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3.2 Factors Influencing The Drainage Basin Pathways (Climate (macro)…
3.2 Factors Influencing The Drainage Basin Pathways
Climate
(macro)
Rainfall
Intensity
Influences infiltration rate
High intensity rain encourages processes like blocking of soil pores through rain splash action, encouraging compaction of upper soil layers, ↓ porosity and infiltration capacity
Tropics generally have high intensity rain resulting in
overland flows
Magnitude & Duration
M: Raindrop size
Humid:
Frequent convective rainfall, occasionally episodic/seasonal events which are large in scale and prolonged (monsoons/cyclones)
Arid:
Rainstorms are brief, episodic and localised but forms a large part of the total rainfall
Amount & Seasonality
Humid:
2500mm with little seasonality in perennial tropics (Af), and more rain during wet seasons of Am/Aw climates
Arid:
Low rainfall with no seasonal regularity, received from occasional and irregular storms (Bsh)
:leaves: Timimoun in Algeria had no rain 98.8% of the time within a 25year period
Affects
groundwater recharge
and influences
water balance
in certain parts of the year
Temperature
High temps → high
evaporation rates
, causing a difference in
water balance between H&A
Humid:
Rainfall total will always be > evaporation rates as rainfall received is a lot (temp affects H lesser than rain)
Arid:
Due to low rainfall, evaporation rates might be higher
:leaves: Comparing Cuba and Brazil's basins, Cuba's humid basin was twice that of Brazil's semi-arid basin and river flow was x5 higher due to low evaporation rates in humid
only gives a broad idea on water balance characteristics in terms of input and output not the individual pathways
Surface & Subsurface Characteristics
(local)
Soil Characteristics
Affects amount of water infiltrated into the soil
Depth/Thickness of Soil
Influenced by rate of weathering
Thickest in
humid
due to active CW , thinnest in
arid
due to limited weathering from the absence of water
Soil Structure
Way in which individual grains are aggregated into pods
Humid:
Particles held together by organic matter causing the aggregate stability of the soil to be
high
and less prone to breakdown, they are also loose and crumbly allowing for rapid infiltration and drainage
Arid:
Usually a thin crust that was compacted by previous rainfall events
Anthropogenic activities can compact the soil, impeding infiltration
Soil Texture
Determined by proportion of constituent grans (sand/silt/clay) which determines porosity and permeability of soil
Coarse
(gravels/sands): Pore spaces are
large
and well-connected , both porous and permeable allowing for ready infiltration
Fine
(silts/clays): Many pores but the are not well-connected and fine, so soil is porous with low permeability
Sand soils > silt infiltration rate
Humid:
Clays located at deeper depths of soil profile due to eluviation,
Arid:
Clays located at the surface, so harder for water to infiltrate downwards due to its low permeability
Soils like those in arid is vulnerable to
slaking
where earth materials disintegrate and crumble when exposed to moisture
Fine materials swell when it absorbs water and breaks down easily in soils of low aggregate stability, collecting in surface pores and sealing the soil surface forming a crust
Crust: A layer of well-packed fine sand/silt/clay horizons or smooth surface pores infilled with silt and clay-sized particles that reduces infiltration
Humid
Thick soil due to active CW
High aggregate stability
Clay/ fine grained layer is deeper in soil so infiltration is faster and more water can be stored as GW
Presence of
antecedent soil moisture
(water from past storms) can impede infiltration of fresh water thus a lower infil capacity
Arid
Thin soil of low aggregate stability due to limited organic matter
Typically stony surface and soils sit on inpermeable bedrock
Crust easily formed due to slaking
Loose stones on surface > Stones embedded in surface has higher infil preventing crust developing around it
Vegetation
(meso)
Facilitates infiltration, increases amount of water infiltrated as lesser water lost to surface runoff, promotes storage and delays runoff
Reduces Infiltration
Intercepts rainfall, ↓ amount of water received at ground surface for infiltration
Increases Infiltration
↓ velocity and impact of raindrops hitting the ground (reduces intensity) , reducing the likelihood of crust formation in arid thus ↑ soil porosity and water infiltrated
Allows raindrops to slowly infiltrate the soil and not all at one go
Roots of vegetation help to break up the soil, creating macropores and subsurface pathways in the soil structure
Helps to return the moisture through evapotranspiration
Humid
Dense vegetation where density of vegetation ↓ with rainfall total
Not all rainfall reaches the ground and the rain that reaches is of
lesser intensity
Arid:
Little vegetative cover
Most rain reaches the ground with
great intensity
Scale (Macro/Meso/Local)
Time (After rainfall / Before)
Anthropogenic Activities
(local)
Land Use
Affects infiltration and runoff rates
Deforestation, Agriculture, Urban activities
Animal grazing or heavy machines can compact the soil and impede infil
Ploughing results in coarse soil structure which promotes infiltration
:leaves: In Burkino Faso, Africa, infiltration rate increased from 1.6 to 6mm/h when the crust was destroyed by ploughing
Arid
Episodic and low amounts of
intense rainfall
(intensity>duration) can result in rapid/intense streamflow or even floods
Sparse vegetation
results in immediate surface runoff and less moisture infiltrates, causing great transmission losses
Stony surfaces with extensive crust
and thin soils sitting on impermeable bedrock results in little water infiltrating and higher runoff
Holes dug by small animals
can increase infiltration by providing direct pathways for water while large animals compact the soil and ↓ infiltration
High temperatures
causes rapid evaporation when rainfall is present causing transmission losses (not as imp as rainfall is not so much)
Cracking and desiccation
results in macropores formed which can develop into large pipes
Drainage Basin:
Infiltration is shallow, runoff is localised (largest pathway is surface runoff IEF that's brief and rapid or in rills&gullies as reticular flow)
NEGATIVE WATER BALANCE :open_mouth:
only after a rainfall event is it temporarily positive
Humid
Constant intense rainfall but modified by
dense vegetation
so runoff generated is not that large
Infiltration rates are high, reducing loss of water from IEF
Thick soil profile of high aggregate stability
and high soil moisture
Fine-grained clay layer located
deeper
in the soil, helping to store water but not impeding infiltration
POSITIVE WATER BALANCE
:smiley:
For seasonally humid, it fluctuates between + and -