2. GSC 1530 Chapter 19 Deserts and Winds

3. Deserts  Deserts (arid) and steppes (semiarid) lands occupy about 30-35 percent of Earth’s land surface – more than any other ecosystem type  Therefore its important to understand these environments and whether deserts are expanding or contracting (see slide)

4. Don’t forget – there are polar deserts (Antarctica and most of the Arctic)!

5. Deserts  Desert – a sparsely vegetated, arid region that typically receives less than ten inches of rainfall per year and where the rate of evaporation greatly exceeds the rate of precipitation  Note no mention of temperature in my definition (see slides)

7. Don’t forget – there are polar deserts (Antarctica and most of the Arctic)!

8. Desert Origin  Scientists recognize five major desert categories  Multiple factors contribute to desert formation – two of the most important are the Earth’s general air circulation patterns and topography (see slides)

9. Rising air expands and cools; encourages rainfall Sinking air is compressed and warmed, inhibits rainfall

10. Rain Shadow Desert Formation

12. Desert Processes/Landforms  Although most think of deserts as water deficient regions, running water (when present) performs most of the erosional activity in deserts  Much of this activity is aided by ephemeral (intermittently flowing) streams that host periodic flash floods during infrequent thunderstorms (see slides)

13. An ephemeral stream is called a dry wash or arroyo in the American west or a wadi In the Middle East

14. Note boulders moved by previous flash floods! Desert thunderstorm over Tucson, Arizona

15. alluvial fans

17. Bajada – coalesced alluvial fans playa lake/deposits

18. Desert Processes/Lanfdorms  Relevance of alluvial fans and bajadas?  See figure Source: Earth, July 2009

20. Inselberg

21. Desert Processes/Landforms  Wind erosion and sediment deposition also significantly influence the desert landscape  Deflation – the lifting and removal of loose material by wind  Remember that fine-grained sediment transported by forceful winds is quite abrasive and can sculpt rocks uniquely (see slides)

26. Desert Processes/Landforms  Persistent deflation may also produce a fairly common surface known as desert pavement (see slides); desert pavement is the predominant surface feature in nearly all deserts – not dunes!

31. Desert Processes/Landforms  Probably the most characteristic desert landforms are the wind-deposited mounds and ridges of sands called dunes

32. Sherman County, Oregon

35. 0100 0 0 vegetation cover wind strength sand supply longitudinal parabolic barchan, transverse no dunes Factors Affecting Dune Formation

36. Major Dune Types

37. Consequences of migrating dunes?

38. migrating dunes, Egypt

39. Desert Processes/Landforms  Loess – deposit of windblown silt and clay size rock particles that mantles the topography like fallen dust  Common source of loess is deserts, but previously glaciated terrains may also host a fine-grained material called rock flour that can be eroded readily by wind  Very thick (up to 300 ft) accumulations exist in China and there are also significant deposits in the United States  Why care? Airborne loess can impact air quality and water quality; loess deposits aid soil development and agricultural productivity (see slides)

40. Loess

42.  Importance of Loess In some regions of the world, loess cave homes are not uncommon

43. October 2001, Geotimes Connection between loess and air and water quality?

44. Possible Influence of Loess?

45. Loess and Plant Productivity Connection between loess and plant productivity?

46. Desertification  Desertification – expansion, or spread, of deserts largely as a consequence of human activities  A worldwide problem, including U.S.  In the U.S. moderate to severe desertification has occurred in parts of the southwest, plains and western states in the last 150 years (see slide)

47. black = desert

48. Desertification  What are the major human activities that could encourage desertification? Hint: any process that would foster removal of the thin topsoil layer in an arid or semiarid region adjacent to a desert would encourage desertification  Causes?

49. Desertification Causes  Clearcutting of native vegetation (drought- resistant trees and grasses like the Tallgrass Prairie) (see figure)

50. Desertification Historic extent of N. American Prairie (Illinois State University) Big Bluestem Grass

51. Desertification  Note the huge difference between the root system of a wheat plant and that of a tallgrass prairie plant!

52. Desertification Causes  Overgrazing by livestock  Poor farming choices (growth of water-hungry crops prone to failure during a drought)  Poor water management (excessive diversions of surface waters/depletion of aquifers)  Off-road vehicle use

53. Desertification  Short- and long-term consequences of U.S. desertification, regionally and nationally?  How could desertification be reduced by actions at the local, state or federal level? (see slides)

55. Dust Bowl storm near Elkhart, Kansas in 1937 Video Trailer for Dust Bowl Documentary