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Characteristics of Deserts General Characteristics of Deserts

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Characteristics of Deserts General Characteristics of Deserts Characteristics of Deserts General Characteristics of Deserts Deserts have a variety of properties • Cold deserts vs hot deserts • Deserts with winter precipitation, deserts with summer precipitation and deserts with virtually no precipitation • Perpetually foggy deserts and deserts with near the maximum possible sunshine • Barren deserts and heavily vegetated deserts • Sand-dune deserts and deserts with rocky plains We will look at • Meteorological characteristics • Physiographic characteristics and substrates • Vegetation characteristics Sahara Desert Patagonia Baja Peninsula Gobi Desert Meteorological Characteristics Warm Versus Cold Deserts COLD DESERT Daily total solar WARM DESERT energy received at top of atmosphere on horizontal surface Coldest Warmest Climate % arid lands Month Month Examples Hot 43 10-30 >30 Central Sahara Great Sandy (Aust) Mild Winter 18 10-20 10-30 Southern Sahara Kalahari Mexico Deserts Simpson (Aust) Cool Winter 15 0-10 10-30 Northern Sahara Atacama Mojave Cold Winter 24 <0 10-30 Canadian Prairie Gobi Turkestan Deserts of China Great Basin Coastal/Foggy Desert • Summer temperatures are not especially high, but the winters are not cold because the ocean is close • Can be very uncomfortable because of high humidity Temperature (solid) and Relative Humidity (dashed), at Different Distances From Namib Desert Coast FOG 5 km from coast 30 km from coast Far inland Seasonality of Precipitation Differences in seasonality of precipitation is due to shifting of global weather patterns throughout year Rule of thumb: Summer precipitation on equatorward side and winter precipitation on poleward side of subtropical desert Desert Temperature Why are deserts often warm or hot? • Lack of evaporation from surface • Lack of vegetation (transpiration) • Not much cloud cover Cloud Cover (%) Desert Winds • Subtropical deserts - High pressure areas are generally characterized by weak winds - Thunderstorms can produce high winds - Mid-latitude disturbances (lows) with high winds can penetrate into deserts • Cold deserts in mid-latitudes, experience mid-latitude cyclones (lows) that can have strong winds • Lack of vegetation – near-surface winds higher • Winds are very desiccating – high temperature coupled with high winds Desert Humidity • Relative humidity • Can be as low as a few percent or as high as 100% (foggy coastal desert) • This depends on both temperature and moisture content of air • Absolute humidity • Compared to hot humid tropics desert air is drier • But deserts don’t always have low absolute humidity compared to non-arid locations Physiographic Characteristics and Substrates of Deserts • Stony soil • Sand dunes and sand “sheets” • Exposed bedrock / mountains • Desert pavement, crusts, varnishes • Salt flats, playas, salars, sabkhas • Dry riverbeds (arroys, wadis) • Clay plains Physiographic Characteristics and Substrates of Deserts Stony soil Stony soil Sand Dunes Sand Dunes Gypsum Sand Dunes White Sands N.P. White Sands Gypsum sand dunes, White Sands N.P. Exposed Bedrock Exposed Bedrock Desert Pavement Pavements • Wind and water can strip small sediments, leaving stony surfaces (right) • Stones can push up from expansion / contraction due to freeze / melt or wet / dry • Crusts can form from chemical action, especially via salts • Varnishes can change the color and albedo of the stony surface. Caused by bacterial and chemical action. Desert Varnish Desert varnish is mostly clay cemented to rock surfaces by microorganisms that are able to take manganese out of the environment, then oxidize and emplace it onto rock surfaces. These microorganisms live on most rock surfaces and may be able to use both organic and inorganic nutrition Desert varnish, Death Valley sources. Varnishes can be broken down by acid rain and lichens primarily. The lack of rain in deserts limit weathering of rocks, thus varnishes are more common here. Desert Varnish Petroglyph carved in desert varnish, Dinosaur National Park Playas • Lower portions of arid basins with internal drainage • Sometimes salty, sometimes wet, sometimes high water table • 10-10,000km2 • ~50,000 on Earth • Large ones are ancient lakes Salt Flat Arroyo Areas With Interior Drainage Mountain – Basin System Substrate Characteristics Vegetation Characteristics §Small trees §Shrubs §Succulents §Grasses §Herbs §Lichens Vegetation Type vs Climate • The type of vegetation found at a given location depends, partially, on the climate • (Right) Vegetation types vs mean annual temperature and precipitation Vegetation Type vs Climate • Another way to relate vegetation to climate is by net radiation and a dryness ratio • Dryness ratio is the ratio of net radiation to energy required to evaporate annual precipitation (Budyko index) • Values >1 indicate arid climates Palo verde tree - Sonoran desert Saguaro cacti – Sonoran desert Grasses and shrubs – Mojave desert Sagebrush and shrubs – Great Basin desert Vegetation Types That are Adapted to Desert Conditions • Phreatophyte – long roots • Xerophyte – mechanisms for conserving water • Halophytes – adapted to saline soils • Psammophytes – grow in sandy soils • Therophytes (annuals) – seeds remain dormant in soil during dry season or dry years (contrast with perennial) • Nephelophytes – absorb dew or fog from leaf surface Desert Vegetation Types Fig. 6.8 Relationship between root structure and water table for 2 types of desert vegetation Where might one find these in the desert? “Xerophytes” “Phreatophytes” Mesquite growing along arroyo where water table is closer to surface Mesquite Tree •Phreatophyte – has both shallow and deep roots •Roots can be up to 80m long •Can drain water tables •Can lead to diurnally flowing rivers (only flow at night since the plants suck up so much during the day) •Areas with dense mesquite can take 2 m out of water table per year •Common in Chihuahuan and Sonoran deserts in North America What mechanisms do xerophytes use to conserve water? • Small leaves • Waxy or hairy leaves • High albedo leaves (light green to gray color) • Leaves oriented with edge to sun • Leaves roll up or fall off during drought • Vegetation types can vary significantly over small distances due to: • Differences in soil / substrate • Water availability • Microclimate • Vegetation types can vary significantly over small distances due to: • Differences in soil / substrate • Water availability • Microclimate Seasonal Variations in Vegetation Seasonal Variations in Vegetation Seasonal Variations in Vegetation Fig. 6.1 Seasonal variations in vegetation cover at 4 locations Interannual Variations in Vegetation Southern Periphery of Sahara Fig. 6.2 Interannual variation of Sahara desert vegetation Why ? True Sahara Semi-Arid Vegetation Types • Savannas (right) – open grassland with scattered shrubs and trees (subtropical) • Steppes (below) – grassland without trees (except along rivers), generally in midlatitudes Global Savanna Distribution Global Grassland Distribution North American Deserts Orography of North America Great Basin desert Sagebrush Common in Great Basin desert and adjacent regions Pinyon Pine (right) and Juniper (bottom) Common at higher elevations in Great Basin desert and adjacent regions Great Basin desert Mojave desert Joshua Tree Found in the Mojave desert Sonoran desert Saguaro Cactus Found in Sonoran desert along with other columnar cacti like organ pipe cactus Palo Verde Tree Found in the Sonoran desert Chihuahuan desert Mesquite Tree Found in Chihuahuan, Sonoran and Mojave deserts Creosote Bush Found in Chihuahuan, Sonoran and Mojave deserts Monthly Temperature and Precipitation Precipitation in North American Deserts Temperature in North American Deserts The Budyko Index (Aridity).
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