Human Impacts on the water cycle
Role of forests in the hydrological cycle
Forests play a very important role within the global and local hydrological cycles. They transmit huge quantities of water into the atmosphere via the transpiration of plants (in which plants release water from their leaves during photosynthesis) and from evaporation from their leaves. These inputs replenish clouds and help produce the rain that sustains the rainforests. Indeed, much of the water that falls in tropical rainforests is recycled within the system if the forest is large enough. Some of this recycling is evaporation from lakes, rivers, or wet soil. However, a lot of it is fast-tracked by plants, and especially trees. Tree roots tap moisture from deep in the soil. This circulation system is driven by releases of moisture into the air through their leaves via transpiration. In the Amazon, 50-80 percent of moisture remains in the ecosystem’s water cycle. 1
Deforestation and the hydrological cycle
When deforestation occurs, this cycle is interrupted. At a local level deforestation can have immediate effects on the hydrological cycle. In tropical forests;
1. Less plants means less evapotranspiration. Subsequently there is a decline in rainfall, subjecting the area to drought.
2. Rainfall can be lost from the area, permanent drying can occur and flood regimes of rivers are altered. 1 This can result in drought in former rainforest areas. Rainforests of Borneo and the Amazon have experienced very severe droughts.
3. The regular flow of clean water from forests and protecting communities from flood and drought can be affected.
4. Interception rates are affected – Tropical forests in particular are multi layered and catch huge volumes of rainfall falling from the sky. This is delivered to the forest floor via stemflow and through fall from leaves. This would stop and water would fall directly onto the forest floor.
5. Infiltration and percolation would increase but this would result in the water table being closer to the soil surface. Rainforests soak up rainfall brought by tropical storms Both by intercepting rainfall and allowing slow infiltration into the soil. This regulates floods and river levels. Without forest cover infiltration rates are affected and more overland flow occurs. This means more destructive flood and drought cycles can occur when forests are cleared.
6. Overland flow increases when forest cover is lost. Rainfall turns into runoff which rapidly flows into streams, raising river levels and creating potential flood risks during wetter periods. During the dry season areas downstream of deforestation can be prone to months-long droughts.
7. Less water is stored in the biosphere as a result of deforestation
8. Transpiration and shading from trees help to cool rainforests - this is lost from deforestation which can increase evaporative losses. In the Amazon, Michael Coe of the Woods Hole Research Center recently reported a difference of 3 degrees Celsius (5.4°F) between the cool of the forested Xingu indigenous park and surrounding croplands and pastures. 14
The global affects of deforestation
Our globe is connected by a global scale climate system. Moisture generated by rainforests travels around the world. According to Mongabay.com Scientists have discovered that rainfall in America’s Midwest is affected by forests in the Congo. 2
Whilst the UN's Food and Agriculture Organization (FAO) and the Center for International Forestry Research (CIFOR) say that deforestation does not cause more flooding, they have found that deforestation does have a role in small floods and topsoil erosion by eliminating the buffering and soil-anchoring effects of forests. In terms of climate change, the water that a single tree transpires daily has a cooling effect equivalent to two domestic air conditioners for a day.
Links to the Carbon Cycle
A result of changes to the hydrological cycle caused by deforestation can have impacts upon the carbon cycle. The obvious link is that the loss of forest cover puts huge volumes of carbon into the atmosphere, replacing biospheric carbon with atmospheric carbon. However, increased drought risk increases fire risk for the scrublands and grasslands that replace the forest cover. These fires also release carbon into the atmosphere. According to Mongabay.com – “The newly desiccated forest becomes prone to devastating fires. Such fires materialized in 1997 and 1998 in conjunction with the dry conditions created by el Niño. Millions of acres burned as fires swept through Indonesia, Brazil, Colombia, Central America, Florida, and other places. The Woods Hole Research Center warned that more than 400,000 square kilometers of Brazilian Amazon were highly vulnerable to fire in 1998. That extent grew in 2005 and 2010 when the Amazon was hit by even worse droughts.” 3
It is also known that carbon dioxide emissions from deforestation add 10 percent or so to global warming by reducing the quantity of CO2 that the world’s forests pull from the atmosphere. 4
In addition, forests release a range of volatile organic compounds that “have an overall cooling effect on our climate,” mostly by blocking incoming solar energy, says Dominick Spracklen of Leeds University.5 Removing forests eliminates this cooling effect and adds to warming.
The image above shows vegetation 'greenness' during the 2010 drought, between July and September, compared to average conditions for the same period between 2000 and 2009. The redder the image the less 'green' the forest. Image from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, courtesy of NASA. 6
Impact of Agriculture on the hydrological cycle
Modern agriculture has a large impact upon the hydrological cycle. In many parts of the world agriculture has replaced natural vegetation with crop cover and pasture, drastically altering the way that water moves through altered drainage basin systems.
We need agriculture to feed the world’s population but this has altered the water balance;
1. Huge quantities of water are used in food production – this water redistributes water away from its natural pathways. Precipitation is trapped and stored in surface reservoirs, groundwater and river water is extracted, then reused on fields to water crops during drier periods. The abstraction of river-water will reduce downstream flow.
2. Water is stored in biomass that is the plants and animals being farmed. In our global agriculture system much of this food enters world markets and thus the water is exported from one country to another.
3. The evapotranspiration regime of areas is affected, rather than natural annual plants cover of native ecosystems agriculture changes when plants are grown and in what quantities. This alters evapotranspiration rates, evapotranspiration from the crops may be reduced if the previous habitat was forest, or it may be increased if an arid area was cultivated and irrigated.
4. Groundwater stores are affected – wells are drilled into the ground and water pumped to the surface for use in irrigation. This can deplete groundwater levels under the ground and damage aquifer resources, this can also cause surface water features such as springs, rivers and marshland to dry up.
5. Surface runoff can increase as post-harvest fields are bare of vegetation. Farmed crops often intercept less precipitation than natural vegetation cover too. Subsequent precipitation can exceed infiltration capacities of the soil resulting in increased overland flow.
6. Drainage patterns are changed too. Farmers intentionally dig drainage ditches within and around their fields to prevent water logging of plants. This means that water moves initially via overland flow and then via small channels into rivers, affecting both the hydrograph and annual regimes of those rivers.
7. Agriculture often reduces vegetation cover and soil compaction from machinery can occur. Both of these can reduce the amount of water that infiltrates into the soil and therefore increase run off.
Rainfall doesn’t only reach rivers by running off over the land surface. Much of the water that falls to earth as rain enters the soil and rock beneath our feet to become ground water. We often forget about this water because, unlike rivers and surface water, it is out of sight. The level beneath the ground at which the rock becomes saturated is called the water table. Water in this saturated zone will flow from where it has infiltrated to a point of discharge. This might be a spring, a river or the sea.
Whilst water can be found in many locations underground, some geological or rock formations are impermeable. This means that water can hardly flow through them, whilst others are permeable (they contain fine holes that allow water to flow). These permeable rocks that contain groundwater are known as aquifers. 12
Groundwater abstraction is the process of taking water from a ground source. It is often pumped through boreholes and wells from underground aquifers, as a source of freshwater. 8 A lot of this water is used for irrigation for crops or to produce drinking water. As water is abstracted the water table (the upper limit of water in the soil or rock beneath the ground) is lowered around the borehole. If rates of abstraction exceed rates of groundwater recharge within an aquifer, the water table can fall across a wide area. Taking too much water, or over abstraction can lead to surface rivers drying up or the level of groundwater aquifers and the water table reducing.
Effects of over abstraction
Over abstracting ground water can have negative effects:
• Wells can dry up – this can occur as over abstraction lowers the water table (the upper limited of ground saturated with water). Unless the well owner can deepen the well or drill a new well, water shortages occur.
• Reduction of water in streams and lakes – Rivers often get a lot of their water from throughflow and groundwater flow from soil and groundwater sources. The amount or proportion of stream water that comes from groundwater inflow varies according to a region's geography, geology, and climate. Removing water from groundwater sources through abstraction reduces the amount available for rivers and streams on the surface. This can cause some surface rivers to dry up.
• Deterioration of water quality – this can occur in coastal regions where saline water can migrate inland and upward when freshwater is pumped out of the ground in these locations. This is known as saltwater intrusion and can contaminate the water supply.
• Land subsidence – This occurs as water is part of the subsurface support for the land above. When water is taken out of the soil and rock, the soil & rock collapses, compacts, and drops.9
Water Abstraction in Europe
Approximately 10 % of Europe’s total freshwater resource is abstracted annually. Overall, the region abstracts a relatively small portion of its total renewable water resources each year, at around 350 km3/year. As a continent, this means that much ground water abstraction is within sustainable levels as much of that water will be recharged from infiltrating rainwater. However, many regional differences exist. 10
In many parts of Europe, groundwater is the main source of freshwater. In some parts of Europe water is being pumped from beneath the ground faster than it is being replenished through rainfall. The results are;
• Sinking water tables,
• Empty wells,
• Higher pumping costs and,
• The intrusion of saltwater from the sea which degrades the groundwater.
These issues are problematic in countries with lower precipitation totals and high potential evapotranspiration such as the Mediterranean coastlines of Italy, Spain, Malta and Turkey. These areas also have to cope where the demands of tourist resorts are the major cause of over-abstraction.
In addition, parts of Greece have to cope with ground water over exploitation as a result of irrigation. According to Europa.eu on the “Greek Argolid plain of eastern Peloponnesus where it is common to find boreholes 400m deep contaminated by sea-water intrusion. In Italy, overexploitation of the Po River in the region of the Milan aquifer has led to a 25m (even up to 40 m) decrease in groundwater levels over the last 80 years” 11
Water in the London and the Thames Basin
The Geology of the London Basin in relation to water sources is dominated by Cretaceous chalk. This is the major aquifer, approximately 60m below the surface of central London trending approximately East to West. This chalk varies in depth due to the presence of numerous small faults which cross-cut basin.
The aquifer of the London Basin is confined or kept in place in the Basin by the London Clay Formation. There are also Fluvial (river) muds and fine sands in many places between the clay and chalk.
The Aquifer is recharged anywhere where the Chalk sticks out at the surface, in places such as the Chilterns to the north and the North Downs to the south. The Chalk allows water to percolate rapidly through the aquifer to accumulate in large volumes beneath central London.
Water use in London
London has experienced periods of water shortage in the past and over abstraction of its ground water occurred because London grew expanded in the 19th century, and the industrial, commercial and public demand for water increased. This meant that groundwater was increasing exploited as a water source.
According to Groundwateruk.org “Progressive increases in abstraction from the confined Chalk in the late 19th century and early 20th century eventually became unsustainable, resulting in steady groundwater level decline. After WWII groundwater abstraction started to decrease, partly as a result of declining well yields and a consequential switch to river derived public supplies. Despite this, confined Chalk groundwater levels continued to fall until the mid 1960s when a dynamic equilibrium existed for several years. By this time, groundwater levels in the centre of the London Basin had fallen by about 65 m, from about 35 m below ground level (m bgl) in 1845 to almost 100 m bgl in 1967.” 13
This decline has been reversed with better management of water sources. The aquifer actually started recharging at a rate of 3m per year in parts of the basin posing a flooding risk to deeps tunnels, Underground escalators and the foundations of tall buildings.
1 - Leigh Redman (2018) Deforestation Impacts on the Earth’s Water Cycle. Accessed 19th October 2018 from http://www.3plearning.com/blog/deforestation-impacts-earths-water-cycle/
2 - Mongabay (2015). Rainforests help maintain the water cycle. Accessed 19th October 2018 from https://kids.mongabay.com/elementary/404.html
3 - Rhett Butler (2012). Impact of deforestation:Local and national consequences. Mongabay. Accessed 19th October 2018 from https://rainforests.mongabay.com/0902.htm
4 - Fred Pearce (2018). Rivers in the Sky: How Deforestation Is Affecting Global Water Cycles. Yale Uniersity. Accessed 19th October 2018 from https://e360.yale.edu/features/how-deforestation-affecting-global-water-cycles-climate-change
5 - Dominick Spracklen (2014.). Climate impacts of deforestation. Leeds Univeristy. Accessed 19th October 2018 from http://homepages.see.leeds.ac.uk/~eardvs/research_archive.html
6 - Jesse Allen (2010). 2010 drought in the Amazon Rainforest. NASA. Accessed 19th October 2018 from https://earthobservatory.nasa.gov/images/50136/2010-drought-in-the-amazon-forest
7 Agricultural expansion and climate change in the Taita Hills, Kenya: an assessment of potential environmental impacts Eduardo Eiji Maeda Helsinki 2011. Accessed 19th October 2018 from https://fennia.journal.fi/article/download/6813/5507/
8 - Oxford Reference (2018). Groudnwater abstraction. OUP. Accessed 19th October 2018 from http://www.oxfordreference.com/view/10.1093/oi/authority.20110803095909737
9 - Howard Perlman (2016). Groundwater depletion. USGS. Accessed 19th October 2018 from https://water.usgs.gov/edu/gwdepletion.html
10 - European Environment Agency (2018). Water abstraction. Accessed 19th October 2018 from https://www.eea.europa.eu/themes/water/water-resources/water-abstraction
11 - European Environment Agency (2018). Water use and environmental pressures. Accessed 19th October 2018 from https://www.eea.europa.eu/themes/water/water-resources/impacts-due-to-over-abstraction
12 -UK groundwater forum. Groundwater basics. Accessed 19th October 2018 from http://www.groundwateruk.org/What-is-Groundwater.aspx
13 - Dr Michael Jones. Rising groundwater in Central London. UK groundwater forum.Accessed 19th October 2018 from http://www.groundwateruk.org/Rising_Groundwater_in_Central_London.aspx
14 - FAO (2005). Forests and Floods - Drowning in fiction or thriving on facts? Accessed 19th October 2018 from - http://www.fao.org/forestry/11722-0aea9fb9406230267eaf9955570ec42f3.pdf
Not used as a source but interesting article on Mexico City and water abstraction - https://www.theguardian.com/science/2004/may/06/thisweekssciencequestions