Deserts: definitions and characteristics
What makes a desert?
1.
Temperature is
occasionally cited in defining deserts, but there are cold deserts as well as
hot deserts. Cold deserts, such as the polar regions,
high altitude regions, the
2. Productivity is sometimes invoked as a definition for deserts
Websters Dictionary– an uncultivated region without inhabitants; a
wilderness; a dry, barren, sandy region, naturally incapable of supporting
almost any plant or animal life. Synonym – waste. The Middle
English root of the word means “forsaken” or “abandoned”.
Biome – one of the major categories
of the world’s distinctive plant assemblages; e.g. tundra, tropical rainforest,
desert.
Productivity
– Primary productivity is the rate at which energy is stored as organic matter
by photosynthesis.
Net primary
productivity
is the rate at which plants store energy or organic matter, not already used up
in respiration. We see it as plant
growth, and this is what is available for consumption by heterotrophs.
|
Ecosystem
type |
Net
primary productivity: g/m2/yr |
Mean |
|
|
|
|
|
Trop
rainforest |
1000-3500 |
2200 |
|
Temperate
evergrn forest |
600-2500 |
1300 |
|
Tundra/alpine |
10-400 |
140 |
|
Desert/semidesert |
10-250 |
90 |
|
Extreme
desert |
0-10 |
3 |
|
Open
ocean |
2-400 |
125 |
|
Coral
reefs |
500-4000 |
2500 |
|
Estuaries |
200-3500 |
1500 |
3. Aridity. The simplest definition holds that a desert is an area
receiving an average of ten or fewer inches of precipitation annually. Yet,
even this is incomplete. The precipitation an area receives depends upon a
number of other factors:
when the
precipitation falls,
how much
at one time,
what the
soil conditions are.
Many factors influence the
value plants derive from moisture they receive; hence, these factors are
instrumental in determining the type of habitat established.
a. A gentle soaking rain benefits the vegetation more than
a violent cloud burst, which results in rapid runoff and moisture loss.
b. Several well-spaced showers are more valuable than a
single rain, even though the same total precipitation may be produced. This has
to do with the phenology of plant development.
c. Moisture received in summer will be more readily
evaporated than that received in cooler weather; conversely, moisture received
in hot weather may be greatly needed by the vegetation for surviving that
particular stressful period.
d. Moisture received in winter may be at a time when most
plants are dormant and are not taking up the available moisture.
e. Less than ten inches precipitation on clay soils may be
held in the soil for a long time for plants to draw upon; whereas, an excess of
ten inches in very sandy, porous soil may quickly vanish to depths beyond where
plant roots can reach.
f. An occasional downpour will not bring an end to desert
conditions, although plants and animals will be quick to take advantage of
it. A place in the That Desert of India
once received 33.5” of rain in 2 days, but it was a very long time before it
rained again. Such water as fall soon disappears and the ground remains dry.
Ten inches or less annual precipitation is an average
figure. The erratic nature of precipitation occurrence is a basic
characteristic of desert precipitation.
An example of the erratic
nature of rainfall in the desert can be seen in
Aridity defines a desert, but it would not be so that a desert is a place
where it never rains. It occasionally rains in most deserts. What matters is not whether or not it
rains, but what happens to the rain as it is falling, and once it reaches the
ground.
1.
The rate at which liquid evaporates depends upon the number of water molecules
in the boundary layer of air.
a. The more water molecules the air contains, the more
slowly the liquid will evaporate.
b. It will evaporate faster the fewer the airborne
molecules there are.
2. How
many water molecules a given volume of air can contain depends on the
temperature.
a. The warmer the air mass, the more molecules it can
hold.
i. Very warm air, 95F (35C) will be saturated when the
saturation vapor pressure reaches 56.2 mb (millibars), meaning water vapor accounts for 5.6% of the
total mass of air.
ii. In much cooler air, 14F (-10C), saturation vapor
pressure is reached at 2.6 mb, with water vapor
accounting for 0.26% of the mass of air.
iii. In other
words, a large fall in temperature reduces the water-holding capacity of the
air significantly.
3. Relative humidity is the amount of
water vapor in the air divided by the amount of water vapor needed to saturate
the air at that temperature, multiplied by 100.
a. (Saturation occurs when no other water molecules
can join a volume of air).
b. Warm air can hold much more water vapor than cool
air; more water vapor is needed to saturate it.
c. The warmer the air, the less likely it is to be
saturated and, therefore, the more readily water will evaporate (or vaporize)
into it.
Evapotranspiration
Plants take water from the
ground and release it as vapor into the air (transpiration). It is difficult to
measure transpiration as separate from evaporation, so the two are often
combined, as evapotranspiration.
Deserts will form if the amount of rain that falls is
less than the amount that evaporates.
Deserts are dry. In particular, their soils are dry. Just how dry depends upon the air temperature, winds, soil type,
and the amount of precipitation. A desert climate is one in which more
water evaporates from the ground than the ground receives in rain or snow.
Over an extended period of
time, the ground cannot lose more water than it receives. Instead of
evaporation, what matters is potential evapotranspiration.
1.
The Actual Evapotranspiration
(AET) is the amount of water actually
lost from a site.
2.
Potential Evapotranspiration
(PET) is the amount of water that
could be lost from a given site, if there was water there to be lost; the
maximum amount of water that will evaporate and be transpired if the supply is
unlimited is called the potential evapotranspiration.
i.
This refers
to the evaporative power of the atmosphere.
ii.
In a region
where water is plentiful, a rainforest, actual evapotranspiration
may equal potential evapotranspiration.
iii.
In deserts,
however, where there is little water there to be lost by evaporation, the
actual evapotranspiration is much less than the
potential evapotranspiration..
Deserts: AET<PET.
iv.
This is close to
the rate at which water would evaporate from an open water surface and can be
measured by using an evaporation pan. A container of a standard size is placed
in the open, exposed to the air, and filled with water. The water depth is
measured at the beginning and the end of a convenient period of time – 1 day, 1
week. The rate of evaporation is calculated from the change in depth. In cool,
moist climates of northern
If
the land is to grow crops, the amount of water supplied by irrigation must
exceed the potential evaporation during the growing season.
v.
If PET is greater than rainfall, deserts will develop.
3.
How dry is dry?
The difference between AET and PET may
be a good measure of aridity.
i.
With the PET/P
ratio, an area greater than 3.0 is said to be semiarid.
ii.
Parts of the
iii.
The area around
iv.
The interior of
the
4.
The
temperatures that matter most to living organisms in the desert is the
temperature at ground level, and in the middle of the day the temp at ground
level is considerably hotter than the surrounding air. In the
It is more than the lack of water that is involved in
producing a desert. In fact, it is more than a lack of water that is involved
in producing a lack of water.
1. Temperature plays
a vital role.
a. High temperatures compound the effects or water
shortages. High temperatures increase the evapotranspiration
rates which, in turn, increases aridity.
b. The high temperature record for the western
hemisphere is 134F on
2. Aridity
contributes to intense daytime heat.
a. There is little atmospheric moisture to absorb or
deflect sun's rays.
b. Much radiation reaches the desert surface and warms it
during the day
c. At night, heat is released into space as the surface
emits infrared radiation that escapes unhindered through the dry atmosphere.
This results in large diurnal fluctuations in temperature. Dry
d.
Aridity and heat are closely related and positively
feed back on each other.
i.
Heat increases evapotranspiration,
and this promotes aridity.
ii.
Aridity promotes increased penetrations of solar
radiation and high surface heating.
3. Temperature fluctuations
1.
Lack of moisture
results in low relative humidity and the formation of only limited cloud cover.
i.
A very high
percentage of possible sunlight is therefore received.
ii.
With little water
vapor in the air, and little cloud cover or plant growth to deflect the sun’s
rays, approx. 90% of the solar radiation possible reaches the ground surface
and lower air layer in the desert, resulting in high air and ground-surface
temperatures.
iii.
(Temperatures at
the ground surface in sunlight often range 30-50F higher than the accompanying
official air temperatures taken under standardized conditions).
iv.
Humid areas
receive about 40% of the solar radiation possible, 60% being deflected before
it reaches the ground and lower air levels.
2.
After sundown in
deserts, heat is rapidly radiated back toward the sky, with about 90% escaping
unimpeded.
i.
In moister
climates, the heat gained at lower levels during the day is less easily lost,
with approx. 50% of it escaping, and the remainder being deflected downward and
held by growth as well as by clouds, water, and dust in the air.
ii.
In moister
climates, then, temperatures fluctuate only moderately from day to night.
iii.
In desert
environments the range between daytime highs and nighttime lows is extreme. The
difference may be 50 degrees or more.
4. Winds are frequent in the desert. Caused by:
a.
General
atmospheric patterns
b.
Local topography
c.
Rapid heating and
cooling of the air near the ground surface.
Due to their frequency and
the air they circulate - often hot and dry - winds constitute a powerful
evaporative force as they sweep across the soil and over living things in their
path.
a.
They also
contribute greatly to the erosion (deflation) of the soil surface.
b.
The dust and sand
they carry often act as agents of abrasion, sandblasting rocks and plants.
c.
Agents of
deposition, moving loose material – soil, dust, sand, dead plants – form one
site to another.
Because of openness of the land,
wind moves relatively unimpeded.
Dust devils,
or whirling winds - rotating air currents occasionally as much as several
hundred feet in height, and carrying dust, sand, and debris - are common
occurrences on hot, still days.
a.
They are caused
when extreme heating of the ground surface results in columns of upflowing air. Surrounding air rushes into this vacuum and
diverts to one side or the other of the rising air, causing a strong, uprising,
whirling column.
b.
Unlike tornadoes,
they rotate upward from the ground surface.
5. Water erosion – one of the most important erosional
factors in the desert is not wind, but water.
1.
Precipitation is
often received thru violent cloudbursts accompanied by rapid runoff.
2.
Even when
precipitation is received in a less violent manner, absorption may still be
impeded, for much of the desert surface is rock or gravel, or exhibits other
factors of a low or non-absorptive character.
3.
In
4.
In deserts,
ground surfaces, more bare than vegetatively covered,
lies vulnerable to these occasional but highly destructive water forces.
5.
Bajada – the material carried in the rapid runoff from
mountainous escarpments, is dropped and spread out in a fan shape, graded from
heavier material higher up the slope to the lightest at bottom. A number of
these contiguous alluvial fans, which eventually coalesce with one another, is known as a bajada.
6.
Washes, arroyos, wadis – are normally dry streambeds occasionally carrying
heavy, brief streamflow, which drain towards the
basin centers, some of which are known as sinks.
7.
Many of these
basins or sinks are undrained, so that water from a
storm not otherwise absorbed into the ground or evaporated before reaching the
low point of the basin collects in an ephemeral lake from which it eventually
evaporates, leaving behind the minerals carried in suspension. On these playas or dry lakes, the buildup of minerals may be extreme, inhibiting plant
growth or supporting a special vegetative community of plants known as
halophytes (salt tolerant plants).
Dry washes, a dominant
feature of the desert landscape, are readily visible on the open, sparsely
vegetated surface. As recipients of runoff, their borders support a heavier, more extensive vegetation than that of the
surrounding area. The wash or stream margins may be marked by the heavy growth
of large cottonwoods, mesquites, willows, and other species of plants where
receipt of sufficient water makes this possible.
The desert is not easily defined, but certain
characteristics can be listed:
1.
low and irregular patterns of precipitation (aridity),
frequently resulting in drought during summer months.
2.
prolonged high temperatures: both air and soil.
3.
high evaporation rates from soil surfaces.
4.
extreme temperature fluctuations.
5.
low relative humidity, high PET.
6.
high solar irradiance, often with cloudless conditions.
a.
mean annual cloud cover in the
b.
coupled with low RH: dry air transmits light and heat more
effectively).
7.
soil high in
minerals, and low in humus
8.
extreme erosion of the ground surface by wind and water.
Such conditions exert a
profound influence on those living things – plant, animal, human – that make
the desert their home.
These
features force perennial plants to survive in soils with limited available
moisture, which thereby limits photosynthesis and productivity, and can place
plant organs under lethal daytime thermal conditions if they do not have
adaptations for coping with high temperatures.
1. Extremely arid - < 70 mm (< 3”) per year:
2. Arid
(typical) – 70-150 mm per year (3-6”): Mojave
3. Semi-arid – 150-300 mm per year (6-12”): Sonoran, Chihuahuan.
4. > 500 mm per year can
still be considered desert if the rain comes in a very restricted time frame.
Single rainstorms can exceed annual averages, as in January 1995 in
5. Fog
is important to both the Atacama (0.04”/yr) and
Polar Deserts
A hot climate and low
rainfall will produce a desert, but how does an extremely cold climate do this?
Answer lies in the temperature of the air and the moisture it can hold.
1.
Air moving toward the polar regions travels at high
altitude, where the air temperature is very low. Because it is so cold, its water vapor
condenses and falls as precipitation in the course of its journey. By the time the air reaches the polar regions, where it descends to surface level, it is
very dry.
2.
Cold temperature because:
a.
Astronomical:
b. When dun does shine, most of its
light and heat are reflected. The reflectivity of a surface is called its albedo. Freshly
fallen snow has an albedo of 75-95% (the proportion
of radiation it reflects). Dry sand has a fairly substantial albedo, but of only 35-45%. A field of grass has an albedo or 10%.
c.
Plants are widely spaced because of lack of moisture.
1.
Some areas may be
totally without vegetation.
2.
Much of the
desert surface lies exposed, subject to erosion by wind and water.
3.
Paucity of
vegetation results in soil low in humus (organic part of the soil).
4.
Desert soils have
acquired large amounts of sodium and potassium salts as well as other water
soluble minerals due to the high rates of mineralization in these areas.
5.
In more moist
regions, minerals tend to be leached downward thru the soil as abundant
moisture soaks thru it.
6.
Water in the
desert is insufficient to soak the soil to any great depth; there, minerals in
suspension may even be sucked upwards thru the soil by capillary action and
pulled to the surface to be deposited as the moisture carrying them is
evaporated.
Soils and Surface Materials – soils result from the weathering of rock; i.e form the action of sun, wind,
and water.
1.
Aridisols (arid soils)
a. High pH (alkaline)
b. Generally less
well-developed than soils of adjacent lands. Little soil profile.
c. Lack of organic matter,
including humus (nutrient holding organic residue that coats soil particles and
reduces soil compaction). <1% soil organic matter (SOL). Moisture
and nutrient retention is therefore diminished.
d. Few nutrients are lost
through leaching because of few precipitation events.
e. Accumulation of salts
because of low penetration of precipitation and high evaporation. (High salts
may inhibit plant growth).
f. Caliche, an accumulation of calcium
carbonate into rock-hard, water-impermeable, root-limiting layer. Can promote
surface run-off of water, that otherwise might hydrate a parched soil. Occurs
where evaporation exceeds precipitation (in areas with a carbonate
substrate/parent rock material: limestone).
2.
Winds modify soil.
a. Due to limited cover of
protecting vegetation, dry clay and silt particles are easily eroded from the
soil surface. Sands may also move. What is left behind is often a surface layer
of closely knitted stones, called desert
pavement.
1. Desert pavement may protect
underlying soil from further erosion.
2. Desert pavement may also
prevent dispersed seeds from reaching soil beneath, and may therefore limit
seed germination and establishment.
3. Desert pavement may inhibit
rain penetration into the soil depths, leading to water (sheet) run-off.
3. Cryptogamic
crusts –
interlacing web of lichens, mosses, cyanobacterial
filaments that hold soil particles in place, and fix nitrogen.
4. Despite severe moisture
deficits, water is an important geomorphological
agent in desert regions.
a. Water moves soil and rocks
(alluvium) down mountain slopes towards washes and playas, creating what we
call bajadas or alluvial fans.
b. Thermal expansion of
moisture trapped in rocks may cause disintegration, and even in hot deserts
rock, shattering can occur when surfaces moistened with dew freeze in the clear
night air.
c. Rain splash and sheet wash
cause pronounced erosion during periods of heavy rainfall because there is
little vegetation to protect the surface.
d. Desert Varnish - the thin patina of lacquer which covers many rock outcrops. This coating tends to be dark red to black in color, and is composed largely of iron and manganese oxides with silica. Capillary rise of salts under influence of high evaporation may play a major role in its development. The varnish can develop in 25 years in the Mojave and the American southwest, but generally it is believed the varnish develops more slowly.
Playas – undrained basins at
base of bajadas. Run-off from bajadas carries fine-textured soil and dissolved
salts to playas.
Increase in salinity
Decrease in soil aeration (fine textured soil)
Decrease in soil oxygen (inhibits colonization by plants)
Subject to cold-air drainage. Cold air at night sinks and collects in the playa.
Playas may be colder than the surrounding and higher bajadas.