3.1 How can dry landscapes be viewed as systems? Flashcards
3.1 How can dry landscapes be viewed as systems?
Key idea ➡ Dryland landscapes can be viewed
as systems.
The components of drylands systems
Drylands landscapes can be viewed within a systems framework. Like other physical systems they consist of components linked by flows of energy and materials. The main components in dryland landscape systems are:
-Geology (rocks and their lithology and structure)
-Sediments (including boulders, screes, sand, silt and clay)
-Water (derived from precipitation (and condensation) and stored in rivers, lakes and aquifers)
-Climate (Including precipitation, temperature, cloud cover and wind)
Aquifer
A body of rock or sediment that stores groundwater and allows the flow of groundwater.
Screes
Sloping masses of rocky fragments
Flows of energy and materials
The components of physical systems are linked by flows, inputs, throughputs and outputs of energy and material.
Latitude
The numbering system used to indicate the location of parallels drawn on a globe and measuring distance north and south of the equator.
Longitude
Distance east or west of the prime meridian, measured in degrees.
Types of dryland systems
-Dryland systems (mid- and low-latitude deserts and semi-arid environment) - Lands where plant productivity is limited by water availability, e.g. Sahara Desert; Australian Desert; Gobi Desert; Atacama
-Polar drylands systems - High-latitude areas frozen for most of the year, e.g. Arctic; Antarctica.
Energy flows
Solar radiation and precipitation are the main inputs to dryland landscape systems. They are balanced by outputs of heat (long-wave radiation from the surface), evapotranspiration and stream flow. The interaction of temperature and moisture with geology, through weathering and erosion, lead to rock disintegration and the formation of erosional landforms such as canyons, deflation hollows and pediments.
Material flows
Mineral materials from weathering and erosion are transported as sediments and eventually deposited by rivers and winds. Sediments are stored, often for millennia, as landforms such as screes, alluvial fans and dunes.
Evapotranspiration
The evaporation of water from soil plus the transpiration of water from plants.
Aridity index
Defining drylands is complex and cannot be based on precipitation levels alone. A more accurate measure of aridity is the ratio of precipitation to evapotranspiration.
Because evapotranspiration is difficult to measure, calculations of aridity are based on the concept of potential evapotranspiration (PET). This is the amount of evaporation that would occur if water were freely available for evaporation all year round. PET takes account of atmospheric humidity, solar radiation and wind.
In drylands annual PET always exceeds annual precipitation. Therefore the moisture that could theoretically be lost through evaporation and transpiration is invariably greater than the amount of moisture available.
Aridity index formula
The United Nations Environment Programme (UNEP) aridity index is a simple way of measuring aridity:
AI = P / PET
where
AI = aridity index
P = mean annual precipitation
PET = mean annual potential evapotranspiration
How the United Nations Environment Programme (UNEP) aridity index identified categories of aridity.
Using this index, the UN identifies four categories of aridity: hyper-arid, arid, semi-arid and sub-humid.
According to the UN classification 37.3% the Earth’s surface is to some extent arid. If we include the sub-humid category, this proportion rises to 47.2%.
Potential influences on drylands landscape systems
-Climate (Weathering)
-Geology
-Latitude and altitude
-Relief and aspect
-The availability of sediment
Potential influences on drylands landscape systems definition
Dryland landscapes are influenced by a range of factors including climate, geology, latitude and altitude, relief, aspect and the availability of sediment.
Climate (Potential influences on drylands landscape systems)
Most drylands have relatively small amounts of annual are substantially higher than precipitation. However, the thermal characteristics of drylands are more variable. Annual temperature ranges are high in continental interiors, even in the tropics.
With the exception of coastal deserts like the Namib in southwest Africa (Walvis Bay) and the Atacama in Chile, temperatures are strongly influenced by latitude. Very high summer temperatures are recorded in the Sahel (Bilma), the Arabian peninsula (Baghdad) and the southwest USA (Las Vegas).
Average maximum precipitation and all have annual values of PET which temperatures are high as far north as the Gobi Desert of central Asia (Ulaanbaatar), but decline steeply in polar regions to just 8 or 9°C.
Weathering (Climate (Potential influences on drylands landscape systems))
Weathering is the in situ breakdown of rocks by mechanical or physical, chemical and biological processes. It is largely determined by climate.
The main processes are insolation weathering, salt weathering and freeze-thaw. These mechanical processes break rocks into smaller particles.
Geology (Potential influences on drylands landscape systems)
Rock lithology and structure are primarily controls on water transfer in drainage basins. Lithology describes the chemical composition of rocks; structure includes the physical characteristics such as jointing, bedding, folding and faulting. Both lithology and structure determine the porosity or permeability of rocks.
Latitude and altitude (Potential influences on drylands landscape systems)
Latitude influences many characteristics of drylands. Tropical and sub-tropical drylands such as the Sahara, Arabian and Australian Deserts owe their existence to the atmosphere’s circulation in low altitudes. These deserts lie on the poleward limb of the convective Hadley cell.
Relief and aspect (Potential influences on drylands landscape systems)
Several drylands such as Patagonia in South America. the Mojave Desert in the USA and the Tibetan Plateau have developed in the lee of mountain barriers. Patagonia is sheltered by the southern Andes, the Mojave Desert by the Sierra Nevada and the Tibetan Plateau by the Himalayas. While windward slopes invariably experience above-average precipitation, leeward slopes, facing downwind, are relatively dry. They lie in a so-called rain shadow.
Generally the higher the mountain range the more pronounced the rain shadow effect. Two factors explain the rain shadow effect:
-As air masses are force aloft by mountains, the air cools, water vapour condenses to form cloud and expends much of its moisture in the form of rain (the so-called orographic effect).
-Air descending on leeward slopes is warmed by compression which lowers its relative humidity and reduces the likelihood of rain
Rain shadow
A region with dry conditions found on the leeward side of a mountain range as a result of humid winds from the ocean causing precipitation on the windward side
Orographic effect
Cooling effect that occurs when air is forced to rise over a mountain, resulting in a wetter windward side and a drier leeward side
The availability of sediment (Potential influences on drylands landscape systems)
Sediment comprises rock particles of varying size derived from weathering and erosion processes, transported and ultimately deposited by rivers, winds and mass movement processes.
In some drylands sediments are scarce and the ground may be covered with tightly packed stones forming desert pavements. This rocky surface is known generally as reg. In fact, most deserts are rocky rather than sandy, suggesting that finer sediments often have been removed by wind action.
Variable relief (The availability of sediment (Potential influences on drylands landscape systems))
In drylands with variable relief, weathering in the mountains may provide an abundant supply of sediment. This sediment, transported by streams and rivers, is often deposited where rivers emerge from mountains onto adjacent plains. The result is alluvial fans and bajadas, which are a common feature in the deserts of the American southwest. Large sediment supplies, often derived from earlier wetter climatic phases, give rise to sand seas, which cover large parts of the Sahara desert.
Sand which forms the world’s highest dunes in the Namib desert has been transported by winds from the Orange River hundreds of kilometres to the south in South Africa.
Desert pavements
In some drylands sediments are scarce and the ground may be covered with tightly packed stones, pebbles and cobbles created by the removal of all the finer grains by wind.
Erg
A generic term used to describe large expanses of sand desert.
The characteristics of different types of drylands landscape
-Polar drylands
-Mid- and low-latitude deserts
-Semi-arid environments
The characteristics of different types of drylands landscape definition
There are three main types of drylands landscapes: polar drylands, mid- and low-latitude deserts and semi-arid environments.
Polar drylands (The characteristics of different types of drylands landscape)
Polar drylands are concentrated north of the tundra in the high Arctic. This is a region of permafrost, free of ice cover. Almost lacking vegetation, it is essentially a cold desert. Barrow Point in Alaska at latitude 71° north recorded an average precipitation of just 134 mm/year for the period 1995 to 2014. Lack of moisture in the atmosphere due to the intense cold is the main cause of low precipitation.
Mid- and low-latitude deserts (The characteristics of different types of drylands landscape)
The world’s great hot deserts straddle the tropics. The largest are the Sahara, Australian and Arabian Deserts. Although these deserts are predominantly rocky, in places vast sand sheets like the Rub’ al Khali in Arabia cover the surface. In tropical deserts there is little seasonal pattern to rainfall, which is sparse, erratic and highly variable from year to year. In summer, surface temperatures may reach 80ºC.
Semi-arid environments (The characteristics of different types of drylands landscape)
Semi-arid environments, with an aridity index of 0.21 to 0.50, are widespread. They include the Mediterranean region; the Sahel, bordering the southern fringes of the Sahara Desert; much of central North America, including the Great Plains; large parts of eastern Australia; the Steppes in Southwest Asia; and Patagonia. Annual potential evapotranspiration (PET) exceeds precipitation in all of these areas but there is a seasonal pattern to rainfall, reliable enough to support agriculture and settlement.
