What is the water cycle?
The water cycle is the path that water takes on the Earth through its different forms (liquid, ice, vapour) and different ‘containers’ (sea, lakes, rivers, soil, vegetation, atmosphere etc.) that are all connected. The specific processes by which the water moves from one form or container to the other include evaporation, condensation, precipitation, freezing and melting. Liquid water turns into vapour when it evaporates from seas, rivers, lakes and wet ground, and also when it is transpired by vegetation. When water vapour reaches the cooler parts of the atmosphere, it cools and condenses into tiny droplets which form clouds. When these droplets grow in size and they become heavy enough to fall, it rains. It may happen that frozen ice crystals form which drop in the form of snow or sleet, or after melting as liquid rain. Therefore, the precipitation again reaches the Earth’s surface and again it is collected in soil, streams, lakes and seas/oceans.
What are the driving factors of the water cycle?
On Earth, the water cycle is influenced by many factors such as the sun’s radiation, which drives the climate in general, but also the land morphology and land cover at a more local scale. Another important natural resource connected to the water cycle is soil. Soils provide a variety of services: food provision, regulation of water and nutrient cycling, support of human infrastructures, buffering of contaminants and preservation of biodiversity. Soil quality is threatened by many factors, including urbanization.
How is water distributed on Earth?
Do you know how much of the global fresh water supply is contributed by rivers? Rivers and streams are important features in the water cycle, contributing to both evaporation and to water collection and transportation to the sea, but they represent a minor part of all the water on Earth. Sea water covers ¾ of Earth’s surface and represents 96.5% of the total water volume on Earth. The ¼ of Earth covered by land has mostly fresh water (or brackish water in desert areas), representing about 3% of all the water on Earth. This percentage includes glaciers, ice and snow (1.74%), groundwater and soil moisture (1.69%) and only a small percentage is represented by lakes, swamps and rivers (0.013%)!
The impacts of human intervention on river ecosystem
Urbanization has strongly affected the flow of rivers in cities. River beds in and around towns have been channelled, drained, diverted or culverted. Many cities have “hidden” rivers which are part of the natural drainage system but now flow underneath concrete streets. For instance, the province of Florence counts 30 covered rivers that could pose serious flooding hazards considering of the increasing frequency of extreme weather events. If, over time, urban development has led to a river’s envelopment and an increase of risk, new concerns such as climate change will amplify these risks in the future and it will be important to give nature back its spaces, and to make cities more resilient.
Over recent years, the restoration of water flows in urban areas and the rehabilitation of aquatic life to a largely ‘natural’ state is of increasing interest – but further promotion and awareness are needed.
Other impacts by human intervention are due to intensive agricultural practices that use fertilizers to produce food and animal feedstock, but also other synthesis products (pesticides) to protect them from pests. The nitrates and phosphates percolation produce water pollution and eutrophication.
Pollution also derives from anthropic activities that may come from different locations along the river and different kinds of pollutants can enter the river. Urban areas are sources of PAHs (polycyclic aromatic hydrocarbons) and heavy metals due to the traffic, that are drained into the water systems, but also to the boats when the rivers are navigable.
Pollutants threaten the life of water organisms, directly and indirectly, for instance by lowering the pH. These effects reduce the natural biodiversity of the river and alter the food chain with effects also for the sea organisms especially in case of migratory fish between oceans and rivers (e.g. eels).
Regarding biodiversity, the living organisms of rivers and water bodies are also affected also by exotic species mostly introduced by man intentionally or unintentionally. Some examples that have threatened the natural autochthonous populations of algae and amphibians are the Luisiana crawfish (Procambarus clarkia, also called killer shrimp) or the red-eared terrapin (Trachemys scripta elegans etc.).
The hydrogeological risk
On both sides of streams and rivers, usually there is a flat area which is occasionally flooded when heavy rains or sudden melting of snow occur. These events together with the rise of the sea level, are the main causes of floods. During the flood, water overflows the river banks, and occupies the adjacent plains. Such water is then absorbed by the soil. During river flooding, water carries sediments that subsequently deposit on top of the soil. This natural process has dramatic consequences in urban area, indeed lot of cities have been built on the flat areas close to a river, above the sediments transported by the river in past flood events, during the geological eras.
The consequences of a flood can be devastating; in Europe and all over the world there are frequent reports of floods in areas with high hydrogeological risk. Risks arise from both the hazards and the vulnerabilities: the morphology of the surrounding landscape, and the characteristics of the sediments of a river, are indicators of the potential energy of water during future floods.
The European Parliament establishes a framework for the assessment and management of flood hazards, aiming at reducing the adverse consequences for human health, environment, cultural heritage and economic activities in the Community. Flood hazard mapping is an exercise to define those areas which are at risk of flooding under extreme conditions, in order to reduce the impact of flooding.
Do communities understand their risks? Moreover, what about the impact of a covered or culverted river?
Flood Hazard Mapping is a fundamental component for an appropriate land use planning in flood-prone areas. It creates easily-read, rapidly-accessible charts and maps which facilitate the identification of areas at risk of flooding and also helps prioritize mitigation and response efforts.
Geographic Information Systems (GIS) are frequently used to produce flood hazard maps. They provide an effective way of assembling information from different maps and digital elevation models (Sanyal & Lu, 2003). Using GIS, the extent of flooding can be calculated by comparing local elevations with extreme water levels.
The hidden rivers determine an increase in water speed and energy and in addition, they constrain the water to flow across a fixed section. Therefore, an under-size bridge or culvert can restrict river flow and increase flood levels locally. Further issue is the decreased perception of water flow in the presence of culverted rivers because of the prevention of the observation of the water course.
Related Learning Units:
During hundred thousand years, Earth planet has recorded cycles of glaciations and warmer periods due to small changes of the Earth’s orbit and declination angle, influencing the amount of solar energy on the surface. During only the last hundred years, Earth’s temperature has recorded on average about 0.9 °C more than in the late 19th century, associated to human activities that release in the atmosphere high amounts of greenhouse gases (GHG) –gases with high heat-trapping capacity. Indeed, the IPCC AR5 report, emphasizes the climate change with climate extremes: an increase of hot nights and days and of the frequency of heat waves, but also an increase in the frequency and intensity of rains. As a consequence, climate change affects the hydraulic risk especially when urbanization and river management alter the surfaces, the water cycle and flow.
A key element of climate change is the impact on the earth’s water cycle, increasing water vapor levels in the atmosphere and making water availability less predictable. This brings dramatic changes on water availability and quality with consequences on accessibility and hygiene.