Deserts and drylands Flashcards
what percentage of worlds pop. lives in drylands and deserts?
40%
what does it mean to have spatially discontinuous precipitation
there are isolated convective cells that cause small intense bursts of rainfall on specific areas
virga
precipitation that evaporates before reaching the land
hortonian overflow
when rainfall exceeds infiltration capacity and depression storage capacity-> predominant in deserts
impact of desert rivers
v. powerful erosional agents, move lots of sediment, can be sand coloured due to suspended load
what parameter defines deserts
aridity
what are drylands
populated desert regions where agriculture may occur
what does P stand for in water balance
monthly precipitation (input)
what does PEt stand for
monthly potential evapotranspiration
what is potential evapotranspiration
amount that could occur from a standardised surface never short of water supply
what scale measures aridity in deserts
general aridity index
what constitutes a hyper arid region
12 consecutive months with no rainfall and no regular seasonality of rainfall
how many deserts in the world
over 51
what percentage of earths surface is dryland?
over 60%
4 principal reasons for moisture deficits which create drylands
atmospheric stability due to subtropical high pressure belts-> consistently descending stable dry air,
continentality,
cold ocean currents,
rain shadows
role of gravel in stony deserts
protect underlying sediments from wind erosion
what can provide a boundary for sand dunes?
river valleys
5 controls on diversity of dryland landscapes
moisture availability, structural settings, parent rocks, ecological characteristics, geomorphic conditions
which moisture balance (P/PEt) value indicates moisture deficiency
under 1.0 is moisture deficit, under 0.5 is drylands
what value indicates a hyper-arid area
less than 0.05
what value indicates a semi-arid area
0.2-0.5
what value indicates an arid area
0.05-0.2
what values indicate a dry-subhumid area
0.5-0.65
why might some subtropical deserts receive some rainfall
seasonally shifting intertropical convergence zone
aridisol
all year round dryness and salinity
alfisol
moderate base saturation and some seasonal moisture
entisol
little- altered sedimentary material and lacks horizonation
mollisol
base and organically rich- scarce
vertisol
deep-cracking clay soil, v dry or saturated
how does soil crusting occur
concentration of minerals in the top layers due to high evaporation rates, raindrop impact or biological crusts
how does salt crusting occur
accum. of gypsum and halite in top layer. these create gycretes and salcretes crusts
duricrusts
the subsurface enrichment of dryland soils by calcium carbonate and silica to form calcretes and silcretes
3 types of plants according to aridity
hydrophytes- saturated soils tolerated, mesophytes- temperate regions, 10-20% moisture, xerophytes- extreme moisture deficiencies
how do lower plants survive in the desert
enter a dormant state before returning to active state when moisture is present
how do grasses survive in the desert
have bulbs and rhizomes so avoid drought by confining growth to the wet season
3 main dryland vegetation systems
savanna, desert, extreme desert
vegetation in savanna
10-30% shrub cover, perennial grasses, succulents…
vegetation in desert
perennial vegetation, rarely over 10% cover, shrubs and grasses, flush of growth after rainfall
veg. in extreme desert
only in most favourable locations like ephemeral channel floors, deep rooted plants dig deep for groundwater
playas
dry lake beds where lakes once existed in wetter climatic regimes
3 key factors in weathering of desert rocks
presence of bare rock surfaces, high diurnal temp. range and excess of evapotranspiration over precipitation
main form of desert rock weathering
insolation (granular disaggregation)
3 mechanical processes in salt weathering
crystallisation, hydration, thermal expansion
when is biomass at lowest levels in a desert?
at the end of the dry season and the beginning of rain again
endoreic
inland-draining river systems
where do endoreic systems usually end
inland salt pans or playas
what % of drylands are covered by aeolian sand deposits and dunes
20%
what is loess
thick deposits of wind-blown silt
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)
what happens to wind velocities as dunes grow?
they can increase due to the compression of flow lines
what determines dune size
sediment availability, dune type, duration of accumulation
what can impact the type of dune formed
direction of sand transport
how do dunes encourage further deposition
by modifying the wind environment to create downwind flow perturbations
what dunes can migrate forward most easily
transverse dunes
where does 90% of sand transport occur
lower 50cm of the atmosphere
ventifacts
smooth surfaces of individual stones
where is the abrasive effect of aeolian sand limited to
the max height of saltation
yardangs
streamlined features smoothed from hills
how were drylands during the last glaciation?
more extensive, world more arid overall, more dust emissions from the Sahara
human impacts on deserts
lowering of groundwater table and conc. of salts in the soil, enhanced desertification, management of ephemeral dryland rivers, global warming
dominant causes of desertification?
water erosion, wind erosion, chemical degradation, physical degradation
