Water and Carbon Cycles Flashcards
(26 cards)
The hydrological cycle 2.1.1
The correct name for the water cycle is the hydrological cycle. This consists of the 1.385million cubic kilometers of water that are found within the Earth-atmosphere system.
Hydrological cycle info
-Being a closed system the mass balance of water is unchanging but where it is stored within the system will change.
hydrosphere
The combined name to water found in liquid and gas forms on, under and above the surface of the planet. This includes groundwater, surface water and atmospheric water vapour. It makes no distinction between saline and fresh water.
cryosphere
The areas where water is in a solid/frozen state. This will be fresh water as very little salt is trapped when water freezes.
Variations in mass balance of hydrological cycle stores 2.1.1
mass balance
The mass balance of a systems describes the total amount in mass of a substance in a system. This is different to the total volume in a system but is often used synonymously. For example the hydrological cycle is usually described by its volume - containing 1.385million cubic kilometers.
The mass balance of the water in the hydrological cycle as a whole is unchanging by mass (though these is variation in volume) but the mass balance of each store varies temporarily and spatially. We can therefore look at each individually. These changes can be very important to us.
Mass balance on the globe
- The largest sphere represents all of the water on the Earth. Although it covers just under ¾ of the world (71%) it is in a relatively thin layer compared to the diameter of the Earth. It totals 1,335,040km3.
-The sphere over Kentucky shows the world’s liquid fresh water (groundwater, lakes, swamps and rivers). 99% of this is in groundwater and is mostly inaccessible.
-Barely visible, this tiny sphere of water is the fresh water in all lakes and rivers (accessible fresh water) on the planet.
Large scale changes
-Long term changes of global water stores have mostly occurred due to increases or decreases in the cryospheric store of water, with water being added or removed from the ocean store.
The longer scale changes are due to glacial and interglacial periods. These changes occur due to Milankovitch cycles (covered in our glacial landscapes topic).
It is theorised that around 650-750 million years ago the Earth was entirely covered in ice in a period called “snowball Earth”. Cryspheric storage would have been much greater than it is today, with oceanic water being below it.
On the other side of this during the Palaeocene and early Eocene (65-35 million years ago) there were the opposite “hothouse” conditions where there was no ice at the poles. Global sea levels would have contained 99% of all the water in these conditions (and there is fossil evidence of crocodiles living within the Arctic circle).
More large scale changes
The position of the tectonic plates is also believed to have had a significant impact on how much water can be stored in the cryosphere.
When the Antarctic Plate moved over the south pole (around 100 million years ago) it allowed for the formation of the Antarctic Ice Sheet, increasing the amount of global ice storage. Antarctic accounts for enough ice to increase sea level by 60 metres. The next greatest land store of ice is Greenland, which would account for a sea level rise of 6 metres.
Thermal expansion due to climate change also accounts for changes in seal level rise as temperature increase leads to an increase in volume (but not mass). Since 1900 the 200mm of sea level rise is largely attributed to thermal expansion more than water being added to the ocean store.
small scale changes
The cryosphere varied throughout the year with seasonal variation in accumulation and ablation, while over the course of the year there is generally a steady-state equilibrium. This does vary greatly between locations with areas such as the Alps, Andes and Tibetan Plateau having significant runoff in summer.
The greatest consistent variation is in the Arctic which in the summer reduces to 50% of it’s winter volume. However, almost all areas of ice have been showing reductions in size due to anthropogenic climate change (long term change).
more small scale changes
Changes to the rate of transfers also vary between seasons. The changes in precipitation vary depending on the global position of the location with temperate climates typically showing increased precipitation towards the winter. But in other locations where there is more extreme topography orographic (relief) rainfall typically leads to an extreme wet season called a monsoon.
These are typical in east Asia (India, Pakistan and China), where a change in wind direction causes water vapour from the ocean to be pushed up mountains, such as the Himalayas. In India over 2/3rds of the rainfall occurs in the three month rainy season.
However, monsoons also occur in eastern Africa, including Kenya, and parts of Australia.
El Nino
In El Niño events this is reversed. The western trade winds weaken and that leads to the convection reversing in direction, with Australia and Indonesia becoming areas of high pressure and receive very little precipitation.
This will also impact areas such as Peru where the change in conditions in the water will cause fish stocks to perish or migrate.
La nina
In normal conditions (La Niña) warm surface winds travel to the west across the Pacific, leading to low pressure and precipitation over eastern Australia and Indonesia.
Causes of precipitation 2.1.4
air pressure
the force exerted by the mass of the air molecules being pulled down by gravity.
The air pressure in different locations on the Earth’s surface will vary depending on the density of the air molecules above it (higher density air molecules will result in higher pressure)
The air pressure will also decrease at altitude. This is because there are less air molecules above the location to “push” down on it.
evaporation
When a liquid (water) is turned into a gas (water vapour) because it is heated.
condensation
When a gas (water vapour) is turned into a liquid (water) because it is cooled (typically because it reaches higher in the atmosphere).
precipitation
When water moves from the atmosphere back to the surface (typically due to gravity).
how precipitation is formed
Air containing water vapour will (typically) rise if it is hotter. This is because the air molecules are further apart than the surrounding air so are more buoyant, meaning the warmer air mass will rise.
As an air mass rises it will expand because there is less air pressure at higher altitudes.
At higher altitudes the air molecules are further apart, due to the decrease in air pressure. This means that there is a reduced chance of collisions between the air molecules, which will decrease the heat per unit volume, so the temperature will be lower.
Water vapour will condense if it reaches the dew point, which is the point in which the air is completely saturated with water vapour.
Air can contain less water vapour as it is cooled so the dew point can be reached by air containing water vapour being cooled (for example at higher altitudes). If it is cooled further then the excess water vapour will condense to form water droplets.
causes of air uplift
For condensation (and therefore precipitation) to occur air has to be cooled beyond the dew point. This can be because of a drop in temperature at the surface, which will lead to fog or can cause water to collect on surfaces (such as water droplets forming on vegetation overnight).
Air uplift can occur for three reasons, which then give name to the types of precipitation:
Convectional rainfall
Frontal rainfall
Orographic/relief rainfall
convectional rainfall
1)The sun heats the Earth’s surface which radiates heat back into the atmosphere.
2)The air molecules move further apart as they are heated making the air mass less dense. This makes it more buoyant forcing it to rise.
3)Cool air moves into take the place of the rising air.
4)Water vapour, from evaporation and transpiration, rises in the hotter air mass.
5)The air expands and cools at higher altitudes, reaching and exceeding the dew point.
6)This forms cumulus (or cumulonimbus in extreme cases) clouds.
frontal rainfall
When warm and cold air masses collide, the denser colder air is forced under the warmer air.
This leads to the less dense warmer air rising at these fronts, along with the water vapour within it.
The air expands and cools at higher altitudes, reaching and exceeding the dew point.
This forms a range of cloud types with cumulus or cumulonimbus clouds forming at the cold front.
orogrpahic rainfall
When an air mass is blown over a topographic barrier (such as mountains or high land) by the prevailing wind from the coast the air mass is forced upwards.
The air expands and cools at higher altitudes, reaching and exceeding the dew point.
After the area of high relief the air mass will descend, meaning it will reduce in volume and warm, so this are is unlikely to receive precipitation.
The feeder-seeder mechanism
Orographic rainfall can produce very heavy rain due to the feeder-seeder mechanism.
In this mechanism precipitation from higher “seeder” clouds falls through lower stratus clouds above the mountainous areas.
The falling rain collects additional water from the droplets in the “feeder” clouds (as they collide).
