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Catching the Rain on a City
By Dr Terry Thomas School of Engineering, University of Warwick.
Cities receive rainfall but also import other water by pipe, river, aquifer and bottle from outside their borders. They export water via storm drains and foul drains near the surface, via the soil beneath them and (via the leaves of plants) to the air above them. These exchanges are often rather unsatisfactory. There are sometimes system failures like floods and water scarcity and there are huge costs which some cities cannot afford. Water ‘use’ in a city is a complex process.
In the countryside, agriculture employs water to cool plants – up to 1000 kg of it per kg of food harvested – and thereby effectively loses that water. By contrast most city uses degrade water rather than consume it, they dirty it chemically, contaminate it biologically, warm it up or do all three. In traditional urban design, rain is mainly regarded as a problem. Film maker’s might love the glitter of lamplight on wet streets, but city engineers have to provide rainwater drainage, householders have to protect their ceilings and walls from rain entry, slum dwellers fear the mud and disease that can follow heavy rain. The conventional wisdom is to duct away rainfall as quickly as possible, although in recent years there has been growing use of run-off buffering so that the storm drainage capacity can be set lower than peak instantaneous rain-flow. Conventional practice is also to disregard rain-flow in making water provision for a city’s inhabitants. This was not always so and may not be so in the near future. Rainwater harvesting, using private or public tanks, was the main form of water supply for many settlements until a couple of generations ago. It was normal practice on Mediterranean islands with their short winter rains, in Monsoon Asia with its short but intense summer rains, and in many semi-arid areas where ground or surface sources of water were unreliable. It was, and is, particularly practised on low islands whose aquifers are often saline and whose small size precludes the formation of streams. The extensive Indian history of managing rainwater harvesting has been documented in two large reports (Dying Wisdom 1997, Making Water Everybody’s Business 2001) by the Centre for Science and Environment in Delhi. However for over fifty years the expectation has been that cities will be fully ‘drained’ and that water will be supplied – to a single quality and by a single provider – to meet every water need. As cities have grown, this water has come from increasingly distant sources by ever longer channels. Moreover groundwater has been tapped so vigorously that in many countries its level or its quality is falling alarmingly. Damaged aquifers have emptied rivers (the once continuous Yellow River in China no longer reaches the sea for months at a time), countered the gains of expanded irrigation that half feeds the world and made even urban water acquisition more costly. Most dramatic has been the arsenic release now affecting many of the Bengali aquifers, but globally there are numerous other examples of the fall in groundwater quality.