The big picture

Water is essential to all life and hence an insufficient supply of water can have a detrimental effect on ecological systems and human societies.

We use the hydrological cycle to help us understand the movement of water around the planet. On a global scale the hydrological cycle is a considered to be a closed system with the amount of water remaining fairly constant. However, components of the hydrological cycle are open systems e.g. a lake is an open system, it exchanges matter with the surrounding environment (e.g. input of surface run-off containing nutrients) and energy (e.g. sunlight). Closed and open systems are discussed more fully in Topic 1 (Systems).

Human activity can alter the natural flow of water and have adverse effects on ecological systems, for example:

• Humans extract water from rivers and streams which lowers the water level and reduces flow rates, altering the environment for aquatic species.
• Humans have drained some wetland areas to increase the amount of available agricultural land. This loss of wetland habitats and replacement with monoculture has resulted in loss of overall biodiversity.

The hydrological cycle

The hydrological cycle is used to describe the movement of water on the planet. Water continually circulates between the atmosphere, land and sea.

Most of the water on earth is saline seawater, which is unfit for human consumption. Only about 2.5% is freshwater. Polar ice caps and glaciers account for the majority of fresh water with only about 0.76% of the earth’s water contributing to rivers, lakes and accessible aquifers.

Figure 1. Distribution of fresh water.

Storages and flows

A model of the hydrological cycle consists of storages (stock) and flows (inputs and outputs). An understanding of water storages and flows can be used to make more effective use of our water resources.

Storages

Major water storages include:

• Oceans
• Surface waters e.g. streams, rivers and lakes.
• Ice caps and glaciers.
• Soil moisture.
• Water vapour and clouds within the atmosphere.
• Groundwater within aquifers.
• Organisms e.g. plants and animals.

Flows

Water moves from one storage to another through various flow routes. The flows can be categorized as either transformations or transfers (discussed in Topic 1). Transformations are a change in state and transfers are a change in location.

Figure 2. Relationship between change in physical state of water and flow processes.

Flow of water between storages

Water flow into the atmosphere

Solar radiation drives the hydrological cycle. Water transforms from liquid to vapour as the sun causes evaporation from land and sea surfaces. Through the biological process of transpiration water moves from the root system in plants to the leaves, where it is lost as vapour to the atmosphere. The processes of evaporation and transpiration are collectively called evapotranspiration and are affected by climatic factors such as temperature and wind speed. There may also be some contribution to water vapour through the process of sublimation, when ice or snow turns directly into water vapour.

As water vapour rises, it cools and condenses onto particles such as dust in the air. The resultant liquid water appears as clouds. These clouds are transported within the atmosphere by wind in a process known as advection.

Figure 3. The hydrological cycle

Water flow out of the atmosphere

The water droplets within clouds grow and join together until they become too heavy and fall as precipitation returning to the surface of the earth, usually in the form of rain. If temperatures are low enough, the process of deposition may occur, where water vapour in the clouds forms snow. About 80% of precipitation falls directly into the sea and the rest falls on land. In addition, when temperatures fall overnight, condensation can occur and the water vapour in the atmosphere is deposited on the ground as dew.

Water flow on land

The movement of water overland is dependent on many factors including, the topography, geology, soil characteristics and vegetation cover. The flow of water over land is referred to as surface run-off. If precipitation occurs in the form of snow, it often remains on the land surface longer than rain. There needs to be a sufficient increase in temperature for the snow to melt from a solid into liquid water and contribute to surface run-off. Conversely if surface temperatures fall sufficiently, the water could become frozen forming ice and the movement of water reduced.

Vegetation can slow the movement of water. The leaves capture the first raindrops and if the rainfall continues the water will reach the ground where some may move into the soil by a process called infiltration.Plant stems also intercept the runoff, reducing the water flow rate which provides more time for infiltration to occur. The plant root system can help to provide channels for the water to move through the soil more easily. Decayed plant matter can act as sponge absorbing the water.

Filtration rates are greater through sandy soils than through fine grained soils such as silt or clay. The latter have smaller pore spaces between the grains for the water to move through, thereby reducing the flow rate. Within the soil, the water can contribute to the soil moisture and by the process of absorption be taken up through the root system into plants.

Figure 4. Movement of water in plants: absorption through the roots and transpiration from the leaves.

Water flow into groundwater

If the subsurface is permeable water will flow from the soil further down. This movement of water occurs under the influence of gravity and is referred to as percolation. The water moves into underground zones called aquifers and contributes to the groundwater storage.

The presence of vegetation which encourages infiltration can potentially increase the amount of stored groundwater.

Water flow out of groundwater

Groundwater may flow directly into the sea or into surface streams and rivers. Hence during drought periods, a reduction in this groundwater flow may have a severe effect on river levels and in turn the river ecology.

Water flow into surface waters

Precipitation contributing to surface run off may feed into streams, rivers and lakes. The movement of water within the stream is referred to as streamflow.

Water flow out of surface water

Water may flow from streams, rivers and lakes into the sea. Some water may evaporate and enter the atmosphere. In addition, there will be some uptake of water by plants and animals. This includes humans who commonly abstract water from rivers and lakes.

During periods of extremely high rainfall or sudden snow or ice melt, the capacity of rivers and lakes can be exceeded resulting in flooding where excessive amounts of water flow over land. The flooding of land can cause great damage to ecosystems and human settlements.

Watch this video by The National Science Foundation which illustrates the hydrological cycle.

Theory of Knowledge

Read the report Earth’s Major Aquifers Are in Trouble and think abou the following:

Hydrological systems can be considered at both a regional and global scale. How does consideration of the scale of problem affect how we tackle the issue?

International-mindedness

The hydrological cycle is a global system so changes in one part of the cycle may affect another country's water supply.

Ocean circulation system

The oceans contain the vast majority of water in the hydrological cycle and cover about 70% of the planet surface. The movement of ocean water occurs through the global conveyor belt and is responsible for distributing heat energy around the world that directly affects regional weather.

The Global Conveyor Belt

The global conveyor belt, also called thermohaline circulation is driven by differences in water density dependent on:

• Temperature (thermo), the colder the water the more dense it is.
• Salinity (haline), the greater the salinity the more dense the water becomes.

Figure 1. Global conveyor belt.

Cold air at the poles cools the water, which becomes denser and sinks. Some water may freeze leaving behind salt that raises the salinity of the water and contributes further to an increase in density. Evaporation of water also leaves behind the salt and raises water density.

The deep cold water moves towards the equator and as the water warms, it becomes less dense and rises to become surface waters. These warmer surfaces waters move towards the Polar Regions, warming the air temperature and the cycle begins again.

Watch the following video clip The Global Conveyor Belt, 2008 by Professor Ritter (NASA):

How does the global conveyor belt influence weather and climate?

Water has a high specific heat capacity which means a small increase in ocean temperatures requires a large amount of energy, in comparison the atmosphere has a low heat capacity. Hence, oceans warm and cool very slowly compared to the atmosphere. As warm water moves towards the poles, it transfers heat to the environment increasing the temperature. Conversely, as cold water moves towards the equator, the water absorbs heat from its surroundings, lowering the temperature and affecting the weather.

Global warming could have a detrimental effect on the global conveyor belt. For example, higher global temperatures will lead to melting of glaciers and ice in the Polar Regions, the resultant increase in warm water input could prevent formation of the water density gradient leading to collapse of the global conveyor belt.

Positive feedback may exacerbate global warming e.g. if the oceans become warmer, they will hold less carbon dioxide, hence with less absorption of carbon dioxide from the atmosphere, global temperatures will increase further.

Watch the following video: The Gulf Stream and Climate Change by Kurz Gesagt:

El Nino and La Nina are the names given to specific combinations of warm/cold oceans and describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific and the impact that it has on climate.

Information can be found here:

• Met Office
• National Ocean Service

Impact of human activity on the hydrological cycle

Humans extract water from various points of the hydrological cycle, thereby altering the natural flow of water. In addition human activity which changes land use can alter the movement of surface water through the system. In some cases the flow of water can be diminished and in other situations can lead to excessive volumes of run-off leading to flash floods.

Flash floods occur when precipitation is at such a rate that the water cannot be absorbed by the ground fast enough leading to sudden flooding.

Changes in land use

Deforestation

Forests are often harvested as a source of timber or cleared to make way for urban growth, industrial development or for agriculture.

Forest vegetation affects the hydrological cycle by:

• Intercepting rainfall which protects the soil from the impact of the raindrops reducing soil erosion.
• Impeding the movement of water which allows time for infiltration.
• Absorbing water through the root system and reducing flow.

The forest acts like a sponge absorbing water and reducing the amount of run-off and therefore the risk of flooding.

Loss of forest effectively removes trees as water stores and run-off is likely to flow through more quickly, reducing the opportunity of infiltration and percolation. Without a plant root system to anchor the soil and with faster flowing water, soil erosion is likely to increase.

Larger amounts of water entering the river may result in flooding downstream. In addition higher sediment loads could reduce the carrying capacity of the river and have an adverse effect on water quality reducing biodiversity.

Forests can also affect the local climate. Through the process of transpiration, the forest increases air moisture which creates a wet micro-climate. In the absence of the forest, there is less water vapour in the atmosphere, therefore less rainfall and a drier climate. Through this process of positive feedback the situation can become further exacerbated and potentially lead to a desert environment.

Figure 2. Positive feedback can lead to a drier climate and less vegetation.

Urbanization

Urbanization is the movement of people from rural areas to towns and cities. This is accompanied by a change from natural to human created landscapes. Large parts of urban areas are covered with impervious (water can not pass through) concrete and tarmac which prevent water from soaking into the ground and percolating into aquifers.

As water moves through the catchment, flowing from roofs and along streets it can become contaminated (e.g. with waste, oil and toxic metals).This run-off is often diverted into drainage systems which discharge into nearby rivers and streams. The increased volume of water entering surface waters can increase the risk of flooding. Aquatic habitats can also become degraded by the polluted run-off.

Figure 3. Influence of urbanization on water flow.

Sustainable urban drainage systems (SuDS) can be used to improve water quality and slow down the movement of water through the catchment and thereby reduce flood risk. Components of a SuDS strategy include:

• Reducing quantity of run-off by encouraging water to pass into the ground and aquifer. E.g. use of porous material (such as gravel, porous concrete blocks and porous asphalt) on driveways and car parks that allow water to pass through into soil and groundwater stores.

Figure 4. Example of permeable surface.

• Slowing the velocity of the run-off by diverting water from roofs and roads into soak-away or infiltration trenches. This reduces the surface discharge and provides protection from polluted water. Green spaces on roadside margins called swales can filter the water removing particulates and other pollutants as well as acting as temporary storage sites for stormwater. Ponds and wetland systems can also be used to intercept run-off and act as retention areas reducing flood peaks and the risk of flash floods. Additional benefits include creation of green spaces and areas for wildlife within urban environments.

Figure 5. Pond that can collect and store surface run off.

Watch the following animation on sustainable drainage by Susdrain org: Ever wondered where the rain goes?

Agriculture

The agriculture sector is the largest user of water. Its use of water is expected to continue to rise due to:

• Increase in population requiring production of more food.
• A change to a more meat based diet.

The production of different foods requires different quantities of water, with meat requiring significantly more water than crops.

Figure 6. Water consumption in food production.

Agricultural activity can encourage excessive abstraction of water, diminishing available water resources for other users. The water supply will not be sustainable if abstraction rates exceed the rate of replenishment.

Excessive irrigation coupled with poor drainage can increase soil salinity. When plants are watered, evaporation occurs and water moves up through the soil drawing salts to the surface. The soil can become sterile and unsuitable to grow crops.

As water flows through agriculture land, it can leach pollutants such as pesticides and fertilizers. Some pesticides are toxic to aquatic organisms. Highly soluble fertilizers such as nitrates can be problematic contributing to eutrophication of aquatic ecosystems. Elevated levels of nitrates in drinking water are linked to a potentially fatal condition in new-born babies called methaemoglobinaemia (also known as ‘blue baby syndrome’). In groundwater, especially in aquifers below arable land there has been a significant increase in levels of nitrates and pesticides.

Livestock produce animal waste, such as manure and slurry that can be washed by surface run-off into nearby streams and rivers. This animal waste contains disease causing pathogens, organic material and suspended solids. The organic material can lower the oxygen levels within the aquatic ecosystems. Suspended solids within the water column can reduce light penetration and therefore reduce photosynthesis and overall primary production. If the particulates precipitate out, they could smother and kill organisms on the riverbed.

Figure 7. Potential impacts of use of water in agriculture.

Availability of water for agriculture could be improved through use of rainwater collection schemes and reservoirs. The amount of water used in agriculture could be reduced by:

• Changing to crop varieties that require less water.
• Changing watering methods to be more efficient and effective and reduce water lost e.g. through drip irrigation systems.
• Increase soil moisture retention and reduction in soil erosion through practices such as use of bunds, terracing, contour ploughing and use of winter cover crops.

Strategies to reduce water pollution from farming activity include:

• Avoid application of fertilizers, pesticides or manure near watercourses or near groundwater abstraction points.
• Avoid application of fertilizers, pesticides or manure during or just prior to rainfall periods.
• Limit application of nitrogen fertilizers to match rates of uptake by the crops.
• Collect and manage animal waste (slurry and manure) to prevent pollution run-off.
• Reduce the use of pesticides by using alternative methods such as integrated pest control (IPM) which includes the use of natural pest predators e.g. spiders, ladybirds and parasitic wasps and changing farming techniques e.g. crop rotation.

International-mindedness

Human impact in one area can have devastating effects downstream in another country sharing the same river system.

Research the issues created by having multiple riparian countries located along the River Nile. Create a structured case study to summarise your research. You can work in pairs if you want to. This may be a useful starting point.