drainage basins Flashcards
what is a drainage basin?
area that supplies a river with its supply of water - below the water table, soil water and any surface flow
how are drainage basins separated?
by high land - watershed
what is the input to a drainage basin?
Precipitation
- lands on the bare surface or, more likely, vegetation cover. This vegetation provides a store: interception store.
- occurs on leaves/branches of vegetation
- some vegetation is a better interception store than others
- density of vegetation cover - study - forests with needle-leaf trees captured 22% of the rainfall, whilst deciduous forests captured 19% - some tropical rainforests intercept as much as 58% of the rainfall
- a lot of the water captured by vegetation surfaces is evaporated back into atmosphere
What is a transfer in the drainage basin?
water makes its way from the leaves of trees to the ground drips or falls by the process of throughfall from one interception store to another until it eventually reaches the ground - the portion of the precipitation that reaches the ground directly through gaps in the vegetation canopy and drips from leaves, trigs and stems - occurs when the canopy-surface rainwater storage exceeds its storage capacity.
- water also flows down the stems of grasses etc., and in very heavy storms it can flow straight down the trunks of trees - called stemflow
What is infiltration and how does it relate to the drainage basin system?
- on reaching the ground, water soaks into the soil by infiltration and the rate of this movement is called the infiltration rate
- this movement of water into soil is controlled by gravity, capillary action and soil porosity - soil porosity is the most important
- soil’s porosity is controlled by its texture, structure and organic content
- coarse textured soils have larger pores and fissures than fine-grained soils and so allow more water flow
- pores and fissures found in soils can be made larger through factors that enhance internal soil structure e.g. the burrowing of worms
what happens to infiltration during a rainstorm event?
declines rapidly and reaches a constant value after several hours of rainfall
- the filling of small pores on the soil surface with water reduces the ability of capillary forces to actively move water into the soil
- as the soil moistens, the clay particles absorb water causing them to expand - reducing size of soil pores
- raindrop impact breaks large soil clumps into smaller particles which clog soil surface pores reducing movement into soil
what is soil storage?
the amount of water stored in the soil
what is vegetation storage?
vegetation requires water to survive - most plants remove water from the soil and store is in the structure of the plant = vegetation storage
plants lose water back to the atmosphere through stomata on their leaves = transpiration
what is infiltration capacity?
if rainfall intensity is greater than the infiltration rate then the soil has reached infiltration capacity and the soil will be saturated
water will build up on the surface as surface storage which is usually in the form of puddles
what is evaporation
most of surface storage evaporates into atmosphere and is lost to the drainage basin
what is evapotranspiration?
total amount of outputs of the system
what is overland flow
the tendency of water to flow horizontally across land surfaces when rainfall has exceeded infiltration capacity of the soil and all surface stores are full to overflowing
what is throughflow?
any lateral movement of soil water downslope is called throughflow and eventually reaches the nearest channel - slower than overland flow - more vegetation = faster rate of through flow as it is aided by root channels
what is percolation?
following infiltration water moves vertically down through the soil and unsaturated rock by the process of percolation - held in pore spaces in the rocks as groundwater
what is groundwater flow?
this is very slow transfer of water through rock - only in limestone areas where there are extensive underground channels can flow be faster
