1.2- Drainage Basins As Open Systems Flashcards Preview

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Flashcards in 1.2- Drainage Basins As Open Systems Deck (44):

What is a drainage basin?

The area of land drained by a river and its tributaries


What separates one drainage basin from another?

An imaginary line called a watershed


The drainage basin is considered to be an open system with

Inputs, and outputs of energy and stores and transfers of water


Drainage basins come in a variety of scales from the very small to continental e.g.

The Mississippi Basin


The drainage basin is part of the hydrological cycle- it starts when

Precipitation (input) first lands on the surface and finishes when water leaves the drainage basin either by evapotranspiration (output) or as the river enters the sea through runoff (output)


Precipitation is an input- explain the main points

- water enters drainage basin as precipitation
- plants and trees may intercept some water, where it may be stored before being evaporated
- it takes time for water to drip through leaves or down the stem (stemflow) to the ground surface
- it is stored as puddles, flows over the ground as overland flow or infiltrates the soil
- some water is taken up by plants before being transpired


What are the 2 main outputs?

1- evapotranspiration (total output of water from the drainage basin directly back into the atmosphere)

2- runoff (all the water that enters a river channel and eventually flows out the drainage basin)


As water travels through the drainage basin it can stop, for a while, in a number of



What are the main stores?

- interception
- surface water
- soil water
- groundwater
- channel storage (water in the river)


Explain how interception is a store

- when precipitation lands on leaves and branches of vegetation
- some vegetation is a better interception store than some others
- deciduous broad leaves intercept more precipitation than the needles of a coniferous tree


Explain how surface water is a store

- refers to when water accumulates on the ground, often in the form of puddles
- while common in a man-made environment, they are naturally rare in natural environments
- water usually infiltrates the ground at a greater rate than it rains- water can only build up on the surface after a long period of rain, an intense rainstorm or an impermeable surface (either man-made e.g. concrete or natural e.g. frozen surface)


Explain how soil water is a store

- simply water stored within the soil
- it occupies the pore spaces between soil particles
- coarse, sandy soils absorb more water rapidly than clay soils which have tiny pore spaces
- when water passes through the soil,the transfer is known as throughflow


Explain how groundwater is a store

- simply water in the ground (rocks) beneath the surface
- some rocks are able to store a lot of water, especially if they are porous like chalk = AQUIFERS


What are the main transfers/flows?

- stemflow
-overland flow
- channel flow


Explain stemflow

- water making its way from the leaves of vegetation to the ground
- this drips/falls from leaf or branch to another until it eventually reaches the ground
- water also flows down stems of grasses and in very heavy storms, it can flow straight down the trunks of trees


Explain infiltration

- water’s ability to sink into the soil
- speed at which this happens is called the infiltration rate/ capacity
- if there is intense or prolonged then the infiltration rate may not be fast enough to absorb all the water


Explain overland flow

- if water is unable to infiltrate it may run off the surface as overland flow flowing across a large surface area (sheetflow) or concentrated into smaller channels called rills
- overland flow on agricultural land is not common in the UK as much of the land is covered by vegetation, although it can sometimes be seen in winter when the soils are bare
- in urbanised areas, particularly on roads, overland flow is extremely common often exacerbating flooding


Explain channel flow

- all of those flows lead water to the nearest river or deep groundwater storage
- the river then transfers the river by channel flow
- the amount of water that leads the basin in this way is called runoff


Any changes to inputs, transfers or stores by people or natural means will have a knock-on effect somewhere else in the system- for example

- deforestation in a river basin will result in less interception and transpiration and eventual increase in channel flow
- a drought may cause a reduction in runoff and decrease in other stores and transfers such as infiltration and soil water storage and ground water store = depleted aquifers


The amount of water in the drainage basin will vary over time depending on the balance between

Inputs and outputs


The water balance in a drainage basin shows the balance between

Inputs (precipitation) and the outputs (runoff and evapotranspiration) together with changes in groundwater storage


The water balance can change with the



In the UK there is usually a _____ _____ during the winter and early springs

Water surplus- evaporation is greater than transpiration = high run off


During the summer there may be a ______ _____ as temperatures rise and vegetation grows quickly- groundwater stores and water from springs may be needed to keep rivers flowing

Moisture deficit


In order to gain a better understanding of the drainage basin system we can use a simple equation called the general water balance equation =

P= Q+E (+/- change in storage)

P= precipitation
Q= runoff
E= evapotranspiration


Regarding water balance, explain the events in chronological order

1- water surplus: Dec- Feb
2- soil moisture utilisation: March-June
3- soil moisture deficiency: July- Sept
4- soil moisture recharge: October-Dec


What occurs in water surplus?

- precipitation is greater than potential evapotranspiration
- soil moisture store is full and there is a soil moisture surplus for plant use, runoff and groundwater recharge


What occurs in soil moisture utilisation?

- potential evapotranspiration greater than precipitation
- the water store is being used up by plants or lost by evaporation


What occurs between soil moisture utilisation and deficiency?

- soil moisture store now used up
- any precipitation is likely to be absorbed by the soil rather than produce runoff
- river levels will fall or dry up completely


What occurs in soil moisture deficiency?

- deficiency of soil water as the store is used up and potential evapotranspiration is greater than precipitation
- plants just have adaptions to survive and crops must be irrigated


What occurs in soil moisture recharge?

- precipitation is greater than potential evapotranspiration
- the soil water store will begin to fill again


What occurs between soil moisture recharge and water surplus?

- soil water store is full
- field capacity been reached
- additional water will percolate down to the water table and groundwater stores recharged


What does field capacity refer to?

Maximum amount of water soil can hold:
- a water surplus can result in saturated soil, high river levels and run-off whereas a deficit leads to dry soil, falling river levels and possibly drought


An important aspect of the water balance equation is

Total runoff (expressed as a % of precipitation) = measure of total precipitation that makes its way into rivers and streams


Runoff % will vary considerably between

different river basins and will depend on a range of factors


What are the different factors that influence water flows within a drainage basin (run off variation)?

- vegetation and land use
- soil type and depth
- bedrock
- rainfall
- climate
- shape of the land
- conditions in river basin
- size and shape of river basin


Explain how vegetation and land use alter run off variation

- amount of vegetation may vary seasonally
- type and amount of vegetation cover will affect the amount of interception storage as well as amounts of throughfall and stemflow
- vegetation tends to reduce the amount of overland flow; bare ground will increase it
- impermeable surfaces created by urban development will also reduce infiltration and increase overland flow and increasing urban sprawl will further drive this


Explain how soil type and depth alter run off variation

- deeper soil able to store more water = high field capacity
- soils with smaller pore spaces in them, such as clay, will reduce infiltration


Explain how bedrock alters runoff variation

- impermeable bedrock such as granite will prevent groundwater flow and encourage throughflow and overland flow
- rocks with air spaces or cracks in them will let water through
- however,if these rocks become saturated they will act as though they are impermeable


Explain how precipitation alters runoff variation

- intense rainfall will tend to increase overland flows and cause a sudden rise in discharge
- gentle rain over a longer period of time allows more time for infiltration


Explain how climate alters runoff variation

- the distribution of rainfall over the year affects how the drainage basin will responds
- some areas may have a very seasonal pattern with rainfall only in one season
- temperature conditions are also important as they will affect evapotranspiration and may determine whether water is stored as ice or snow


Explain how shape of the land alters run off variation

- steep slopes will encourage overland flow
- if the drainage basin has many surface storage stores such as lakes, ponds and hollows, this will reduce river flow


Explain how conditions in the drainage basin alter run off variation

- these may have considerable effects on water flows
- ground that is frozen or has a baked surface crust will produce more surface runoff
- if the ground is already wet, overland flow will occur sooner


Explain how the shape and the size of the river basin alter run off variation

- larger basins result in more runoff
- shape of basin can affect the time taken for rainfall to reach the river channel
- a high density of tributaries can produce high flows