Water and Carbon Cycles


Parts of a system

 Inputs- matter or energy which is added to a system

Outputs- matter or energy leaving a system

Stores/components- a build-up of matter or energy

 Flows/transfers- matter or energy moving from one store to another

Boundaries- the limits of a system.

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Types of system (open/closed)

Systems can be open or closed. In an open system both energy and matter can enter and leave the system, these are the inputs and outputs. Eg, drainage basins are open systems, energy from the sun enters and leaves the system and water is inputted as rain and outputted by river discharge into the sea.

In a closed system matter cannot enter or leave the system, it is only able to cycle between stores, however energy is able to both enter and leave. Eg. The carbon cycle is a closed system, energy can be inputted eg from the sun or photosynthesis and outputted from respiration- but the amount of carbon on earth does not change as matter cannot be created or destroyed.

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Dynamic Equilibrium

If the inputs and outputs of a system are balanced then the system is known to be in equilibrium, they flows and processes continue to happen- but in the same way at all times so there are no ultimate changes in the system. 

However in reality there will always be lots of small variations in the inputs and outputs of a system eg rain entering a drainage basin is always changing. But these variations are small and so in the long term the inputs and outputs remain balanced on average. Hence the system is in dynamic equilibrium. Long-term changes in a system to the balance of inputs and outputs can cause changes to a system so a new equilibrium has to be established.

These changes can trigger different types of feedback: positive and negative 

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 positive feedback are mechanisms which amplify the change in the inputs and outputs. This means that the system responds by increasing the effects of the change, moving the system further from its previous state. Eg, if temperatures rise, ice in plar regions will melt, less ice cover means that less of the suns energy is reflected, so more energy is absorbed by earth, this then causes a further rise in temperature.

Negative feedback mechanisms counteract the change in inputs and outputs, this means that the system responds by decreasing the effects of the change- so the system remains closer to its original state. Eg, Lots of CO2 emitted, atmospheric CO2 increases, this extra CO2 causes plants to increase growth, plants remove and store more CO2, and so the amount of CO2 is reduced.

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The Earth's Systems

The earth can be seen as a closed system- where energy is the input from the sun and is outputted back into space but matter is not input or output into space. But this whole system can be broken down into smaller subsystems.

Cryosphere; systems in place in frozen environments

Lithosphere; the outermost part of the earth, the crust and the upper part of the mantle

Biosphere; the part of the earths system where living things are found, and includes all living parts of the earth

Hydrosphere; includes all of the water on earth- in liquid, solid or gas form. Saline or fresh.

Atmosphere; the layer of gas between the earth and space, held in place by gravity. 

All of the subsystems within the earth’s whole system are interlinked and connected by the cycles and processes that keep the earths system as a whole running as normal. Matter and energy both move through all subsystems, the output of one cycle is the input of the next etc. because of the way that matter and energy move from one system to the next the earths system is said to be a cascading system. Due to this a change in one system can affect what happens in the others. 

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Stores of Water

The hydrosphere contains 1.4 sextillion litres of water, most of this is saline water in the ocean, and less than 3% is freshwater. Of this 3%- 69% is frozen in the cryosphere in sea ice, ice shelves, ice sheets, ice caps, alpine glaciers and permafrost, 30% is stored as groundwater as terrestrial water in the lithosphere, 0.3% is liquid freshwater on the earth’s surface, 0.04% is stored as fresh water vapour in the atmosphere.

Water must be physically and economically accessible for humans to be able to use it- ie groundwater is hard to access and so is not always cost effective to extract. Due to these factors only a small amount of water on earth is available for humans to access. Water is able to change between different states and does this using the suns energy (in the form of latent heat which is gained or lost), and for it to freeze or condense it has to loose energy.

Water is continuously cycled between different stores, known as the global hydrological cycle, which is a closed system with no inputs or outputs of water.

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Changes in the magnitude of stores

The amount of water present in each store varies over a range of scales- local to global. The magnitude of each store depends on the amount of water flowing between each of them. These flows occur at a range of spatial and temporal time scales.

Evaporation; occurs when liquid water changes state into a gas, and becomes water vapour, it gains latent heat energy (mostly from solar radiation) this evaporation increases the volume of water stored in the atmosphere. The magnitude of the evaporation flow varies by location and season, if there is more solar energy, or a large supply of warm dry air and water the rate of evaporation will be high. However if there is little water and the air is already saturated (humid and unable to absorb more water) evaporation rates will be the opposite.

Condensation; occurs when water vapour changes state to become a liquid- and loses energy to the surroundings. This happens when air containing water vapour cools to its dew point , the temperature at which it changes from a gas to a liquid, for example at night when temperature drops. Water droplets can either stay in the atmosphere or flow into other subsystems- eg when water forms dew it reduces atmospheric stores. The magnitude of the condensation flow depends on the amount of water vapour in the air and if there is a large or rapid drop in temperature increasing temperature.

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Changes in the magnitude of stores

Cryospheric processes; accumulation (input of snow and ice into the glacial system) and ablation (the output of water from a glacier) change the amount of water stored as ice in the cryosphere, the balance of these processes change with temperature. During periods of a global cold, inputs into the cryosphere are greater than the outputs- water can be transferred as snow and less water is transferred away by melting. During a warmer period the magnitude of the cryosphere reduces as losses from melting are larger than the input of snow. The earth is still emerging from a glacial period which reached its peak 21000 years ago, this is shown as there is still large quantities of ice sheets, sea ice and alpine glaciers. Variations in cryospheric processes happen over different timescales as well as changes in global temperature occurring over thousands of years- there are also annual temperature variations which are short term and seasonal.    

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Changes in the magnitude of stores

Cloud formation and precipitation; cloud formation and precipitation are essential parts of the water cycle as precipitation is the main flow of water from the atmosphere to the ground. Clouds form when warm air cools- meaning the water vapour condenses into water droplets who gather together as clouds, and fall as rain. There are other things which cause warm air to cool leading to rain, including:

  • Other air masses- warm air is less dense than cool air and so when warm meets cool, the warm is forced upwards and cools down as it rises causing frontal precipitation.
  • Topography- when warm air meets mountains it is forced to rise and cool, causing orographic precipitation
  • Convection- when the sun heats the ground, moisture evaporates and rises in a column of warm air which rises and cools causing convection precipitation.

Water droplets from condensation are too small to form clouds on their own, and so in order for clouds to form there has to be tiny particulates of other substances like dust to act as cloud condensation nuclei which gives the water a surface to condense on, so the air forms clouds rather than just dispersing. Cloud formation and rainfall can vary seasonally and by location.

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Drainage Basins

Drainage basins can be seen as open, local hydrological cycles. A rivers drainage basin is the area surrounding the river where the rain falling on land flows into that river, or the river catchment. The boundary of each drainage basin is known as the watershed. These systems are open and have inputs and outputs, the water enters the system as rain and leaves by evaporation, transpiration, and river discharge.

Inputs- precipitation details all of the ways in which moisture leaves the atmosphere- eg, rain, hail, snow, dew.snow, dew.

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Drainage Basins


interception (where rain lands on vegetation or structures before it can reach the soil) this creates a significant store, especially in wooded areas. This store is only temporary because the water collected may evaporate, or fall as through fall.

Vegetation storage is the water which has been taken up by plants, and is contained in them at any one time.

Surface storage includes water in puddles (depression storage), ponds and lakes.

Soil storage includes moisture in the soil

Groundwater storage is water stored in the ground, either in soil moisture or rock. The water table is the top surface of the zone of saturation- the point at which all of all of the pores in the rock are filled with water. Porous rocks which hold water are called aquifers.

Channel storage is water held in a river or stream channels.

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Drainage Basins

Flows (how water moves from one place to another)-

infiltration, water seeping into the soil, rates are influenced by soil type, soils structure and how much water is in the soil.   Overland flow/ runoff- is water flowing over the land, it can flow over the whole surface or just in channels- this occurs when rain is falling faster than the ground can absorb it.    Throughfall is water dripping from one leaf of a plant to the next to the ground.  Stemflow is water running down the stem of a plant or tree trunk    Throughflow is water moving slowly downhill through the soil, this is faster through ‘pipes’ which are made up of cracks in the soil or animal burrows.     Percolation is water seeping down through water into rocks and the water table groundwater storage.     Groundwater flow is water flowing slowly below the water table to the permeable rock, water flows slowly through most rocks, but flows quicker through highly permeable rocks, such as limestone which have lots of gaps for the water to travel through.  Baseflow is groundwater flow that feeds into rivers through river banks and beds.    Interflow is water flowing downhill through permeable rock above the water table.    Channel flow is the water flowing through the river or stream itself/ the river discharge.

Outputs (how water leaves the system)-   evaporation- water turning into vapour and entering the atmosphere, transpiration- evaporation from leaves, evapotranspiration- the process of evaporation and transpiration together, river discharge.

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The Water Balance

Is worked out from the inputs (rain) and outputs (channel discharge + evapotranspiration), and the balance effects how much water is in the basin. The general water balance in the UK shows us seasonal changes. In wet seasons, precipitation is greater than evapotranspiration, creating a surplus of water, the ground stores fill up with water and so there is more surface runoff and a higher river discharge- so the levels of rivers increase. In drier seasons the rate of precipitation is lower than that of evapotranspiration. The ground stores of water become depleted as more water is up taken by plants and some flows into the river channel but cannot be replaced. This means that at the end of the dry season there is a deficit of water in the ground, and become recharged in the next wet season.

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River discharge is the volume of water that flows in a river per second in cumecs or m3/s. High levels of runoff increases the discharge of a river because more water makes it into the river and increases its volume. Hydrographs are graphs of river discharge over time, they show the volume of water flowing at a certain point in a river changes over a period of time. Flood hydrographs show river discharge around the time of a storm event, they only cover a relatively short time period (hours or days).


Flashy hydrograph = short lag time, reaches peak quickly (rapid runoff and little capacity)

Subdued hydrograph = longer lag time

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1)      Peak discharge, the highest point of the graph when river discharge was at its greatest

2)      Lag time, the time between peak rainfall and peak discharge. This delay happens as the water has to have time to reach the river, if there is a short lag time the peak discharge may be increased because more water reaches the river during a short stretch of time.

3)      Rising limb, the length of the map up until peak discharge, this is where rainwater begins to reach the river channel.

4)      Falling limb, the length of the graph after the peak discharge, the discharge of the river begins to decrease after the rainfall event because less water is flowing into the river, when this becomes shallow to the end it shows that water is still flowing from stores after it has stopped raining.

5)      Base flow, the basic flow of the river- detracting the water entering the river channel from the rainfall event.

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Factors Affecting Flood Hydrographs (physical)

Size of the drainage basin- larger drainage basins can catch more rain ad so they have a higher peak discharge than smaller basins, however smaller drainage basins are more likely to have a shorter lag time as the water has a shorter distance to travel.

Shape of drainage basin- circular basins are most likely to have a flashier hydrograph this is because all points on the watershed has an equal distance to reach the river, and so lots of water will come to the same spot at the same time.

Slope- water flows more quickly downhill on a steeper slope, and has less chance to infiltrate the soil so will have a short lag time.

Rock and soil types- impermeable rocks don’t store water or let water infiltrate, increasing surface run off, and the lag time is shorter.

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Factors Affecting Flood Hydrographs (human)

Changes in tree cover- deforestation decreases the amount of interception, and the lack of vegetation on the ground makes it harder for water to infiltrate the ground- this means that more water travels to the river by overland flow. On top of this material from soil erosion can be carried to the river and contributes to its sedimentation, decreasing the size of the river channel.

Growth of urban areas- the increase in impermeable surfaces decreases the amount of water which can be taken up by the soil, and the use of infrastructure such as drains transports the water quickly out of urban areas and to the river, leading to a short lag time.  

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Changes to the Water Cycle (physical)

Storms and precipitation- intense storms generate more rain and large peak discharges. The increase of water causes flows (runoff) and stores (groundwater) to increase in size.  Some flows, like infiltration, may not be quick enough to cope with the size of the input fast enough which increases surface run off.

Seasonal changes and vegetation- the size of the inputs, flows and stores in the water cycle vary with the seasons. During the winter temperatures may fall below 0 causing water to freeze, this can reduce the size of the flows through drainage basins while the frozen store of water increases, when temps then increase the flows may increase as the ice melts.  Plants show seasonal variations, this affects the rates of interception which is higher in summer when the amount of leaves and vegetation is highest, meaning that water movement to the channel is slower in summer.

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Changes to the Water Cycle (human)

Farming practises- infiltration is a key part of the water cycle, farming practises can affect this in many ways: ploughing, breaks up the surface so more water is able to infiltrate into the soil reducing runoff. Crops, increase infiltration and interception compared to bare ground, evapotranspiration also increases which could lead to more rainfall. Livestock, trample the soil decreasing infiltration and increasing runoff. Irrigation, can increase runoff if the soil becomes saturated, and if water is extracted from groundwater for irrigation the water table may fall.

Land use change- deforestation reduces the amount of water intercepted by vegetation increasing the amount of surface water, dead plant material also helps to hold the water so it infiltrates into the soil rather than just running off. However if this was to be removed infiltration would decrease. The construction of new urban areas creates an impermeable layer over the land, preventing infiltration, this increases runoff and results in water passing through the drainage basin rapidly, making flooding more likely.

Water abstraction- more water is abstracted to meet demand in areas where population density is high, this reduces the amount of water in stores such as groundwater storage, reservoirs and lakes. During dry seasons even more water is abstracted from stores for consumption and irrigation so stores become more depleted.

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