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There are several reasons why global water supply is not even.

Factors affecting water supply

Access to a fresh water supply is affected by the following factors:

1. Climate. Low levels of rainfall and high temperatures lead to water deficits. When rainfall is low, there is less water available. When temperatures are high, water evaporates and so there is less available to use. Water surpluses are common where rainfall is high and temperatures are lower.
2. Geology. Rainfall flows down to the rocks beneath the ground. Some rocks are permeable and allow water to flow through them. Permeable rocks can lead to less surface water. For example, limestone landscapes often have dry rivers - the rivers only exist when rain has just fallen. Permeable rocks form aquifers, which means they are stores of water. 70% of the water supply in the south east of England comes from the chalk aquifer beneath the surface. Other rocks are impermeable. These rocks do not hold water, but they can trap it in the layers above.
3. Pollution. Some places have plenty of water, but pollution has made it unsafe to use. Untreated sewage and waste water from factories cause problems. Groundwater is usually cleaner, although pollutants can travel down into the ground. Gold mining near Johannesburg, South Africa, has led to uranium, arsenic and sulphuric acid polluting streams and rivers.
4. Over-abstraction. When water is taken from aquifers, groundwater levels fall. If the amount of water taken is greater than the amount of water falling as rain, it is called over-abstraction. The Sonoran desert in Arizona has experienced over-abstraction as water is taken for irrigation and urbanisation. The land here is subsiding and water is becoming scarcer.
5. Limitedinfrastructures. Pipelines are needed to safely move water from place to place. Sealed pipes reduce the potential for leaks and pollution. Some places do not have these pipes in place. They can be expensive to lay as they need to be buried underground. Water pumps also form part of the water infrastructure. Water pumps in villages across Kenya were recently fitted with transmitters that send text messages if the devices break down.
6. Poverty. Nearly 1 billion people in Africa do not have access to clean, safe water. This locks them in a cycle of poverty - they cannot afford water so they become ill and when they become ill then cannot work and earn money.