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 Figure 6 and Table 1. The oceans of the world hold the bulk of the water on Earth. Thus the water resources of the world are largely saline (salty).
Table 1 Total available water in the various storage components of the hydrological cycle
|Storage component||Volume of water (1012 m3)||Volume (%, approx.)|
|Oceans||1 350 400||97.6|
|Ice caps and glaciers||26 000||1.9|
|Groundwater and soil moisture||7150||0.5|
From Table 1, it can be seen that only 2.4% of the Earth's water is non-saline, and that much of this is locked up as ice.
Calculate the volume of surface fresh water available in the non-polar regions using the figures in Table 1. What percentage of the total water available is this?
Surface freshwater available from:
As a percentage of the total water resource available (1 383 794.7×1012 m3)
which is less than 0.01% – this is a very small amount indeed!
One thing which is not shown in Table 1, but which I'm sure you have thought about, is the unevenness in the distribution of fresh water throughout the world. When I was working in Kuwait we had a proposal from an organisation representing oil tanker owners. They wanted to sell fresh water from Norway and Wales, brought as ballast in the tankers coming to collect crude oil. For a variety of reasons, the proposals were rejected.