Water for people.pdf - WHO Thailand Digital Repository

T H E N A T U R A L W A T E R C Y C L E / 8 9■ infiltration and recharge to aquifer systems are reduced andsurface runoff is increased in both volume and rate due togrowth in the impermeable surface areas leading to increaseddownstream flooding;■ declining water levels and land subsidence may occur due togroundwater mining;■ pollutant loads to water courses and surface water bodiesincrease from surface runoff and sewage outfalls especiallyduring storms in urban areas;■ leakage to groundwater occurs from old and poorly maintainedsewers;■ extensive soil and groundwater are contaminated by industrialleakages, spills of hazardous industrial chemicals and poorlyplanned solid and liquid waste disposal practices;■ increased artificial surface water infiltration and recharge fromsource control devices lead to poor groundwater quality; and■ habitats and the diversity of species are reduced in receiving waters.Hibbert (1967) surveyed the results from thirty-nine studies, mainlyin the United States, on the effects of altering the forest cover onbasin water yield. He showed, in general, that reduction in forestarea increases yield and that reforestation decreases yield. Theincrease was a maximum in the first year of complete felling, withan upper limit equivalent to a depth of 4.5 mm a year for eachpercent reduction in forest cover. As the forest regrows, theincreased streamflow declines in proportion to the rate of forestrecovery. Dunne and Leopold (1978) reached similar conclusionsfrom these and other findings and added that the effect of reducingforest cover may be far less important in arid regions. Results fromninety-four paired basin studies in different parts of the world werereported by Bosch and Hewlett (1982). They found that for pineand eucalyptus forest there was an average of 40 mm change inyield per 10 percent change in cover, while the correspondingfigures for hardwood and scrub were 25 and 10 percent. Laterresearch has generally agreed with these results, but the emphasishas switched from relatively simple studies of water quantity tothose of the processes involved (Kirby et al., 1991), including basinhydrobiogeochemistry, in attempts to understand the mechanisms inoperation. But it seems that the further the functioning of a basinis unravelled, the more complex and detailed the processes appearand the greater the number of questions and uncertaintiesgenerated (Neal, 1997).Desalinated water resourcesWith population growth and concerns about water scarcity growing,several countries, especially in the Near East region, are developingdesalination plants to convert saline water (e.g. sea-water, brackishwater or treated wastewater) into freshwater.The deterioration of existing groundwater resources, combinedwith the dramatic decline in costs, has given new impetus to thisold technology, once considered an expensive luxury. The globalmarket for desalination currently stands at about US$35 billionannually, and could double over the next fifteen years.The process of desalting has a great deal to contribute torelatively small-scale plants providing high-cost water for domesticconsumption in water-deficient regions. For irrigation, however,costs do constitute a major constraint. Therefore, except in extremesituations, desalinated seawater has not been used for irrigation,and the contribution of desalinated seawater on a global scale tototal resource availability is very small.In 2002 there were about 12,500 desalination plants around theworld in 120 countries. They produce some 14 million m 3 /day offreshwater, which is less than 1 percent of total world consumption.The most important users are in the Near East (about 70 percent ofworldwide capacity) – mainly Saudi Arabia, Kuwait, the United ArabEmirates, Qatar and Bahrain – and North Africa (6 percent), mainlyLibya and Algeria. Among industrialized countries, the United States(6.5 percent) is a big user (in California and parts of Florida). Mostof the other countries have less than 1 percent of worldwidecapacity. It is expected that the demand for desalinated seawater willincrease in those countries that already apply this option, and willalso appear in other regions and countries as their less expensivesupply alternatives become exhausted. However, safe disposal ofgenerally toxic chemical by-product of desalination is still a concern.Among the various desalination processes, the following are themost interesting for large-scale water production: Reverse Osmosis(RO), Multi-Effect Distillation (MED) and Multi-stage Flash Distillation(MSF). The latter is used mainly in the oil producing countries of theMiddle East. Currently, RO offers the most favourable prospects, as itrequires less energy and investment than other technologies. A lot ofenergy is needed to desalinate water, although the form and amountof energy input depends on the process used. For RO, for example,about 6 kilowatts per hour (kWh) of electricity is required for each m 3of drinking water produced. For distillation processes such as MEDand MSF, the energy input is mainly in the form of heat (70°C to130°C hot water or steam). For MED, specific heat consumption is inthe range of 25–200 KWh/m 3 and for MSF, 80–150 kWh/m 3 .Gulf States such as Saudi Arabia, the United Arab Emirates andKuwait use dual-purpose power and desalination plants on a grandscale. Jordan, Israel and the Palestinian Authority are increasinglyseeing a viable and economic solution to ensuring future water