Specification content:
- Systems concepts and their application to the water cycle.
- Global distribution and size of major stores of water.
- Processes driving change in the magnitude of these stores over space and time, including flows and transfers: evaporation, condensation, cloud formation, causes of precipitation and cryospheric processes at the hill slope, drainage basin and global scale with reference to the varying time scales involved.
1. The water cycle on a global scale
Water exists in three states: as a solid, a liquid and a gas.
The diagram below demonstrates where all of the water on Earth is stored.
- At a global scale, the water and carbon cycles are closed- no matter is theoretically lost from the boundary layers. However, at smaller scales, the systems are largely open or a combination of open and closed systems.
- At a global level, the water is being moved from one "sphere" to another. From the atmosphere, to the hydrosphere, to the lithosphere, to the atmosphere, to the biosphere or cryosphere.
The diagram below demonstrates where all of the water on Earth is stored.
Where is all of the water?
We are going to study the individual drainage basin, but form a global perspective, different parts of the world not only have different levels of rainfall, but they also have different levels of underground water storage. These are called “aquifers”- vast underground storage.
Water enters the rocks underground directly when water enters the ground, or slowly as the water drains through the overlying soil. Different soils can store different amounts of water-this is known as the soil moisture budget.
The upper level of saturated rock is known as the water table. This rises and falls in relation to local conditions.
We are going to study the individual drainage basin, but form a global perspective, different parts of the world not only have different levels of rainfall, but they also have different levels of underground water storage. These are called “aquifers”- vast underground storage.
Water enters the rocks underground directly when water enters the ground, or slowly as the water drains through the overlying soil. Different soils can store different amounts of water-this is known as the soil moisture budget.
The upper level of saturated rock is known as the water table. This rises and falls in relation to local conditions.
2. Where is all of the water?
Each of the water stores below effect and are affected by each other. They all interlink, and they are linked by processes in the water cycle.
Cryospheric water.
Sea Ice:
The magnitude of the stores varies over space and time.
Cryospheric water.
- Fresh water which is locked up in land ice, glaciers and permafrost.
- Snow falling on glaciers and ice sheets becomes compressed and enters long term storage, forming layers of glacial ice.
- Glaciers grow and shrink in mountainous regions depending on climatic conditions. On a glacier the line of equilibrium marks the altitude where annual accumulation and melting are equal. Most glaciers globally re shrinking and retreating.
Sea Ice:
- Arctic ice and Antarctica- much is frozen but grows and shrinks depending on the time of year.
- Sea ice does not create sea level rise when it melts because its volume is already in the oceans just in a different state.
- Ice shelves are platforms of ice where ice sheets spread into the oceans
- Icebergs are large chunks of ice that have broken off of a ice shelf. When they first enter the water they raise sea level, but not when they melt.
- Major ice sheets on Greenland and in the Antarctic make up more than 99% of the freshwater ice on Earth.
- The Antarctic ice sheet is so big that it is the area of the USA and Mexico combined!
- If all of the melted on a global scale, the volume of water added to the system, could result in a sea level increase of 60m!
- The oceans dominate the available water on earth making up 97% of the water on planet earth.
- The oceanic water is divided between major oceans and smaller seas. It is thought that less than 5% of all of this has been exposed.
- Ocean water contains dissolved salts. They allow the water to remain as liquid below 0C.
- The oceans are alkaline but this is reducing which has been linked to the increased of carbon in the atmosphere and in the oceans.
- This includes 4 main categories: surface water, groundwater, soil water, biological water.
- Includes: free flowing water of rivers, ponds and lakes.
- Rivers are a store and a transfer of water, they transfer water from the ground, soils and atmosphere to another store which may be lakes, wetlands and oceans.
- Rivers make up a really small proportion of the total planetary water. The Amazon river is the largest river by discharge in the world. It accounts for 1/5 of the worlds total river flow.
- Lakes are fresh water found in hollows. They are classed as lakes if they are two hectares or greater. If it is smaller than this, it is a pond. Most lakes are freshwater and in the Northern Hemisphere at higher latitudes. Canada has approximately 31,752 lakes. The largest lake is the Caspian Sea which was once an ancient ocean. Its around 5.5 million years old. Some lakes are also very deep. The deepest being Lake Baikal in Siberia which is 749m on average and at its deepest point is 1637m!
- Wetlands are areas of marshland, peat fen or water where there is a dominance of vegetation. Water covers soil for all of some of the diurnal range, and wetlands may support a wide variety of species including land based and aquatic species.
- The largest wetland system is the Pantanal in S. America. It extends millions of km including western Brazil, east Bolivia and eastern Paraguay. It is a very important ecosystem as it provides economic benefits through water purification and groundwater discharge and recharge. It also supports transports systems.
- Groundwater is water that collects underground in the pore spaces in the rock. This water extends deeper than 4000m in parts. The water collects in gaps and fractures in rocks- where the rock becomes completely saturated is known as the "water table". See above!
- Soil water is the water that us held together by air in the upper layers of unsaturated water. It affects weather and climate, run-off potential and flood control. It also affects slope failure (mass movement- think back to coasts and Walton-on-the-Naze), engineering, and water quality.
- Soil moisture is a key variable in controlling evaporation, evapotranspiration and plant respiration.
- It affects the weather and precipitation because of the feedback influences related to the land-atmosphere exchange.
- This is all of the water stored in biological organisms and biomass.
- The amount stored varies by vegetation cover and type. Areas of dense forest particularly in the Tropics, can hold much more water than the hot dry climate of deserts such as the Sahara desert.
- Biological water affects the amount of transpiration, through the stomata in leaves, the amount of water that is stored, and the climate. There are many feedback loops associated with vegetation, as the more deforestation there is for example, the less evapotranspiration there is, so the less precipitation there is. And so the cycle continues.
- Plants and animals have adapted to their climates to ensure that they store enough water to survive, both in hot climates, and in cold ones.
- This exists as a gas, liquid and solid (ice crystals).
- Water vapour is a really important greenhouse gas.
- Water vapour is a clear colourless gas. Atmospheric water vapour is important as it absorbs, reflects and scatters incoming solar radiation, and it stabilises the atmospheres temperature. This is how it affects the enhanced greenhouse effect because it absorbs heat.
- How much water vapour the atmosphere can hold in the air very much depends on the temperature. Cold air holds less water vapour than warm air. This means that Antarctic air is very dry, and the tropics are really humid.
- If the amount of water vapour goes up, air temperature will increase. So, a global rise in temperature will result in more water vapour globally, which in turn enhances atmospheric warming. This is why water vapour is a greenhouse gas!!!! This links to the carbon cycle!
- Clouds are a mass of water droplets and ice crystals. Water droplet formation requires condensation nuclei which are particles of dust or pollution that the water clings to. Therefore, you often get more rainfall in urban areas because there are more particles for droplets to form.
- The amount of water in the atmosphere are affected by Lapse rates. Lapse rates are the amount the temperature changes as the air rises, and are dependent on local conditions including how saturated the parcel of air is.
- For water to move around the water cycle it also needs energy to allow it to change its state from solid, liquid or gas. This, in the form of latent heat is absorbed and sometimes it is released in order for the water particles to change state. These sensible heat processes are really important to the water cycle. (see below)
The magnitude of the stores varies over space and time.
3. The hydrological cycle
Evapo-transpiration.
Evaporation can occur from open water or from wet surfaces. Total evaporative losses also include water vapour transpired by vegetation, taken up by root systems and released through the stomata of the leaves. Taken together these processes are often referred to as evapotranspiration.
Rates of evaporation are controlled by the surface energy balance, temperature, relative humidity and wind speed. Rates of transpiration are also affected by plant type and growth condition. Both evaporation and transpiration are maximised when water is not limited, this is known as Potential Evapotranspiration, and values (Actual Evapotranspiration) may fall below this level due to reduced soil moisture or due to closure of plant stomata under moisture stress.
Rates of evaporation over the ocean exceed terrestrial rates because over the land actual evapotranspiration is less than potential. This result in a net transfer of atmospheric moisture to the continents as moist air moves across the continents driven by global air mass circulation.
Runoff generation.
The atmospheric moisture which is transferred to the continents is returned to the oceans as runoff either surface runoff or as groundwater flow. Overland flow and river flow is relatively rapid whereas transit times to the ocean for deep groundwater can be thousands of years. Infiltration is a key process partitioning precipitation between runoff and water which either enters the soil as soil water storage/soil throughflow or percolates to bedrock and becomes groundwater flow. Surface flow is generated when rainfall intensity exceeds infiltration capacity (Infiltration excess overland flow) or when rain falls on soils where the soil water store is full and water table is at the surface (saturation excess overland flow). Understanding of runoff and the partitioning of moisture at the surface is central to terrestrial water management since both water resources and flood hazard are intimately associated with our ability to manage and respond to these flows. (RGS.org.uk)
Heat Transfers.
What is a Sensible Heat Transfer?
Heat is transferred back to the atmosphere from the Earths surface as terrestrial radiation. This takes the form of long wave radiation. Heat transfers may involve processes such as convection and conduction. Together these are sensible heat transfers.
A further heat transfer process involves Latent Heat. During evaporation water changes from liquid to gas. The heat that is used during this process is stored as latent heat. When condensation takes place the gas is converted to water droplets and clouds releasing the latent heat.
Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law). Conduction is the process by which heat energy is transmitted through collisions between neighbouring molecules. Think of a frying pan set over an open camp stove. The fire's heat causes molecules in the pan to vibrate faster, making it hotter.
During evaporation water changes from liquid to gas. The heat that is used during this process is stored as latent heat. When condensation takes place the gas is converted to water droplets and clouds releasing the latent heat.
Precipitation.
Atmospheric moisture is returned to the terrestrial system through precipitation. Vertical motion of air masses in the atmosphere controlled by global circulation of air masses, local radiation balance and by interactions with topography cause cooling and condensation of atmospheric moisture. These processes generate frontal, convective and orographic rainfall respectively. For an excellent summary of processes controlling precipitation see chapter one, in Shaw et al. (2010).
Evaporation can occur from open water or from wet surfaces. Total evaporative losses also include water vapour transpired by vegetation, taken up by root systems and released through the stomata of the leaves. Taken together these processes are often referred to as evapotranspiration.
Rates of evaporation are controlled by the surface energy balance, temperature, relative humidity and wind speed. Rates of transpiration are also affected by plant type and growth condition. Both evaporation and transpiration are maximised when water is not limited, this is known as Potential Evapotranspiration, and values (Actual Evapotranspiration) may fall below this level due to reduced soil moisture or due to closure of plant stomata under moisture stress.
Rates of evaporation over the ocean exceed terrestrial rates because over the land actual evapotranspiration is less than potential. This result in a net transfer of atmospheric moisture to the continents as moist air moves across the continents driven by global air mass circulation.
Runoff generation.
The atmospheric moisture which is transferred to the continents is returned to the oceans as runoff either surface runoff or as groundwater flow. Overland flow and river flow is relatively rapid whereas transit times to the ocean for deep groundwater can be thousands of years. Infiltration is a key process partitioning precipitation between runoff and water which either enters the soil as soil water storage/soil throughflow or percolates to bedrock and becomes groundwater flow. Surface flow is generated when rainfall intensity exceeds infiltration capacity (Infiltration excess overland flow) or when rain falls on soils where the soil water store is full and water table is at the surface (saturation excess overland flow). Understanding of runoff and the partitioning of moisture at the surface is central to terrestrial water management since both water resources and flood hazard are intimately associated with our ability to manage and respond to these flows. (RGS.org.uk)
Heat Transfers.
What is a Sensible Heat Transfer?
Heat is transferred back to the atmosphere from the Earths surface as terrestrial radiation. This takes the form of long wave radiation. Heat transfers may involve processes such as convection and conduction. Together these are sensible heat transfers.
A further heat transfer process involves Latent Heat. During evaporation water changes from liquid to gas. The heat that is used during this process is stored as latent heat. When condensation takes place the gas is converted to water droplets and clouds releasing the latent heat.
Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law). Conduction is the process by which heat energy is transmitted through collisions between neighbouring molecules. Think of a frying pan set over an open camp stove. The fire's heat causes molecules in the pan to vibrate faster, making it hotter.
During evaporation water changes from liquid to gas. The heat that is used during this process is stored as latent heat. When condensation takes place the gas is converted to water droplets and clouds releasing the latent heat.
Precipitation.
Atmospheric moisture is returned to the terrestrial system through precipitation. Vertical motion of air masses in the atmosphere controlled by global circulation of air masses, local radiation balance and by interactions with topography cause cooling and condensation of atmospheric moisture. These processes generate frontal, convective and orographic rainfall respectively. For an excellent summary of processes controlling precipitation see chapter one, in Shaw et al. (2010).
Precipitation does not fall in the same amounts throughout the world, in a country, or even in a city. Here in Georgia, USA, it rains fairly evenly all during the year, around 40-50 inches (102-127 centimeters (cm)) per year. Summer thunderstorms may deliver an inch or more of rain on one suburb while leaving another area dry a few miles away. But, the rain amount that Georgia gets in one month is often more than Las Vegas, Nevada observes all year. The world's record for average-annual rainfall belongs to Mt. Waialeale, Hawaii, where it averages about 450 inches (1,140 cm) per year. A remarkable 642 inches (1,630 cm) was reported there during one twelve-month period (that's almost 2 inches (5 cm) every day!). Is this the world record for the most rain in a year? No, that was recorded at Cherrapunji, India, where it rained 905 inches (2,300 cm) in 1861. Contrast those excessive precipitation amounts to Arica, Chile, where no rain fell for 14 years, and in Bagdad, California, where precipitation was absent for 767 consecutive days from October 1912 to November 1914.
The map below shows average annual precipitation, in millimeters and inches, for the world. The light green areas can be considered "deserts". You might expect the Sahara area in Africa to be a desert, but did you think that much of Greenland and Antarctica are deserts?
On average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches (0.76 meters).
The map below shows average annual precipitation, in millimeters and inches, for the world. The light green areas can be considered "deserts". You might expect the Sahara area in Africa to be a desert, but did you think that much of Greenland and Antarctica are deserts?
On average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches (0.76 meters).
- Cloud formation and precipitation vary over the globe.
- The driving force behind cloud formation and precipitation is the atmosphere circulation model (see left).
- There are three interconnecting cells which run from the equator to the poles.
- At the Equator high temperatures results in high evaporation, moist air rises, cools, condenses to form large cloud banks. This results in large volumes of rainfall at what is called the Inter Tropical Convergence Zone (ITCZ).
- The air moves towards the higher latitudes. As it does, warm tropical air converges with cold arctic air.
- Going to the poles, where cold air is transported back down towards the Horse latitudes.
Condensation.
Condensation is the process by which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation. The phase change that accompanies water as it moves between its vapor, liquid, and solid form is exhibited in the arrangement of water molecules. Water molecules in the vapor form are arranged more randomly than in liquid water. As condensation occurs and liquid water forms from the vapor, the water molecules become organized in a less random structure, which is less random than in vapor, and heat is released into the atmosphere as a result. Even though clouds are absent in a crystal clear blue sky, water is still present in the form of water vapor and droplets which are too small to be seen. Depending on weather conditions, water molecules will combine with tiny particles of dust, salt, and smoke in the air to form cloud droplets, which grow and develop into clouds, a form of water we can see. Cloud droplets can vary greatly in size, from 10 microns (millionths of a meter) to 1 millimeter (mm), and even as large as 5 mm. This process occurs higher in the sky where the air is cooler and more condensation occurs relative to evaporation. As water droplets combine (also known as coalescence) with each other, and grow in size, clouds not only develop, but precipitation may also occur. Precipitation is essentially water in its liquid or solid form falling from the base of a cloud. This seems to happen too often during picnics or when large groups of people gather at swimming pools.
Why is the atmosphere colder as you go higher?
The clouds formed by condensation are an intricate and critical component of Earth's environment. Clouds regulate the flow of radiant energy into and out of Earth's climate system. They influence the Earth's climate by reflecting incoming solar radiation (heat) back to space and outgoing radiation (terrestrial) from the Earth's surface. Often at night, clouds act as a "blanket," keeping a portion of the day's heat next to the surface. Changing cloud patterns modify the Earth's energy balance, and, in turn, temperatures on the Earth's surface.
As we said, clouds form in the atmosphere because air containing water vapor rises and cools. The key to this process is that air near the Earth's surface is warmed by solar radiation. But, do you know why the atmosphere cools above the Earth's surface? Generally, air pressure, is the reason. Air has mass (and, because of gravity on Earth, weight) and at sea level the weight of a column of air pressing down on your head is about 14 ½ pounds (6.6 kilograms) per square inch. The pressure (weight), called barometric pressure, that results is a consequence of the density of the air above. At higher altitudes, there is less air above, and, thus, less air pressure pressing down. The barometric pressure is lower, and lower barometric pressure is associated with fewer molecules per unit volume. Therefore, the air at higher altitudes is less dense. As the total heat content of a system is directly related to the amount of matter present, it is cooler at higher elevation ... fewer air molecules exist in a certain volume of air higher up. This means cooler air.
(Source: USGS.gov)