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examBoard: Cambridge
examType: IGCSE
lessonTitle: Distribution of Earths Fresh Water
Water is often called the "blue gold" of our planet. It covers about 71% of Earth's surface, but not all of this water is available for us to use. Understanding how water is distributed across our planet helps us make better decisions about how to manage this precious resource.
Key Definitions:
When we look at Earth from space, we see a blue planet. But this can be misleading when thinking about water we can actually use.
Of all water on Earth, approximately 97% is saltwater found in oceans and seas. Only about 3% is freshwater. This means that despite Earth appearing to be water-rich, the amount of water readily available for human use is actually quite limited.
Humans, animals and plants need freshwater to survive. We can't drink saltwater and most crops can't be irrigated with it. Converting saltwater to freshwater (desalination) is possible but expensive and energy-intensive. This makes the 3% of freshwater incredibly valuable.
The small percentage of freshwater on Earth isn't all readily available to us. It's distributed across different reservoirs and much of it is difficult to access.
If we take that 3% of freshwater and break it down further, we find that it's not evenly distributed in forms we can easily use:
About 68.7% of Earth's freshwater is locked up in ice caps, glaciers and permanent snow, particularly in Antarctica and Greenland. While this is the largest freshwater reservoir, it's largely inaccessible for direct human use.
Around 30.1% of freshwater exists as groundwater. This includes water in aquifers and soil moisture. Groundwater is a critical resource for many communities worldwide, accessed through wells and boreholes.
Only about 1.2% of freshwater is found in lakes, rivers, swamps and atmospheric water vapour. Despite being the smallest portion, this is the most accessible freshwater source for most living things.
Surface water includes all freshwater that's visible on the Earth's surface. Though it represents a tiny fraction of all water, it's vital for ecosystems and human societies.
Lakes contain about 87% of all surface freshwater. Major lakes like Lake Baikal (Russia), the Great Lakes (North America) and Lake Victoria (Africa) are crucial freshwater reservoirs. Lake Baikal alone contains about 20% of the world's unfrozen freshwater.
Rivers contain only about 2% of surface freshwater but are vital for transporting water across land. Major river systems like the Amazon, Nile and Ganges support billions of people and countless species. Rivers are constantly being replenished through the water cycle.
Beneath our feet lies a vast reservoir of freshwater that many people don't think about. Groundwater is stored in aquifers - layers of permeable rock, sand, or gravel that hold water.
Groundwater is replenished slowly when surface water seeps down through soil and rock layers. This process, called infiltration, can take anywhere from days to thousands of years depending on local conditions.
Humans access groundwater through wells and boreholes. In many regions, groundwater is the primary source of drinking water. About 2.5 billion people depend solely on groundwater for their daily water needs. In rural areas without piped water systems, groundwater is often the only reliable water source.
Groundwater faces several threats including over-extraction (taking water out faster than it's replenished), pollution from agricultural chemicals and industrial waste and saltwater intrusion in coastal areas. Once an aquifer is contaminated, it can take decades or centuries to recover.
The largest portion of Earth's freshwater is frozen in ice caps and glaciers. These massive ice formations serve as long-term water storage systems.
Ice caps and glaciers are often called Earth's "water towers" because they store water in winter and release it slowly during warmer months. This natural regulation helps maintain river flows year-round in many regions.
The Antarctic ice sheet contains about 61% of all freshwater on Earth. If it melted completely, global sea levels would rise by about 58 metres. This ice has accumulated over millions of years.
The Greenland ice sheet holds about 7% of the world's freshwater. It's been losing mass at an accelerating rate due to climate change, contributing to sea level rise.
Mountain glaciers, though smaller than the polar ice sheets, are critical water sources for many river systems. The Himalayan glaciers, for example, feed major rivers like the Ganges, Indus and Brahmaputra, supporting over a billion people.
The Ogallala Aquifer is one of the world's largest aquifers, underlying eight states in the central United States. It supplies about 30% of all groundwater used for irrigation in the US and provides drinking water to about 2 million people. However, the aquifer is being depleted at an alarming rate - in some areas, water levels have dropped by more than 30 metres since pumping began in the 1950s.
The water in the Ogallala accumulated over thousands of years, but modern pumping is removing it much faster than natural processes can replenish it. Some estimates suggest parts of the aquifer could be effectively depleted within decades. This case illustrates how even seemingly vast water resources can be vulnerable to overuse and highlights the importance of sustainable water management practices.
The distribution of freshwater around the world is highly uneven, both in terms of natural availability and human access to clean water.
Some regions have abundant freshwater resources. Canada, for example, has about 20% of the world's freshwater but less than 0.5% of the global population. In contrast, many countries in the Middle East and North Africa have very limited freshwater resources. Water scarcity affects about 40% of the global population and this figure is projected to rise.
Human activities significantly impact freshwater distribution. Dams and reservoirs store water and alter river flows. Irrigation diverts water from rivers and aquifers to farmland. Urbanisation changes how water moves across landscapes. Climate change is altering precipitation patterns and accelerating glacier melt, further complicating water distribution patterns.
As the global population grows and climate patterns shift, managing Earth's limited freshwater resources becomes increasingly challenging.
Several factors are putting pressure on the world's freshwater resources:
Once the fourth-largest lake in the world, the Aral Sea in Central Asia has shrunk to about 10% of its original size since the 1960s. This environmental catastrophe occurred because the rivers that fed the sea were diverted for irrigation, primarily for cotton production.
As the sea dried up, fisheries collapsed, local climate patterns changed and toxic dust from the exposed seabed caused health problems for nearby communities. The Aral Sea disaster demonstrates how human alteration of natural water distribution can have devastating and far-reaching consequences. It serves as a powerful reminder of why sustainable water management is essential.
Understanding how freshwater is distributed across our planet helps us appreciate its scarcity and value. While water covers most of Earth's surface, only a tiny fraction is available as freshwater. Of this freshwater, most is locked in ice or stored underground, leaving just a small amount readily accessible in lakes and rivers.
As climate change alters precipitation patterns and human demand for water continues to grow, managing our limited freshwater resources sustainably becomes increasingly important. By understanding where our water comes from and how it moves through the environment, we can make better decisions about how to protect and conserve this essential resource for future generations.
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