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Dynamic Earth: Physical Geography > Deserts and drylands > Flashcards

Deserts and drylands Flashcards

(61 cards)

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1
Q

what percentage of worlds pop. lives in drylands and deserts?

A

40%

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2
Q

what does it mean to have spatially discontinuous precipitation

A

there are isolated convective cells that cause small intense bursts of rainfall on specific areas

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3
Q

virga

A

precipitation that evaporates before reaching the land

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4
Q

hortonian overflow

A

when rainfall exceeds infiltration capacity and depression storage capacity-> predominant in deserts

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5
Q

impact of desert rivers

A

v. powerful erosional agents, move lots of sediment, can be sand coloured due to suspended load

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6
Q

what parameter defines deserts

A

aridity

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7
Q

what are drylands

A

populated desert regions where agriculture may occur

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8
Q

what does P stand for in water balance

A

monthly precipitation (input)

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9
Q

what does PEt stand for

A

monthly potential evapotranspiration

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10
Q

what is potential evapotranspiration

A

amount that could occur from a standardised surface never short of water supply

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11
Q

what scale measures aridity in deserts

A

general aridity index

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12
Q

what constitutes a hyper arid region

A

12 consecutive months with no rainfall and no regular seasonality of rainfall

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13
Q

how many deserts in the world

A

over 51

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14
Q

what percentage of earths surface is dryland?

A

over 60%

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15
Q

4 principal reasons for moisture deficits which create drylands

A

atmospheric stability due to subtropical high pressure belts-> consistently descending stable dry air,
continentality,
cold ocean currents,
rain shadows

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16
Q

role of gravel in stony deserts

A

protect underlying sediments from wind erosion

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17
Q

what can provide a boundary for sand dunes?

A

river valleys

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18
Q

5 controls on diversity of dryland landscapes

A

moisture availability, structural settings, parent rocks, ecological characteristics, geomorphic conditions

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19
Q

which moisture balance (P/PEt) value indicates moisture deficiency

A

under 1.0 is moisture deficit, under 0.5 is drylands

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20
Q

what value indicates a hyper-arid area

A

less than 0.05

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21
Q

what value indicates a semi-arid area

A

0.2-0.5

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22
Q

what value indicates an arid area

A

0.05-0.2

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23
Q

what values indicate a dry-subhumid area

A

0.5-0.65

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24
Q

why might some subtropical deserts receive some rainfall

A

seasonally shifting intertropical convergence zone

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25
aridisol
all year round dryness and salinity
26
alfisol
moderate base saturation and some seasonal moisture
27
entisol
little- altered sedimentary material and lacks horizonation
28
mollisol
base and organically rich- scarce
29
vertisol
deep-cracking clay soil, v dry or saturated
30
how does soil crusting occur
concentration of minerals in the top layers due to high evaporation rates, raindrop impact or biological crusts
31
how does salt crusting occur
accum. of gypsum and halite in top layer. these create gycretes and salcretes crusts
32
duricrusts
the subsurface enrichment of dryland soils by calcium carbonate and silica to form calcretes and silcretes
33
3 types of plants according to aridity
hydrophytes- saturated soils tolerated, mesophytes- temperate regions, 10-20% moisture, xerophytes- extreme moisture deficiencies
34
how do lower plants survive in the desert
enter a dormant state before returning to active state when moisture is present
35
how do grasses survive in the desert
have bulbs and rhizomes so avoid drought by confining growth to the wet season
36
3 main dryland vegetation systems
savanna, desert, extreme desert
37
vegetation in savanna
10-30% shrub cover, perennial grasses, succulents...
38
vegetation in desert
perennial vegetation, rarely over 10% cover, shrubs and grasses, flush of growth after rainfall
39
veg. in extreme desert
only in most favourable locations like ephemeral channel floors, deep rooted plants dig deep for groundwater
40
playas
dry lake beds where lakes once existed in wetter climatic regimes
41
3 key factors in weathering of desert rocks
presence of bare rock surfaces, high diurnal temp. range and excess of evapotranspiration over precipitation
42
main form of desert rock weathering
insolation (granular disaggregation)
43
3 mechanical processes in salt weathering
crystallisation, hydration, thermal expansion
44
when is biomass at lowest levels in a desert?
at the end of the dry season and the beginning of rain again
45
endoreic
inland-draining river systems
46
where do endoreic systems usually end
inland salt pans or playas
47
what % of drylands are covered by aeolian sand deposits and dunes
20%
48
what is loess
thick deposits of wind-blown silt
49
how are sand seas formed
rate of arrival of sand exceeds rate of loss due to change in surface roughness (slows down wind and causes deposition)
50
what happens to wind velocities as dunes grow?
they can increase due to the compression of flow lines
51
what determines dune size
sediment availability, dune type, duration of accumulation
52
what can impact the type of dune formed
direction of sand transport
53
how do dunes encourage further deposition
by modifying the wind environment to create downwind flow perturbations
54
what dunes can migrate forward most easily
transverse dunes
55
where does 90% of sand transport occur
lower 50cm of the atmosphere
56
ventifacts
smooth surfaces of individual stones
57
where is the abrasive effect of aeolian sand limited to
the max height of saltation
58
yardangs
streamlined features smoothed from hills
59
how were drylands during the last glaciation?
more extensive, world more arid overall, more dust emissions from the Sahara
60
human impacts on deserts
lowering of groundwater table and conc. of salts in the soil, enhanced desertification, management of ephemeral dryland rivers, global warming
61
dominant causes of desertification?
water erosion, wind erosion, chemical degradation, physical degradation

Dynamic Earth: Physical Geography (11 decks)

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