Geoscience Reference
In-Depth Information
4
STORAGE
LEARNING OBJECTIVES
When you have finished reading this chapter you should have:
An understanding of the role of water stored below the ground (in both the saturated and unsaturated
zones).
An understanding of the role of snow and ice acting as a store for water.
A knowledge of the techniques for measuring snow and ice and water beneath the ground.
A knowledge of the techniques used to estimate the amount of water stored as soil moisture,
groundwater and snow and ice.
The water balance equation, explained in Chapter
1, contains a storage term (
S
). Within the hydro-
logical cycle there are several areas where water
can be considered to be stored, most notably soil
moisture, groundwater, snow and ice and, to a
lesser extent, lakes and reservoirs. It is tempting to
see stored water as static, but in reality there is
considerable movement involved. The use of a
storage term is explained in Figure 4.1 where it can
be seen that there is an inflow, an outflow and a
movement of water between the two. The inflow
and outflow do not have to be equal over a time
period; if not, then there has been a
change in storage
(
S
). The critical point is that at all times there is
some water stored, even if it is not the same water
throughout a measurement period.
This definition of stored water is not perfect as it
could include rivers as stored water in addition to
groundwater, etc. The distinction is often made on
the basis of flow rates (i.e. how quickly the water
moves while in storage). There is no critical limit
to say when a deep, slow river becomes a lake, and
likewise there is no definition of how slow the
flow has to be before becoming stored water. It relies
on an intuitive judgement that slow flow rates
constitute stored water.
The importance of stored water is highlighted by
the fact that it is by far the largest amount of fresh
