Water for people.pdf - WHO Thailand Digital Repository

C I T I E S : C O M P E T I N G N E E D S I N A N U R B A N E N V I R O N M E N T / 1 7 5costs are also augmented by the layouts of many illegal settlementsand other site characteristics such as unclear plot boundaries andunstable soils (Roche et al., 2001). However, this is not always thecase and some illegal settlements or illegal subdivisions develop withcareful attention to site layouts that allow for future infrastructureinstallation. In addition, the agencies or companies whoseresponsibility it is to provide piped water and provision for sanitationand drainage may be reluctant to install the needed infrastructure inillegal settlements or may be prohibited from doing so.Other water problems related to uncontrolled city expansioninclude:■ difficulties in protecting water sources as new urbandevelopments occur within watersheds and surface water sourcesbecome polluted;■ damage to drainage systems, such as land clearance anddeforestation, which greatly increases silt loads that clogdrainage channels, thus increasing the risks of flooding (Vlachosand Braga, 2001);■ increases in wastewater, storm and surface runoff flows with theexpansion of impermeable surfaces and extraction, use anddisposal of available water sources, which often bring increasedrisks of flooding and reduced infiltration and recharge of aquifers(Ellis, 1999);■ new possibilities for disease vectors, as the expansion of thebuilt-up area, water reservoirs and drains, together with landclearance and deforestation can drastically change the localecology (natural foci for disease vectors may become entrappedwithin the suburban extension, and new ecological niches for theanimal reservoirs may be created; within urban conurbations,disease vectors may adapt to new habitats and introduce newinfections among the urban population [see chapter 5 for moredetails]); and■ the impact on peri-urban agriculture, as most major cities grewwithin fertile agricultural areas, with the result that expandingcities often cover agricultural land while urban water demand canpre-empt water previously used by farmers.Water inputs and outputsUrban water and sanitation benefits and costsIt is difficult to draw up any ‘balance sheet’ regarding the benefitsand costs of urban-based demand for water and other ruralresources. This section concentrates on the environmental costs buturban-based demands for rural resources is also an important (andin fact often the most important) basis for rural incomes andlivelihoods, as the rural incomes produced by urban demand formthe basis for prosperous, well-managed farms, fisheries and forests.In discussing cities’ environmental impacts there is a tendency toforget the key role of cities in providing lower-income nations withstronger, more robust economies.Water resource availabilityMany urban centres face difficulties in obtaining sufficientfreshwater and this is even the case in cities where half or more ofthe population is not adequately served with safe, sufficientsupplies. Many cities have outgrown their capacity to provideadequate water supplies – as all nearby surface water sources havebeen tapped and/or as groundwater resources are being drawn onmuch faster than the natural rate of recharge. Acute problems mayoccur during periods of low rainfall. Problems of water scarcity areparticularly serious in the many urban centres of relatively aridareas.Surface water sources on which cities draw are often of poorquality – for instance, saline because of return water from irrigation,contaminated with agricultural chemical and human and livestockwastes, or heavily polluted by urban areas, industries or otherupstream users. Groundwater resources are also often contaminatedfrom wastewater discharges or from overuse resulting in saltwaterintrusion (coastal aquifers). Consequently, cities face problems infinancing the expansion of supplies to keep up with demand, as thecheapest and most easily tapped water sources have been pollutedand drawing on newer sources implies much higher costs per unitvolume of water (Bartone et al., 1994).Historically, Latin American cities have relied on their closestsources of water which in most cases were freshwater streams, lakesor springs, but many large cities such as Bogota (Colombia), Lima(Peru) and Mexico City (Mexico) have had to invest heavily in theconstruction of dams and pipelines to supplement local water supplieswith water from more remote sources (Anton, 1993). By the 1990s,among sixty-seven urban centres in twenty-nine nations in Africa(including most of the continent’s largest cities), 58 percent wereusing rivers 25 or more kilometres away, and just over half of theurban centres that relied on rivers depended on interbasin transfers.Comparisons between the water sources used in thirty-eight of theseurban centres in the 1970s and the 1990s found an increasing