Water supply depends on factors in the water cycle:Rates of rainfall, Evaporation, The use of water by plants (transpiration), River, Groundwater flows. 1% of water is available to use, most of it is trap in glaciers and ice capsIt is estimated that the amount of water supply will decrease in the coming years due to all the water flowing from rivers after evaporation and infiltration into the ground.The water supply is not evenly distributed across the globe.Three quarters of the annual rainfall occurs in areas containing less than one third of the population. One quarters of the annual rainfall occurs in areas containing two thirds of the population. Water stress occurs when per capita water supply is less than 1700 m3 per year, an area that suffers from water stress is subject to frequent water shortages2.3 billion people live in water-stressed areas.Water Use: Agriculture is the largest user, consuming almost two thirds of all water drawn from river, lakes and groundwater. Since 1960 crop irrigation has increased by 60% to 70%. Industry uses about 20%. Municipal uses about 10%. Population growth, urbanisation, and industrialisation have increased the use of water. As world population and industrial output increase, so does water consumed. Causes of Water Shortages: Population Growth: As with many of the world’s resources, they are coming under increasing pressure as the world’s population grows. The world’s population now stands at about 7 billion, all of whom are placing increasing on water resources as they develop and get richer.Pollution: As the world’s population grows so does the demand for agricultural and industrial products. Increased the use of fertilizers and pesticides, which often run off into rivers and lakes or leach down to groundwater stores. Demand for industrial and at times relaxed environmental regulations mean more chemicals and metals are being released into our water sources. Sewage treatment also often lags behind population growth so increasingly our rivers and lakes are being polluted by sewage.Domestic Demand: The demand from households is not only increasing because there are more households in the world, but also because the amount of water they want is increasing with development. Agricultural Demand:  With a growing population, global warming and the movement in to less favorable agricultural regions, the demand from agriculture is only likely to increase in the future. Industrial Demand: As the world’s population grows and becomes richer our demand for industrial products grows. Sewage: With rapid urbanization taking place in many cities around the world, infrastructure often does not keep up with new arrivals. The growth of informal settlements without proper sewage treatment can mean that human waste is often pumped directly into water sources. However, this is not only a problem in LEDCs, in London the sewer system cannot cope and an estimated 39 million tones of sewage are dumped in the River Thames annually.Climate Change: Climate change is impacting the availability of water in many ways. Global warming maybe releasing freshwater from glaciers and ice shelves, but unfortunately much of it is running directly into the oceans. The subsequent rising sea levels are threatening many coastal freshwater wetlands as well as increasing the risk of saltwater intrusion into aquifers. Warmer temperatures are increasing the amount of evaporation from rivers and surfaces stores.Political: In many countries or regions, water sources are shared e.g. the River Nile flows through eleven countries. At times some countries control large percentages of the shared resource, leading to shortages for other countries. Groundwater Depletion: If water is used unsustainably i.e. more is taken out than is being recharged then aquifers can sufferfrom salinization and saltwater intrusion. If you increase the concentration of water it can become too salty for human use. Also if you drain aquifers near coastal areas, then they can become full of saltwater, again making them useless. Problems Caused by Water Shortages and Water Pollution:Drought:  Drought is below average supply of water over a prolonged period. Because drought is below average supply of water, even relatively wet countries like the UK can suffer from drought.Crop Failure: If there is a shortage of water and farmers cannot irrigate their crops then they begin to die.Livestock Deaths: If livestock don’t have enough water to drink they will begin to die. Famine: If cops are failing and livestock are dying then people will become undernourished and suffer from famine.Groundwater Depletion (subsidence and saltwater intrusion): If aquifers begin to dry up or are used unsustainably, then the ground above can subside (collapse) or the aquifer can suffer from salinization or saltwater intrusion. Subsidence is a problem common in parts of Mexico City.Conflict: If there is a limited supply of water and water resources are shared conflict can arise. Many of the ongoing border disputes between Israel and Palestine are blamed on water shortages.Refugees: If there is drought and famine then people are forced to relocate or face death. Unfortunately many of the countries that suffer from drought and famine have poor neighbors so refugees will be arriving in countries that are least able to cope. Disease: Dirty water can attract mosquitoes, which can increase diseases like dengue and malaria. Dirty water can also cause the spread of diseases like hepatitis A and typhoid as well things like diarrhea.Eutrophication: Run-off from farms containing fertilizer can lead to eutrophication. Eutrophication is the excess growth of algae causing water to not oxygenate properly or receive enough light. This can cause plants and animals to suffocate and die.Biodiversity Loss: Dirty water and eutrophication can cause loss of biodiversity in wetland environments, but also just like humans can die of thirst and starvation, so can plants and animals. Big animals like elephants, which require large amounts of water often die in African droughts.Possible solutions to water shortages: Irrigation Projects: Countries that have regional shortages of water or variable rainfall can use irrigation systems to redistribute water and water the land. The largest and most famous irrigation project been undertaken is Libya’s man-made river scheme, which aims to turn parts of the desert green. The water is being accessed from under the ground and redistributed around the country. Reduced Leakage: Leakage is a huge problem, especially in countries with old pipe networks. One water company in the UK is estimated to lose 295 million litres a day. In the UK there are targets for water companies to reduce leakages, unfortunately many companies are currently missing these targets.Dam Construction: Dams are controversial because they can bring many disadvantages as well as advantages but if they are built sustainably they can create artificial stores that can collect water in rainy seasons and distribute during drier periods.Water Metering: Charging people per unit of water used, rather than charging a flat fee can drastically reduce wastage and make people consider how and when they use water.Construction of Wells: Many countries cannot afford to have piped water to every residence so wells become important in accessing groundwater supplies. As long as wells are used sustainably they can be a vital source of water in many LEDCs and arid countries.International Cooperation: When water is shared, it is necessary to have sustainable policies to reduce the tragedy of the commons, when all countries or regions take water for their needs and forget about the overall impacts. The countries along the Nile are trying to create such agreements, but Egypt is hostile to any plans to redistribute.Water scarcity: Where water supplies are inadequate, there are two types of water scarcity affect LEDC’s:Physical water scarcity: This occurs where water consumption exceeds 60% of the usable water supply. To help meet needs some countries like Saudi Arabia and Kuwait import much of their food and invest in desalinization plants. Where the demand for water is greater than the supply of water. Physical water scarcity does not have to be an arid environment, because there demand for water in arid environments (deserts) is not normally low meaning that there is no shortage.Economic water scarcity: Where there is water available, but for some economic reason it is not possible to fully utilize the source of water. This might because extraction or transportation costs are too high, or because the water is polluted and it is not possible to treat it.In LEDC’s access to adequate water supplies is most affected by the exhausted of traditional sources, such as wells and seasonal rivers.In many poor countries farmers use, on average, twice as much as water per hectare as in industrialised countries, yet their yield can be three times lower. Water needs to be adequate quality for consumption. The World Health Organisation estimates that around 4 million deaths each year are due to water related diseases especially cholera, hepatitis, malaria.Too many people lack access to safe and affordable water supplies and sanitation in developing countries.Organic waste from sewages, fertilizers and pesticides from agriculture may affect water quality. Also heavy metal and acids from industrial processes and transport.  Global water supply and sanitation. Urban areas have more access to the global water supply: Many-piped water system do not have a good quality of water, leading to more people relaying on bottled water brought in markets for personal use for example in major cities in Columbia, India, Mexico, Venezuela and Yemen.Fewer people have adequate sanitation than safe water, and the global provision of sanitation is not keeping up with the growing population.