Geoscience Reference
In-Depth Information
The meteorological definition of drought has become
clouded by the subject of
desertification
, particularly
since the UN conference on the subject in 1977 in
Nairobi. This concern was sparked by the protracted
drought, resulting in desiccation, in much of the Sahel
zone. In turn, the removal of vegetation, increasing the
surface albedo and lowering evapotranspiration, is
believed to result in decreased rainfall. The problem for
climatologists is that desertification involves land
degradation as a result of human activities, primarily in
areas of savanna and steppe surrounding the major
desert regions. These areas have always been subject to
climatic
fluctuations
(as distinct from climatic
change
)
and to human impacts (e.g. deforestation, mismanage-
ment of irrigation, overgrazing) initiating changes in
surface cover, which modify the moisture budget.
areal extent and frequency (or recurrence interval)
of rainstorms. Orography intensifies the precipitation
on windward slopes, but there are geographical
differences in this altitudinal effect. Global patterns of
precipitation amount and annual regime are determined
by the regional atmospheric circulation, the proximity
to ocean areas, sea-surface temperatures, and the
atmospheric moisture budget. Droughts may occur in
many different climatic regions due to various causal
factors. In mid-latitudes, blocking anticyclones are a
major factor. The primary cause of protracted drought
in the African Sahel seems to be climatic fluctuations.
DISCUSSION TOPICS
Trace the possible paths of a water molecule
through the hydrological cycle and consider the
measurements that need to be made to determine
the quantities of water involved in the various
transformations.
What processes lead to phase changes of water in
the atmosphere and what are some of their conse-
quences?
SUMMARY
Measures of atmospheric humidity are: the absolute
mass of moisture in unit mass (or volume) of air, as a
proportion of the saturation value; and the water vapour
pressure. When cooled at constant pressure, air
becomes saturated at the dew-point temperature.
The components of the surface moisture budget are
total precipitation (including condensation on the
surface), evaporation, storage change of water in
the soil or in snow cover, and runoff (on the surface or
in the ground). Evaporation rate is determined by the
available energy, the surface-air difference in vapour
pressure, and the wind speed, assuming the moisture
supply is unlimited. If the moisture supply is limited, soil
water tension and plant factors affect the evaporation
rate. Evapotranspiration is best determined with a
lysimeter. Otherwise, it may be calculated by formulae
based on the energy budget, or on the aerodynamic
profile method using the measured gradients of wind
speed, temperature and moisture content near the
ground.
Condensation in the atmosphere may occur by
continued evaporation into the air; by mixing of air of
different temperatures and vapour pressures, such that
the saturation point is reached; or by adiabatic cooling
of the air through lifting until the condensation level is
reached.
Rainfall is described statistically by the intensity,
What is the significance of clouds in the global water
balance?
Compare the moisture balance of an air column and
that of a small drainage basin.
What are the various statistics used to characterize
rainfall events and for what different purposes are
they important?
Consider how an annual water budget diagram
might differ between a wet year and a dry year at
the same location.
FURTHER READING
Books
Baumgartner, A. and Reichel, E. (1975)
The World Water
Balance: Mean Annual Global, Continental and
Maritime Precipitation, Evaporation and Runoff
,
Elsevier, Amsterdam, 179pp. [Statistical assessment of
the major components of the hydrological cycle; one
of the standard summaries.]
Brutsaert, W. (1982)
Evaporation into the Atmosphere:
