Can rain water be stored outside?

How much rain water could be collected if all of the roofs in the world captured and stored water?

• Answer:

  Sufficient to just satisfy the needs of the human population for about 9 days/year. The average precipitation over land is around 715 mm or 0.715 m. The surface area of earth is 510,072,000 sqkm = 5.1*10^8 sqkm = 5.1*10^14 sqm. 30% of the earth is land. According to the World Bank's World Development Report, 2009, 90% of the humans lived in 10% of the land area i.e. 3% of the total surface area. Assuming 20% of this area is buildings (and not parks, roads, open spaces etc) and assuming it rains uniformly throughout the landmass (a ridiculous assumption), total rainfall over buildings (considering all buildings have a flat roof) would be (0.715) * (0.2) * (0.03) * (5.1*10^14) cubic meter. = 2.2 * 10^12 cubic meter A human being uses around 100 gallons of water everyday i.e approximately 360 litres of water. Hence, yearly usage = 130 cubic meter. Total human population = 7*10^9 Total annual water usage = 9*10^13 cubic meter. The rainfall captured is (2.2 * 10^12)/(9*10^13)*100 = 2.44% of human use. References: http://en.wikipedia.org/wiki/Rain http://en.wikipedia.org/wiki/Earth http://www.sciencedaily.com/releases/2008/12/081217192745.htm http://www.globalchange.umich.edu/globalchange2/current/lectures/freshwater_supply/freshwater.html