Physical geography

Water cycle

Enquiry question 2: What factors influence the hydrological system over short- and long-term timescales?

5.4 Deficits within the hydrological cycle result from physical processes but can have significant impacts

5.4A - The Causes of Drought

Causes of meteorological drought:

• Drought's physical causes are only partially understood. They are somewhere in the intricate interactions between the atmosphere, seas, cryosphere, biosphere, and land that form the world's climates

• Droughts can range from short-term, localised precipitation shortfalls to longer-term patterns caused by climate change

El Ni​ño Southern Oscillation (ENSO):

Temperature anomalies are regarded to be the key to ENSO, which in turn is supposed to cause droughts.

Conditions in the Pacific Basin are normal:

• The trade winds blow from east to west along the equator in non-El Nio years

• The warm water is pushed westward by the air

• Upwelling, thermocline

• Warm, moist air rises, cools, and condenses, resulting in the formation of rain clouds

During an El Nio event, the weather conditions are as follows:

• The trade wind pattern is interrupted; it may slacken or even reverse, which has an impact on ocean currents.

• Reversal of the air circulation loop

La Nina:

• La Nia occurrences can sometimes, but not always, occur after an El Nio event. They are caused by the accumulation of cooler-than-normal subsurface water in the tropical Pacific. This scenario has the potential to cause catastrophic drought conditions, particularly on South America's western coast

• Strong air circulation and warm water going east to west

5.4B - Human Activity and Drought

Desertification in the Sahel:

The Sahel area of Africa extends from Mauritania to Ethiopia

Desertification is the slow transformation of once-productive land into a desert-like terrain. It frequently takes place on the outskirts of existing deserts in semi-arid territory. It is not always irreversible

Desertification is mostly caused by natural processes. They started a downhill spiral:

• Changing rainfall patterns, with seasonal and yearly rainfall becoming less dependable. The odd drought year might last many years

• The vegetative cover gets strained and dies, exposing the dirt

• Wind and the occasional heavy rain damage the bare earth

• When it does rain, it usually falls in short, strong bursts. This makes capturing and storing it challenging for the remaining soil

Human factors function as a feedback loop. Drought consequences are exacerbated by humans' over-abstraction of surface water from rivers and ponds, as well as groundwater from aquifers. The following are key human elements that are encouraging this:

• Population growth: Rapid population increase puts pressure on the land to produce more food. Migrants escaping one catastrophe region contribute to the creation of another

• Overgrazing: when there are too many goats, sheep, and cattle, the vegetative cover is destroyed

• Overcultivation: intensive usage of marginal land depletes the soil, preventing crops from growing

• Deforestation: occurs when trees are taken down for fuel, fencing, or houses. The roots no longer bond the soil, causing erosion

5.4C - Drought and Ecosystems

Forests:

Forests have a large influence on the hydrological cycle. They are responsible for a lot of interception, which implies less infiltration and overland flow. High amounts of transpiration characterise forests

Drought, like wetlands, harms forests, but it is humans and deforestation that pose the greatest threat to their existence. Drought not only causes direct physiological harm in coniferous forests but also makes pines and firs more susceptible to fungal infections. Tree mortality is increasing. The same holds true for the tropical rainforest, with the exception that the increasing mortality caused by drought appears to be having a bigger impact on huge trees. There is also the concern of what increasing tree mortality may do to this vital carbon storage in the long run

Because ecosystems play such an essential part in the hydrological cycle, it is critical to ensure that their ecological resilience is not overstressed by either harmful human activities or natural disasters such as droughts and floods

Wetlands:

Wetlands now occupy around 10% of the Earth's land area, and they were thought to be wastelands until about 50 years ago, only useful for draining and infilling to create construction space. Wetlands, on the other hand, are now acknowledged to fulfil a variety of vital roles, ranging from functioning as temporary water storage to recharging aquifers, from huge filters collecting pollutants to providing nurseries for fish and feeding spots for migrating birds

Drought may have a significant influence on wetlands. There will be reduced interception (as vegetation gets stressed), as well as less infiltration and percolation, with less precipitation. Water levels will drop. Evaporation will rise as well. Wetlands' vital services will be reduced as a result of this, as well as the decrease in transpiration

While droughts are a concern to wetlands, the most serious threat to their survival remains artificial destruction

5.5 Surpluses within the hydrological cycle can lead to flooding, with significant impacts for people

5.5A - Meteorological Causes of Flooding

Surpluses in the hydrological cycle almost always result in flooding. Flooding is caused by the following weather factors:

• Storms that are severe enough to cause flash floods (with a short lag time), as in semi-arid areas but more prevalent in mountainous places

• Extended periods of heavy rain, such as during the Asian monsoon and the passage of deep depressions over the UK

• Rapid snowmelt during an exceptionally warm spring, such as in the Siberian plains

Bangladesh is a particularly food-prone country, owing to its floodplains and deltas formed by great rivers such as the Ganges, Padma, and Meghna. Meltwater from the Himalayas and the summer monsoon swell these rivers twice a year. Flash floods are common in the hilly areas between the rivers and behind Chittagong

Tidal flooding occurs as a result of storm surges or when heavy river flows collide with unusually high spring tides in estuaries. A storm surge is created by extremely low air pressure, which causes the high-tide sea to rise. The 'raised' sea is then driven towards the coast by strong onshore winds, frequently smashing coastal defences and flooding significant regions

The likelikhood of flooding is also increased by other physical circumstances:

• In low-lying locations with impermeable surfaces, like as cities and towns

• Where impermeable rocks lie under the ground surface

• When ice barriers unexpectedly collapse and glacial lakes' waters are freed

• When volcanic activity causes meltwater beneath ice sheets to be discharged abruptly (jökulhlaups)

• When earthquakes force dams to fall or landslides to impede waterways

5.5B - Human Actions and Flooding

During the twentieth century, economic and population expansion forced many floodplains to be built on and many natural landscapes to be transformed for agricultural, industrial, and urban reasons. These similar activities, all of which are tied to altering land use within river catchments, usually enhance the danger of flooding, none more so than urbanisation

Examples:

• Tarmac impermeable areas

• Wells dug to provide water to towns

• Groundwater sprinkling on arable crops

• Dams constructed to deliver water to settlements

• Plowing compacts the soil

• Grazing animals trample on the earth

• Rain is intercepted and moisture is transpired by woodlands; roots provide strong soil structure. This is destroyed by deforestation

• Water can sink into natural grasslands, which have been replaced by developed grazing

• Bridge supports installed in rivers, floodplain ramps

• Water is fed into the waterway via sewers

River mismanagement:

• Channelisation is an efficient method of increasing river discharge and lowering flood danger. The problem is that it only shifts the river downstream. The increasing discharge may overload some other place

• Dams: obstruct the flow of sediment down a river, causing the reservoir to gradually fill with silt; downstream, river bed erosion increases

• River embankments: structures built to guard against floods of a specific magnitude.When a flood surpasses their capacity, they might fail. When this occurs, the extent of floods is always increased

5.5C - Impacts of Flooding

Environmental:

Flooding's environmental effects receive far less attention, maybe because there are certain benefits:

• Recharged groundwater reservoirs

• Enhanced interconnectivity of aquatic environments

• Replenishing of the soil

• Floods cause breeding, migration, and dispersion in many species

Socio-economic:

• Injury and death

• Water-borne illness transmission

• Property damage caused by trauma, notably housing disruptions in transportation and communications

• Water and energy supply disruptions

• Crop damage and supply shortages

• Ordinary life, including employment, is disrupted

Floods in the UK:

In recent years, the United Kingdom has experienced significant flooding, most notably in the summer of 2007 and the winter of 2015-16

These extraordinarily catastrophic floods were caused by the same underlying cause, which was persistent heavy rain but at various periods of the year. Large sections of the UK got more than double the typical amount of rainfall for this time of year during the 2016 floods. Cumbria's Carlisle and Cockermouth were among the hardest impacted