Geoscience Reference
In-Depth Information
Atmospheric moisture budget
Learning objectives
When you have read this chapter you will:
Be familiar with the major atmospheric components of the hydrological cycle,
Know the main controls of evaporation and condensation,
Be aware of the spatial and temporal characteristics of moisture in the atmosphere, evaporation and
precipitation,
Know the different forms of precipitation and typical statistical characteristics,
Know the major geographical and altitudinal patterns of precipitation and their basic causes,
Understand the nature and characteristics of droughts.
This chapter considers the role of water in its various
phases (solid, liquid and vapour) in the climate system
and the transfers (or cycling) of water between the major
reservoirs - the oceans, the land surface and the atmos-
phere. We discuss measures of humidity, large-scale
moisture transport, moisture balance, evaporation and
condensation.
Approximately 70 per cent of the total
fresh
water is
locked up in ice sheets and glaciers, while almost all of
the remainder is ground water. It is an astonishing fact
that rivers and lakes hold only 0.3 per cent of all fresh
water and the atmosphere a mere 0.04 per cent (Figure
4.1). The average residence time of water within these
reservoirs varies from hundreds or thousands of years
for the oceans and polar ice to only about ten days for
the atmosphere. Water cycling involves evaporation,
the transport of water vapour in the atmosphere, conden-
sation, precipitation and terrestrial runoff. The equations
of the water budget for the atmosphere and for the
surface are respectively:
A THE GLOBAL HYDROLOGICAL CYCLE
The global hydrosphere consists of a series of reservoirs
interconnected by water cycling in various phases.
These reservoirs are the oceans; ice sheets and glaciers;
terrestrial water (rivers, soil moisture, lakes and ground
water); the biosphere (water in plants and animals); and
the atmosphere. The oceans, with a mean depth of 3.8
km and covering 71 per cent of the earth's surface, hold
97 per cent of
all
the earth's water (23.4
10
6
km
3
).
∆Q = E - P
D
Q
and
∆S = P - E - r
