the water cycle

define an open system

when systems receive inputs and transfer outputs of energy or matter with other systems

define a closed system

when there is no transfer of matter into or out of the system

define dynamic equilibrium in a system

when inputs equal outputs despite changing conditions

are the carbon and water cycles open or closed systems

• on a local scale, they are open systems

• on a global scale they are closed systems

describe the inputs of water to a drainage basin system

rainfall (there are three types);

• convectional- due to heating by the sun, warm air rises, condenses at higher altitudes and falls as rain

• relief- warm air is forced upward by a barrier such as mountains, causing it to condense at higher altitudes and fall as rain

• frontal- warm air rises over cool air when two bodies of air at different temperatures meet, because they warm air is less dense and therefore lighter. it condenses at higher altitudes and falls as rain

describe the outputs of a drainage basin system

• evapotranspiration- evaporation occurs when water is heated by the sun, causing it to become a gas and rise into the atmosphere. transpiration occurs in plants when they respire through their leaves, releasing the water they absorb through their roots, which then evaporates due to heating by the sun

• streamflow- all water that enters a drainage basin will either leave through the atmosphere, or through streams which drain the basin. These may flow as tributaries into other rivers or directly into lakes and streams

describe the flows of water in the drainage basin system

• infiltration- water moves from above ground into the soil. grass crops and tree roots create passages for water to flow through from the surface into the soil, so the rate of infiltration increases. if precipitation falls at a greater rate, then overland flow will occur

• percolation- water moves from the ground or soil into porous rock. the percolation rate is dependent on the fractures in the rock and the permeability of the rock.

• through flow- water moves through the soil and into streams or rivers. speed of flow depends on type of soil (clay soils have smaller pore spaces to a slower flow rate) (sandy soils drain quickly because they have larger pore spaces)

• surface runoff (overland flow)- water flows above the ground or in rills (small channels similar to streams, that are unlikely to cary water during periods where there is not any rainfall)

• groundwater flow- water moves through rocks

• streamflow- water moves through established channels

• stemflow- flow of water that has been intercepted by trees, down stem of a plant

describe the stores of water in a drainage basin system

• soil water- water stored in the soil which is utilised by plants

• groundwater- water stored in pore spaces of rocks

• river channel- water stored in a river

• interception- water intercepted by plants on their branches and leaves before reaching the ground

• surface storage- water stored in puddles, ponds, lakes etc.

what is the water table

the upper level at which pore spaces and fractures in the ground become saturated.

when is the water balance used

it is used to express the process of water storage and transfer in a drainage basin ssytem

it uses a formula

the water balance is an important factor in explaining droughts or floods

what physical factors will change the water balance

seasonal variations of temperature and precipitation

how does deforestation change the water cycle

there is less interception by trees so surface runoff increases. the soil is no longer held together by roots, so soil water storage decreases. there are fewer plants so transpiration decreases

how do storm events affect the water cycle

• large amounts of rainfall quickly saturate the ground

• no water can infiltrate into the soil, increasing the surface runoff

how do seasonal changes affect the water cycle

• spring- more vegetation growth so more interception by vegetation

• summer- likely to be less rain in summer. ground may be harder and therefore more impermeable encouraging surface runoff

• autumn- less vegetation growth so less interception. seasonally more rainfall

• winter- frozen ground may be impermeable and encourage runoff. snow discourages runoff and takes time to melt, slowing down the processes that occur within the water cycle

how does agriculture affect the water cycle

• pastoral farming relates to livestock. livestock trample the ground, reducing infiltration

• arable farming relates to crops. ploughing increases infiltration by creating a looser soil, which decreases surface runoff. however, digging drainage ditches increases surface runoff and streamflow

• hillside terracing (for rice padi fields)- increases surface water storage and therefore decreases runoff

• irrigation (the movement of water by human intervention) can lead to groundwater depletion

how can urbanisation affect the water cycle

• roads and buildings creates impermeable surfaces with drains that reduce infiltration but increase surface runoff, reducing lag-time and increasing the flood risk.

what does the soil water budget show

it shows the annual balance between inputs and outputs in the water cycle and their impact on the soil water storage.

it is dependent on the type, depth and permeability of the soil and bedrock. Once the maximum capacity of the soil is reached, rainfall after this will not infiltrate into the ground and flooding is likely.

describe the seasonal variation of the water budget in autumn

• there is a greater input from precipitation than there is an output from evapotranspiration as deciduous trees lose their leaves and cooler temperatures mean that the plants photosynthesise less. soil moisture levels increase and a water surplus occurs.

how does winter affect the soil water budget

potential evapotranspiration from plants reaches a minimum due to the colder temperatures and the precipitation continues to refill the soil water stores. infiltration and percolation will also refill the water table

how does spring affect the soil water budget 

plants start to grow again and potential evapotranspiration increases as temperatures get higher and plants start photosynthesising more. there is still a water surplus 

how does summer affect the soil water budget

hotter weather leads to utilisation go soil water as evapotranspiration peaks and rainfall is at a minimum. the output from evapotranspiration is greater than the input from precipitation and so the soil water stores are depleting. a water deficit may occur.

how much of global water do the oceans contain

97%

how much water in the world is freshwater

2.5%

what are aquifers

underground water stores that are unevenly distributed on a global scale

what happens in the global atmospheric model 

• different zones of rising and falling air leads to precipitation through convectional rainfall.

• this creates a low pressure zone, which leads to very heavy rainfall

what is the El Niño and La Niña effect

El Niño- occurs every 2-7 years and causes warm temperatures in a predictable way

La Niña- occurs every 2-7 years and causes cooler temperatures in a predictable way

it is likely that climate change will increase the probability of more El Nino’s in the future

how could changes to the cryosphere change the water cycle

• almost all of the world’s glaciers are shrinking, causing sea levels to rise.

• if all the world’s glaciers and icecaps were to melt, sea levels would rise by around 60 metres

how do farming practices impact the water cycle

• ploughing breaks up the surface, increasing infiltration

• arable farming (crops) can increase interception and evapotranspiration

• pastoral (animal) farming compacts soil, reducing infiltration and increasing runoff

• irrigation removes water from local rivers, decreasing their flow

how do land use changes affect the water cycle

• deforestation reduces interception and evapotranspiration but infiltration decreases (dead plant material in forests usually prevents infiltration)

• contraction reduces infiltration and evapotranspiration, but increases runoff

how does water abstraction (water removed from stores for human use) affect the water cycle

• reduces the volume of water in the surface stores e.g. lakes

• water abstraction increases in dry seasons (e.g. water is needed for irrigation)

• human abstraction from aquifers as an output to meet water demands is often grater than inputs to the aquifer, leading to a decline in global long-term water stores

simply list some human impacts to the water cycle

• farming practices

• change in land use

• water abstraction

simply list some natural processes that cause changes in the water cycle

• seasonal changes

• storm events

• droughts

• El Niño and La Nina

• cryospheric changes 

what does a flood hydrographic show

it shows the rainfall for the drainage basin of a river and the discharge of the same river on a graph.

key words include: rising limb, lag time, bankfull discharge, falling limb, stormflow, baseflow

define discharge

the volume of water passing through a cross-sectional point of the river at any one point in time, measured in Cubic Metres Per Second (Cumecs). Made up of baseflow and stormflow

what is the rising limb on a flood hydrograph

the line on the graph that represents the discharge increasing 

what is the falling limb of a flood hydrograph

the line on the graph that represents the discharge decreasing

what is lag time on a flood hydrograph

the time between peak rainfall and peak discharge

what is baseflow on a flood hydrograph 

the level of groundwater flow

what is stormflow on a flood hydrograph

comprised of overland flow and through flow

what is tankful discharge on a flood hydrograph

the maximum capacity of the driver. if discharge exceeds this then the river will burst its banks and be in flood

what is a flashy hydrograph

short lag time and high peak discharge, most likely to occur during a storm event, with favourable drainage basin characteristics 

what is a subdued hydrograph

long lag time and low peak discharge

list natural factors that would act to create a flashy hydrograph

1. high rainfall intensity- higher discharge potential from the river and more likely for soil to become saturated, increasing surface runoff and decreasing lag time.

2. antecedent rainfall (rainfall that occurs before the studied event e.g. rain the day before)- increases surface runoff as ground is saturated and soil has reached its field capacity

3. impermeable underlying geology- decreased percolation and therefore greater levels of through flow

4. high drainage density- many tributaries to main river, increasing speed of drainage and decreasing the lag time

5. small basin- rainfall reaches the central river more rapidly, decreasing lag time

6. low temperatures- less evapotranspiration so greater peak discharge

7. precipitation type- snow or hail takes time to melt before moving towards the river, so rainfall increases the flooding risk

8. vegetation cover- forested areas intercept more rainfall, decreasing the flood risk, but exposed areas will transfer water to the river more rapidly, decreasing lag time.

list human factors that would create a flashy hydrograph

1. urbanisation- more impermeable surfaces, so runoff increases and surface storage and infiltration are reduced

2. pastoral farming- ground trampled so less interception and more surface runoff 

3. deforestation- less interception by trees, so water reaches the ground and river more quickly. more surface runoff and greater flood risk

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