Download Deserts - formation, location and climate

Desert environments in
Namibia
their formation, location and
climate
Key Question 1.1 What are the characteristics of a desert environment that make it
extreme?
*
The climatic, biotic and soil characteristics of a desert environment.
*
The links between climate, biotic and soil characteristics.
Key Question 1.2 How is human activity causing pressures on the desert
environment?
*
The threats that are posed by
(i)
population growth;
(ii)
mineral exploitation;
(iii)
farming;
(iv)
tourism.
*
The positive and negative outcomes of human activity.
Key Question 1.3 What are the strategies that can be used to manage human
activity in deserts?
*
Strategies that attempt to
(i)
conserve the desert environment;
(ii)
alleviate the impacts of human activity;
(iii)
control the use of the desert environment
(iv)
monitor the impacts of human activity.
*
The role of local, national and international groups in the
management of the desert environment.
What is a desert?
Deserts are arid places. Their aridity can be defined solely by total annual
rainfall, but more usually the relationship between rainfall and evaporation
is examined.
Arid
Semi-arid Hyper-arid -
having less than 250mm of rain per year
having between 250 and 500mm of rain per year
having less than 100mm of rain per year
By this measure, the Namib Desert in Namibia is hyper-arid, and the
Kalahari Desert is semi-arid.
Deserts occur for a number of reasons, which are examined on the next
slide.
Why do deserts occur?
There are four main reasons for deserts to form:
1. sub-tropical high pressure
2. rain shadow
3. continentality
[1] As can be seen on the diagram
4. cold ocean currents
showing the global pattern of
atmospheric circulation, areas at
about 25° - 30° N and S of the
Equator experience falling air or
high pressure. This causes the
air to warm and become drier,
meaning cloudless skies and
aridity.
2. When air rises over a
mountain range the air
cools and water vapour
condenses to form clouds
and rain. On the lee side air
falls, warms and creates
clearer skies and a rain
shadow area with lower
precipitation rates. Winds
from the south and east
have to cross the
Drakensberg mountains in
South Africa helping to keep
the Kalahari dry.
3. Remoteness from the sea is a major cause
of aridity. Places in maritime locations tend to
have a much higher rainfall than those in
continental interiors. This helps the Kalahari
be so dry. The places most affected by
continentality are the cold winter deserts of
the Asian interior.
4. Where cold currents lie off the Western
coasts of continents winds tend to blow
offshore, cold water is drawn to the surface
(upwelling), and any air which crosses the
current is cooled releasing rain before
getting to shore. Air warms over the land,
keeping the area arid. Fog may represent
the only water available. In Namibia the
cold current is the Benguela Current.
The Namib Desert is a desert because of its sub-tropical location in the high
pressure belt, and because of the upwelling of cold water in the Benguela
Current offshore.
The Kalahari Desert is also in a sub-tropical location and is far from moist
winds in a mid-continent location.
Climate in Namibia - Rainfall
As can be seen from the map, the driest
areas in Namibia are the west and southwest, i.e. the Namib Desert, whilst the southeast and east, i.e. the Kalahari Desert are
semi-arid. The Caprivi strip is the rainiest
area.
cold Benguela
Current
The main factors influencing the rainfall
pattern are the cold ocean current offshore
(the Benguela Current) and the winds. Winds
from the west are dry whilst the wet winds
from the east cross 4000km of land before
reaching Namibia.
Although most desert
storms are of low intensity,
there are occasional
spectacular storms which
reflect the high variability
index found in arid regions.
Rainfall variability is about
10% in places like the UK.
As can be seen rainfall in
Namibia is much more
undependable.
A place like Swakopmund
with an average annual
rainfall of 15mm may
receive 50mm in one
storm event.
Swakopmund
Rainfall variability
Mariental, the capital of the Hardap Region in the Republic of Namibia,, experienced
a major flood over the weekdend of February 24th to the 27th, 2006. The entire lower
town to the west of the railway line was completely flooded when the inflow to the
Hardap Dam, the largest water reservoir in Namibia, increased its volumes to 110%
full. The sluices were opened, and that caused a breakdown to electricity, the
sewerage system, loss of lives etc. 5 people were killed and over 2000 displaced
from their homes.
Three pages of images of the Mariental floods in 2005
Evapo-transpiration
The highest rates of evapo-transpiration are
to be found in the Kalahari desert in southeast Namibia. Fog along the coast reduces
evapo-transpiration rates here whilst
convectional clouds again reduce the rates
in the north.
The highest rates of evapo-transpiration
coincide with the summer rainfall season,
reducing the effectiveness of the low rainfall
still further.
Temperature
Temperatures can be quite cold in the
desert because of clear skies in the
winter time and at night. Daytime
temperatures in summer are very hot.
Diurnal ranges can be very high.