Isn’t all water recycled round the planet in the water cycle, so that whatever we use is continually being replaced? And with two thirds of the Earth surface covered in water, haven’t we got plenty of it?
Unfortunately, it’s not quite that simple. Yes, water does move round in a cycle, and it’s not a case of “when it’s gone, it’s gone” like with oil reserves. But, the water cycle involves more than just evaporation into the atmosphere, where it only remains for about 9 days on average [4], and then precipitation back to the Earth’s surface. Water is stored in various types of reservoir, such as the oceans, rivers, glaciers, soil moisture and groundwater, and only 2.5% of the water on Earth is freshwater. Although we’ve got plenty of seawater and it can be put through a desalinisation process to remove salt and turn it into a freshwater supply, this uses a lot of energy so is too expensive to use in many parts of the world. Of the small amount of freshwater that we have got, almost 70% is locked up in glaciers and ice caps. This means that only around 1% of the water on the planet is usable by humans [6]. Of this, 99% is groundwater, stored below the Earth’s surface, and only a tiny amount is stored in rivers and lakes. Rivers are where most of the water we use comes from [6].
Although whatever water is used will be returned to the environment, it won’t be immediately available again. It will have to go back through the water cycle and be returned to one of the reservoirs that we can access before it can be used again. Some groundwater supplies are accessible, but they are rapidly becoming depleted because water is being extracted from them faster than it can be replaced by the natural cycle [7, 1]. Water supplies from other types of reservoir are also being lost – as an example, Lake Chad in Africa has been declared an ecological catastrophe because so much water was extracted for use by the local population that the lake shrank by 90% between 1963 and 2001 [5]. On average across the world, 630 cubic metres of water per person are taken from water sources each year for human use [3]. Most of this is used for irrigation in agriculture [8] so it follows that the amount of water withdrawn varies hugely between countries depending on how much crop irrigation is needed.
There is also a big difference in availability of water supplies between different regions of the world. In drier climates, the most obvious problem is the lack of access to water sources, and that can be even more difficult if there are economic problems in a country. In wetter areas, there might be enough water but it can get polluted by waste disposal and chemicals used in agriculture and industry, so that the water needs extra treatment to make it usable. Pollution can occur all over the world in various ways, and has knock-on effects for the health of humans and other species, especially if there’s no water treatment system set up.
In summary, although we have plenty of water on Earth, very little is safe freshwater and it is being removed from the environment more quickly than it can be replaced. Wastage of water, even in individual households, contributes to this. Reduce your water consumption easily with a shower timer to limit your time in the shower. Once the water has gone down the plug, it’s going to take time before it can be used again, and in that time more and more water will need to be extracted. Even in the UK we see the effects of this – hosepipe bans in the summer come about because there’s not enough rain falling to replenish the water reservoirs quickly enough [2]. As the global population grows, so will this problem of “overextraction”. So, are there any solutions? Well, there’s desalinisation of seawater, but as already mentioned that uses a lot of energy and is expensive. So it’s not good for the environment either, and many countries are trying to cut their emissions. Alternatively, maybe water could be moved from places with extra water supplies to areas with water shortages. This requires expensive transport systems and there could be political and organisational difficulties if water has to be brought in from other nations. Also, it would be very difficult to stop some of the water evaporating while it’s being stored or transported. Or, water can be properly treated and managed, and efforts can be made by everyone to protect and reuse what is available. Essentially, we’ve got it – we just need to look after it.
References
[1] American Geophysical Union, 2010. http://www.agu.org/news/press/pr_archives/2010/2010-30.shtml
[2] BBC, 2010. http://news.bbc.co.uk/local/manchester/hi/people_and_places/newsid_8797000/8797287.stm
[3] Food and Agriculture Organization, 2005. www.fao.org/nr/water/aquastat/maps/index.stm
[4] Physicalgeography.net, 2011. http://www.physicalgeography.net/fundamentals/8b.html
[5] Underwatertimes.com, 2009. http://www.underwatertimes.com/news.php?article_id=10205417863
[6] United States Geological Survey, 2011a. http://ga.water.usgs.gov/edu/earthwherewater.html
[7] United States Geological Survey, 2011b. http://ga.water.usgs.gov/edu/gwdepletion.html
[8] Waterencyclopedia.com, 2009. http://www.underwatertimes.com/news.php?article_id=10205417863
