TOPIC 5: THE WATER CYCLE AND WATER INSECURITY

hydrological cycle

• also known as the water cycle

• the continuous movement of water on, above and below the surface of the earth

• closed system at a global scale so the total amount of water on earths remains constant

• no external inputs or outputs of water

drivers of the global hydrological cycle

• solar energy: sun heats the water on the earths surface causing it to change state and evaporate into the atmosphere → water is also drawn from the soil by plants and evaporation from leaves and stems by the process of evapotranspiration

• gravitational potential energy: the force that causes precipitation to fall from the sky and water to flor downhill, both on the surface and through the soil, and back to the ocean

stores of the hydrological cycle

• oceans: water stored in liquid form, with only a minute fraction as icebergs

• cryosphere: water found in a largely solid state → some formed from melt water

• terrestrial: water stored in rivers, streams, lakes and groundwater → also known as blue water → visible part of the cycle → green water → water is stored as vegetation, the invisible part of the cycle

• atmosphere: water largely exists as vapour with the carrying capacity directly linked to temperature → can contain liquid water or ice crystals in high altitudes in clouds

fluxes and flows in the hydrological cycle

• flow: movement or transfer of water from one store to another, describing the pathway the water takes → precipitation, surface runoff, infiltration, evaporation, transpiration

• flux: rate of flow of water between different stores → the quantity of water that is transferred per unit of time

relative importance and size of water stores

• size and importance of each star depends on the amount of water flowing between them

• over longer time scales, some stores grow while others shrink

Relative importance and size of water fluxes and flows

• vary spatial and temporally

• largest and most important flux/flow is the evaporation of ocean water into the atmosphere → precipitation and condensation as a result also an important flux

• far smaller and less important is evaporation of water held in surfaces and vegetation in the atmosphere

• evaporation and precipitation arguably most important in establishing overall balance and also influences fluxes on a more local scale, regulating how water is distributed and where it is stored locally

evaporation

• occurs when liquid water changes state into as gas becoming water vapour → normally gains energy from solar radiation → increases amount of water stored in the atmosphere

• scale of flux varies by location and season

• lots of solar energy, large supply of water, warm, dry air → amount of evaporation will be high (vice versa)

condensation

• occurs when water vapour changes state to become a liquid, losing energy to the surroundings

• happens when air containing water vapour cools to its dew point → the temperature at which it will change from gas to liquid

• water droplets can stay in the atmosphere or flow to other subsystems → decreases amount of water stored in atmosphere

• scale demands on amount of water vapour in the atmosphere and temperature → lots of water vapour + large or rapid drop in temperature = levels will be high

precipitation

• the main flow of water from the atmosphere to the ground

• clouds form when warm air cools down, causing the water vapour in it to condense into water droplets, which gather as clouds → when the droplets get big enough, they fall as precipitation

• these water droplets are too small to form clouds on their own and for clouds to form, there have to be tiny particles of other substances such as dust or soot to act as cloud condensation nuclei → give water a surface to condense on → encourages cloud formation rather than dispersal of moist air

• vary seasonally → more rainfall in winter than summer in the uk

• vary spatially → generally higher in the tropics than at the poles

cryospheric processes

• such as accumulation → build up of snow and ice and ablation → melting of snow and ice → balance varies with temperature

• during periods of global cold → inputs greater → water transferred to the cryosphere as snow and less water is transferred away due to melting

• periods of warmer global temperature → scale of ecosphere store reduces as losses due to melting being larger than inputs of snow

• earth is emerging from a glacial period → reached its maximum around 21000 years ago

• climate change is shrinking these ice stores → Antartica, Greenland, artic, alpine glaciers

• variations happen over different time scales → glacial and interglacial periods → or in shorter timescales → annual temperature fluctuations during the winter and summer

residence times

• the average length of time a water molecule will spend in the reservoir or store

• varies greatly from store to store

• calculating residence times important tool for developers and engineers →→ consult a reservoirs resident time when ovulating how quickly a pollutant will spread through the reservoir → stores with a slower turnover tend to be more easily polluted as the water is in situ for a longer length of time

• may also influence how communities use an aquifer

non-renewable stores of water

• constant circulation and replenishment of stores without any losses means water is generally considered a renewable resource

• however, some stores are non-renewable as they are not replaced in short period of time

• fossil water: water that has been contained in an undisturbed place, salt groundwater in an aquifer for a millennia or longer such as the Sahara desert → may be extracted for human purposes such as agriculture industry and consumption → little no significant recharge

• ablation → melting of glaciers, due to climate change, is reducing storage of water as ice → not replaced → cryosphere losses

drainage basin

• catchment area from which a river (and tributaries) gets its water

• boundary marked by a ride of high land called the watershed

• can be any size

• subsystem within the global hydrological system and is an open system

• input —> precipitation

• output —> ocean: streamflow, discharge —> atmosphere: evaporation, transpiration

• series of stores that hold water, linked by flow or transfers

• amount in drainage basin varies over time

parts of a drainage basin: watershed, catchment area, source, confluence, tributary, mouth

• watershed: ridge of highland surrounding a drainage basin that marks the boundary between the drainage basin

• catchment area: area within the drainage basin

• source: beginning or the start of a river

• mouth: point where the river connects to the sea

• confluence: pain where two rivers or stream join

• tributary: stream or small river which joins a larger stream or river

drainage basin hydrological cycle components: inputs and outputs

• input: precipitation - any form of moisture falling from the atmosphere and can be in the form of snow, rain, hail, skeet, dew or frost

• output: evaporation - physical process y which moisture is lost directly into the atmosphere from water surfaces and soil as water vapour

• output: transpiration - biological process by which water Is lost from a plant through stomata in its leaves and transferred to the atmosphere

• output: evapotranspiration - combined effect of evaporation and transpiration representing the most important aspect of water loss to the atmosphere

drainage basin hydrological cycle components: stores

• surface storage: rainfall that is temporarily retained and does not immediately add to the streams flow such as rain that sits on ponds

• channel storage: the storage of water in streams or rivers

• water table: marks the boundary between unsaturated and saturated zones within the earth and can fluctuate based on rainfall amounts and other factors

• groundwater storage: water held below the water table in aquifers

• depression storage: storage of water in hollows and holes in the ground to form puddles

• soil moisture storage: the water that is held in the spaces between soil particles

• atmosphere: water is stored as droplets in clouds

• oceans and seas: water is stored in liquid form in the oceans and seas

• snow and ice: water is stored in solid form as snow and ice in ice sheets and glaciers

drainage basin hydrological cycle components: flows

• infiltration: process whereby water soaks into or is absorbed by the soil and is a vertical movement - capacity depends on soil texture, vegetation cover, existing soil moisture and time —> rate of infiltration (infiltration capacity) depends on antecedent moisture conditions and the soils porosity

• channel flow: water that flows along the river itself and is fed by three transfer processes: groundwater flow, overland flow and through flow

• direct runoff: encompasses any water that reached the river channel quickly, could include overland flow, routes such as artificial pipes and fast subsurface pathway —> tends to be quite fast

• through flow: the lateral transfer of water down slope through the soil via natural pipes (line of roots or soil weaknesses) and percolines (lines of concentrated water flow between soil horizons to the river channel) —> while slower than direct overland flow, this shallow transfer can occur quite rapidly in porous, sandy soils

• percolation: water seeps down through the soil into the water table via permeable rocks (those with joints (pervious) or those that are porous)

• groundwater flow: water flowing slowly below the water table through permeable rock; this feeds into rivers through riverbanks and riverbeds and this is called baseflow —> slow transfer of water through rocks but in limestone areas where there are extensive underground channels the flow can be fastest

interception flows

• interception loss: vegetation surfaces catches falling precipitation (this is called interception), some of this water will evaporate and become an output of the drainage basin so interception loss is water evaporated form the plants surface

• through fall: precipitation which drips of leaves and does reach the ground

• stemflow: intercepted precipitation which runs down the stems of plants to reach the ground

• vegetation slows down and reduces water transfer

overland flows

• overland flow: movement of a sheet of water across the ground

• infiltration-excess occurs when rainfall intensity is so great that not all water can infiltrate, irrespective of how dry or wet the soil is

• saturation-excess occurs when rainfall continues for a long time and thus, the entire soil becomes saturated and overland flow begins

• this is a rapid and fast transfer

physical factors within drainage basins: climate change

• inputs: amount of precipitation and seasonal patterns —> in some climates (monsoon, mediterranean, continental climates) strong seasonal patterns of rainfall or snow will have a major impact on when inputs are received —> if precipiation falls as snow, it can act as a temporary store and then a large flux of water may be released into the system when experiencing rapid thawing

• flows: more infiltration-excess overland flow when precipitation is greater or more intense; it is difficult for rain to infiltrate if the rain is too intense —> convectional thunderstorms are short, heavy bursts of precipitation and are confined to a small area and can cause a sudden rise in the channel flow from increased direct runoff —> passing of a depression will give a longer period of steady rainfall extending all over a drainage basin which allows time for infiltration —> if the ground is frozen, water flows over the surface and cannot infiltrate —> warmer weather results in more leaves, causing more interception and evapotranspiration and so less water reaches the ground, so infiltration and percolation are reduce

• outputs: evaporation and transpiration is greater when hotter —> cold temperatures slow or prevent evapotranspiration

physical factors within drainage basins: vegetation

• inputs: vegetation transpires and so if there’s lots of it, higher transpiration rates which will increase local rainfall

• flows: large forests intercept lots of rain, slowing infiltration, direct runoff and through flow —> contains roots that text net and break up the soil which create cracks and fissures in the soil, allowing for more rapid infiltration and increased infiltration capacity

• outputs: extensive tree cover increases evapotranspiration and reduces channel flow

physical factors within drainage basins: soils

• flows: more spaces such as sandy soils allow more water to infiltrate, reducing overland flow but increasing through flow —> some soils, pipes develop as water flows along the lines of roots or burrows increasing through flow rates —> compacted soils inhibit infiltration leading to greater amounts of overland flow

• output: clay soils reduce infiltration and so increase evaporation from the ground and runoff —> saturated soil creates more overland flow removing water more quickly form the drainage basin —> deeper soil able to store more water so this will reduce overland flow and output of water from the system

physical factors within drainage basins: geology

• flows: porous describes the physical structure of the rock where interconnected voids or pores within it structure which allows the rock to be permeable making it easier for water to flow through, the more permeable it is —> tend to led to less overland flow and increased through flow and groundwater flow

• output: impermeable rocks such as granite prevent infiltration and percolation, producing more overland flow and greater number of streams leading to water moving out of the system more quickly

physical factors within drainage basins: relief

• inputs: orographic (relief) rainfall created on high ground —> shallow slopes prove infiltration as water has time to penetrate into the ground

• flows: very steep slopes tend to encourage overland flow and reduce infiltration

• output: steeper slopes move the water out of the drainage basin quickly reducing the amount of output through evaporation

human factors: water storage reservoirs

• dams increase surface water stores and evaporation and makes access to fresh water easier for humans

• reduces downstream river discharge

• lake Nasser behind the Aswan Dam in Egypt estimated to have evaporation losses of 10 - 16 cubic metres every year representing a loss of 20-30% of the Egyptian water volume from the river nile

human factors: water abstraction

• reduces groundwater levels and changes river flows

• growth of global population results in increased water demand

• where precipitation levels sufficient, water can come from rivers themselves which will lower the levels of water in channels

• where precipitation levels are low, alternative supply is groundwater - this supply of water in porous rocks underground is known as an aquifer

• excessive abstraction - water taken too quickly and does not recharge naturally, is unsustainable and can lead to the depletion of groundwater stores

• in some locations, reduced industrial activity has increased groundwater rebound increasing the risk of groundwater flooding if the water table reaches the land surface

human factors: water abstraction case studies

• China - groundwater used to irrigate 40% of farmland and to provide 70% of drinking water in northern and northwestern regions with extraction increasing by 2.5bn m³/year and consequently, groundwater levels in arid north china plan dropping by as much as a metre per year

• Aral Sea - began shrinking in the 1960s when soviet irrigation schemes for cotton growth took water form the Syr Darya and Amu daryl rivers greatly reducing the amount of water reaching the sea; by 1994, levels fallen by 16m, surface area declined by 50%, volume declined by 75% and salinity levels increased by 300%

• London - reductions in water-using manufacturing activity led to less groundwater abstraction but also groundwater levels have began to rise; this is called groundwater rebound. this lets to surface water flooding, flooding of cellars and basements as well as increased leakage into tunnels such as in the London Underground

human factors: changing land use

• infiltration is up to 5X greater under forests when compared to grasslands

• conversion to farmland reduces interception, increases soil compaction and more overland flow

• changing from natural landscapes to urbanised landscapes increases impermeable surfaces leading to an increase in overland flows and a reduction in infiltration - can lead to flooding as rive levels rise quickly leading to a short lag time and high peak discharge

• UK - Winchester and maidenhead (2014 floods) and Carlisle, York and Manchester (2015 floods), River Severn, Tewkesbury 2007

human factors: deforestation

• removal of trees leads to a reduction in evapotranspiration and increase in overland flow which increases flooding potential

• leads to a decline of surface storage and a decrease in lag time between peak rainfall and peak discharge

• speeds up the cycle

amazon rainforest water cycle