stored volume of water is stable in large lakes and reservoirs and also in aquifers, where the inflow and outflow rates are naturally low. As the size of the system increases, so also does the stability.
In our discussion of the hydrological cycle we have presumed that the system is completely stable, so both inflow and outflow are zero. This assumes that water present in the hydrological cycle was formed at a very early stage in geological time.
However, there is a theory that suggests that water is continuing to form in the Earth's core. Water is formed in small quantities, of course, by a number of artificial processes (e.g. in car exhausts). We have also assumed that no water is lost to the system by escaping from the Earth's gravitational pull.
The concept of storage is vital to the supply of water, since a major problem of supply revolves around the provision of water at the right time. Water is, of course, in highest demand in dry weather and we seek constantly to exploit or increase the existing storage potential in the cycle.
The relative contributions to total storage are summarised in the diagram. The oceans of the world hold the bulk of the water on Earth. Thus the water resources of the world are largely saline (salty).
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