Deserts
Unfortunately your text no longer includes a section on hot deserts! Of course in Canada this isn’t a big deal, but globally they are major landforms! The only hot desert area in Canada is in the southern Okanagon Valley of B.C.
**There is a video version of this lecture here: https://youtu.be/TDWVXMbjPqk.
**The exam is based on the content in these notes, so please print them off to study from.
The majority of the Earth’s surface (71%) is covered by oceans. Of the 29% that is land, deserts (both hot and cold) make up 33% of the land mass. Note that the Arctic and Antarctic are cold deserts (low temperature and low levels of precipitation); we discussed it in Chapter 17. In terms of low precipitation, Antarctica is the largest desert in the world.
The largest hot desert, however, is the Sahara desert, in northern Africa, covering 9 million square kilometers.
Hot deserts are defined by high temperatures and low levels of precipitation. The US Geological Survey has a great introduction to deserts, landforms, processes, etc.
In hot deserts, precipitation, when it falls, is usually short and intense. Because the rainfall happens so quickly, the water cannot be absorbed by the soil by infiltration fast enough. The rain simply comes too quickly for the water to soak in. The ground may, technically, be able to absorb all the rainfall, but the rain happens too quickly. (Although, rremember, most deserts are also rock pavements – the finer sediment is blown away by deflation – so they allow for little infiltration, anyway). The net result is that most rain water runs off. Consequently, directly after a rainfall, flash floods are common.
Hot deserts often have well-developed (but almost always dry) streambeds that become raging torrents during these isolated storms. These intermittent streambeds are variously called washes, arroyos, or wadis (depending upon where you are in the world). Streams/rivers that are dry most of the time, except during rare rainstorms, when they can be rushing torrents. These flash floods can generate tremendous power to move vast amounts of sediment, from fine silt to large boulders.
A playa lake is an intermittent lake – full after a rainstorm, but dry most of the time. Often it has a white, crusty surface from salt crystals left behind after evaporation of water.
Where an intermittent stream canyon (a.k.a. wash, arroyo or wadi) empties onto a valley floor, the velocity of the stream decreases, and much of the sediment is deposited as an alluvial fan. See Figure 15.26 4CE, p. 476 (Figure 15.17, 3CE, p. 472) “An alluvial fan.” Coarse materials are deposited at the top of the fan; finest sediment is sorted near the bottom of the fan. Fans also contain much groundwater.
Deserts are often characterised by landforms that have eroded at different rates. Less resistant layers erode away, leaving behind more resistant rocks as arches, pedestals, balanced, rocks, etc.
Over vast areas, the entire level of the landscape may have dropped because of erosion, except for isolated buttes, pinnacles and mesas, which have been protected by resistant “caps” on top.
Deserts often have landforms created by streams (like stream canyons and alluvial fans) that seem out of place. more than 99% of the time the landscape is dry! But during the <1% of the time when water is flowing, it’s REALLY flowing in flash floods. So these streams can reshape the landscape dramatically within very short periods of time.
Desertification refers to an expansion of desert areas or deterioration of drylands in arid, semi-arid, and dry sub-humid areas, creating desert-like dead zones that can spread and merge, primarily because of human activity, but also from climatic changes Drylands currently cover about 46% (±0.8%) of the global land area and are home to 3 billion people, in 110 countries worldwide, most of them among the poorest in the world. Almost 70% of Africa is desert or dryland; 75% of Africa’s agricultural drylands are already degraded.
Climate variability and human-caused climate change, particularly through increases in both land surface air temperature and evapotranspiration, and decreases in precipitation, are key. The major human drivers of desertification are expansion of croplands, unsustainable land management practices, and increased pressure on land from population and income growth. Poverty is limiting both capacities to adapt to climate change and availability of financial resources to invest in sustainable land management
As the process occurs, biodiversity (a whole range of plants and animals) is typically lost, replaced buy only one or two species. The results on poor populations can be catastrophic. These people, occupying approximately one quarter of the planet, are facing major problems which include soil degradation and vegetation loss, leading to the deterioration of arable land and, eventually, to chronic food insecurity.
Wikipedia has a decent article on desertification causes, effects, and attempts to counter the process.
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