Precipitation
More frequent intense rainfall leads to more street, basement, and sewer flooding and stormwater runoff to various disposal systems. In most parts of the world, whether average precipitation totals increase or decrease with climate change, more intense rainstorms are expected.More intense rainstorms will increase nutrient loads, eutrophica-tion, taste and odor problems, and loading of pathogenic bacteria and parasites (Cryptosporidium and Giardia) in reservoirs. More intense precipitation will lead to more combined sewer overflow events that, depending on the city, pollute coastal waterways or other nearby bodies of water. More frequent intense rainstorms will also increase the sediment load in some rivers and reservoirs, and this may decrease the water quality of water diverted for water supply or further restrict periods of diversion. More intense and frequent rainstorms also can result in more flooding and erosion, which will lead to destruction of infrastructure such as bridges and approach embankments to bridges. The timing of rainfall may change, causing further disparities between supply and demand, e.g., with later rainfalls in places like the Seattle region (Chinn, 2005). An example of a city facing future climate stress from direct precipitation and loss of glacier mass is presented in the case study of Santiago de Chile (Box 5.3).
Santiago de Chile: Adaptation, water management, and the challenges for spatial planning
Santiago de Chile, with its population of six million concentrated in the Maipo river basin on the western flanks of the Andes, is regarded as a Latin American city that compares well with others in terms of poverty, security, economic activity, and other urban indicators. Although the country contributes little in terms of global greenhouse gases, and is highly active in CDM projects, it faces considerable adaptation challenges, e.g., due to the vulnerability of its agro-business sector and its coastal cities (CONAMA, 2005, 2008). Santiago’s future is linked to these changes, but faces more specific local adaptation challenges. Perhaps the most important of these is water management. The catchment is fed year-round from Andean glaciers since localized precipitation is highly concentrated in the June-July winter period. The projections to 2070 under an A2 scenario suggest a potential 40 percent reduction in precipitation, compounded by reductions in glacial flows and rising evapotranspiration tied to higher temperatures of 2-4 °C (CONAMA, 1999; CONAMA, 2006). Pressures will grow to change the current water management system and meet the adaptation challenge as a consequence of increasing conflicts over water access. The expected population by 2030 exceeds eight million people (MINVU, 2008). This is likely to correspond to urbanization processes that displace agricultural interests in the region as the metropolitan area expands into productive land, also areas of increased risk and areas that provide important environmental services for the watershed.
The adaptation challenge to tackle this scenario lies in three fields: water markets, equitable distribution and water conflicts, and climate change governance within spatial planning. National and local government has only partially addressed these concerns to date.
Box Figure 5.5: Maipo Basin, Santiago de Chile.
Box Figure 5.6: San Carlos channel (early nineteenth century) entry into the Mapocho River, an artificial channel that draws water from the Maipo River and takes it north into the Mapocho.
THE WATER MARKET
The water market is based on water rights that are purchased and transacted (Water Code, 1981, modified 2005), based on a minimum streamflow condition and a total availability calculated by the national water authority (DGA). There is currently insufficient supply for new consumptive rights to be made available in the Maipo basin (DGA, 2003). Meantime, there is pressure from more powerful interests to buy out smaller rights holders, such as small-scale irrigation associations. The market is also unable to respond to fluctuations in the hydro-logical cycle, including the El Nino phenomenon for example, since rights are fixed and are awarded in perpetuity. In consequence, water availability decreases, existing rights will not be able to be extracted, and no new uses will be catered for.
EQUITABLE DISTRIBUTION AND WATER CONFLICTS
The limitations of the existing market, its weakness to respond to the natural cycles in the water basin, and the anticipated scarcity due to climate change, present a major adaptation challenge. Conflicts will increase particularly between residential, agricultural, and mining demands and environmental services. Assuming that the high residential value of land will lead to wine investments and horticulture moving to other regions, the question remains as to how a potential 40 percent reduction in water availability will be met by a population in the metropolitan area that is 30 percent larger than at present. The city’s location is in a Mediterranean biodiversity hot spot, where current levels of "green" space per habitant (3.2 m2/cap; CONAMA, 2002) are well below the WHO recommendation of 9 m2/cap. This raises the issue of water use for maintaining the region’s ecosystems, and for increasing public spaces to enhance urban quality of life (particularly in the lower income municipalities of the city) and reducing, for instance, the heat island effect. This will require a significant shift in water management in many areas with:
• reduced agricultural irrigation capacity
• watering of, and species selection in public spaces and domestic gardens
• a storm water drainage system that seeks to shift water downstream of the city as swiftly as possible during peak events (rather than capture and storage)
• broad-based demand reductions.
CLIMATE CHANGE GOVERNANCE WITHIN SPATIAL PLANNING
The 2005 national climate change strategy concentrates on productive sectors, particularly mitigation and CDM commercial opportunities, but fails to put much weight on adaptation issues (CONAMA, 2005). It also fails to explicitly consider urban centers in spite of over 80 percent of Chileans living in urban areas, with over 40 percent of the national population living in the Santiago metropolitan region (RMS). This has changed slightly with the publication of the 2008 national action plan (CONAMA, 2008). The plan focuses on seven fields for action, water being one of them.
Although urban change, except coastal city risk, is not an explicit focus of the plan, all seven issues relate to urban transformations. Their incorporation into planning instruments is going to be a primary challenge for climate change adaptation: development strategy, metropolitan and local regulatory plans, and local development plans. To date, these documents have not included climate change considerations explicitly, largely because of the sectoral approach to public sector management.
Climate change adaptation will demand a coordinated response from government agencies, within the context of a regional adaptation plan. Although the DGA manages the water market, the water planning dimension must be brought within the administration of the territorial authority, the Regional Government, as part of a strategy able to engage with the priorities of the national plan (less fisheries) and the multiple public and private actors who are direct stakeholders, from rights holders (agriculturalists, mining firms and others), to the environment commission, the housing and urbanization ministry, the public works ministry, and municipal authorities. It would appear that the limitations of this natural resource market for the climate change challenge to be faced this century are already evident.
More frequent and intense droughts may affect reservoir and groundwater storage, as well as rainwater capture systems. Reduced precipitation may also result in less groundwater recharge and lower summer streamflows (Pitre, 2005; Earman et al., 2006). Reduced precipitation will also contribute to increased pumping costs due to deeper groundwater levels and will also contribute to increased conflicts over water related to baseflow in streams, maintenance of water rights, and restriction of new water users (Pitre, 2005). Reduced snowfall results in less water stored in snowpack reservoirs that provide water to some cities, and thus will change the temporal patterns of flow to reservoirs and supply systems. Reduced snowfall can also challenge many water systems that implicitly or explicitly depend on seasonal storage in snow form, ultimately requiring the development of even more constructed storage to provide supply reliability. Reduced snowfall and warmer water temperatures may aggravate demands for various instream and non-consumptive water uses, such as maintenance of fisheries, ecosystems, river amenities, and recreation, as well as various industrial and cooling needs. Increased precipitation and related peak flows may affect coastal and inland shipping by seasonally reducing water depths in channels, reducing passage heights under bridges, and limiting passage through weirs and locks (Klein et al., 2005b), which can impact the shipping of wastewater treatment system outputs.
Sea level rise and storm surges
Salt water will encroach on coastal surface water sources, groundwater, and ecosystems. For example, increased sea levels will result in higher pressures of the ocean on submarine and coastal outcrops of coastal aquifers that will result in additional seawater intrusion. An increase in sea level will lead to an increased probability of flooding of sewers and wastewater pollution control plants (WPCP) and a reduced ability to discharge combined sewer overflows (CSO) and WPCP effluent by gravity. High storm surge levels lead to more street, basement, and sewer flooding. Higher sea levels, when inundating polluted areas (brownfields), can cause harmful release of pollutants. Higher sea levels can inundate fresh and saline wetlands and threaten the stability of canals and levee systems, which can have impacts on water supplies, water quality, and flooding. For example, a projected sea level rise of about 1.5 m along coastal California by 2100 with related levee failures and obstruction of fresh water flow in the Sacramento Bay-Delta system would jeopardize the fresh water currently passed through the delta for irrigation and drinking water supply to the cities and farms south of the delta (CADWR, 2008a). Higher sea levels will also increase the probability of water and wastewater damage due to surge action. Rise in sea level will result in reduced sediment transport, and may require increased dredging of sluices, weirs, groynes, locks, and canals, filling of wetland areas and raising and reinforcing levees and embankments (Klein et al, 2005b).
Surface-water impacts
These threats, in turn, may lead to further hydrological challenges, such as: loss of reservoir storage owing to competition for reservoir space for flood control, ecological flows, recreation, or agricultural supplies; reduced natural storage of water supplies on seasonal to decadal time scales due to declining ice and snowpack reserves; loss of inflow into the reservoirs owing to increased droughts; loss of storage owing to unscheduled releases from increased precipitation, runoff, or transition of runoff from snow to rainfall; reduction of diversion flows owing to competition with ecological flows during dry periods or droughts; and increased runoff (including urban runoff) preventing adequate water quality of stream flow diversions for water-supply needs through entrainment of increased total dissolved solids or agricultural and urban contaminants.
Degradation of groundwater aquifer systems used for urban supply
Climate change may eventually affect groundwater aquifers that supply water for cities through seawater intrusion, land subsidence, lateral and vertical migration, and capture of contaminants. Flow of degraded waters between different parts of aquifers, or different aquifers, tapped by boreholes and wells may compromise aquifers used for water supply, even where not all the aquifers are directly impacted by climate change. Climate change may also increase the need for injection systems and surface water delivery pipeline systems in lieu of coastal pumpage to prevent seawater intrusion (Hanson et al., 2008).
Regional-scale changes
Because many urban systems are part of larger regional water systems, the effects of climate change will also yield regional-scale challenges, such as reduction in snow-melt in watersheds that provide crucial supplies or storage mechanisms for urban supplies; loss of groundwater storage in supply basins owing to urbanization or legal constraints as regional competition for water supplies and wastewater disposal options increase; and disruption of delivery or competition for imported water.
Impacts on informal urban water systems
Given the meager capital resources and lack of centralized planning associated with development and maintenance of most informal water systems, climate change will have additional impacts on urban informal water systems. Even small perturbations of water sources, informal conveyances, and wastewater disposal options by climate change are likely to challenge these informal systems, and larger disturbances by extreme climatic events such as storms and heat waves typically will not have been accommodated in informal system designs. Such climate change stresses will bring management challenges, since informal water supply systems are complex structurally and institutionally, and since decision-making tends to focus on the short-term. The biggest impacts of climate change on the informal water supply sector have to do with the maintenance of sources in terms of both quantity and quality. Here, adaptations go beyond the management of particular systems to larger issues faced by cities and regional institutions. For example, increasing temperatures may adversely affect the health of populations not served by organized sanitary systems.
Interactions of climate change, urban water, and other sectors
Water is a cross-cutting theme in urban life and function and, as such, it is at the nexus of many issues regarding how climate change will challenge cities. The effects of climate change on urban water will impact other urban sectors; equally, climate change effects on other sectors will, in many cases, impact the urban water systems.
Mexico City’s formal and informal water supply
The importance and complexity of water problems in Mexico City make it particularly vulnerable to the negative impacts of climate change. Water management has been a critical factor in the evolution of Mexico City. This megacity of more than 18 million inhabitants grew in a hydrological basin composed of five shallow lakes. Historical urban growth was on the lower part of the basin on top of the lakes and it has extended to the slopes of the surrounding piedmonts. The city has overtaken most of the former lakebeds and it has suffered major floods throughout its history. Despite numerous efforts to control this problem, flooding continues to be a major hazard in Mexico City and its solution requires an integrated approach together with other water problems.
Water supply is a multidimensional problem in Mexico City. The city has gone from a high level of self-sufficiency to a high level of dependence on two external watersheds. Current water use in Mexico City is approximately 63 m3/s. Close to 66 percent (43.5 m3/s) is extracted from aquifers and the remainder is imported from the Lerma basin (6.0 m3/s) and the Cutzamala basin (13.5 m3/s) (Ezcurra et al., 1999). Importing water to Mexico City from those two basins has had a significant impact on them. The mean annual input of rainwater into the basin in Mexico City is 23 m3/s. It is estimated that only 50 percent of that water recharges the aquifers. Deficiencies in the operation of the distribution system cause leaks estimated to be about 30 percent of the water managed by the city. Considering that some of that water makes it to the aquifers, the estimated total recharge of the aquifers is 28 m3/s, equivalent to approximately only 50 percent of the water extracted every year. Water extraction from aquifers has caused a subsidence problem since the early 1900s. The city has sunk at different rates in different parts, but it reaches its extreme in the old historical center, where some parts have sunk up to 9 m during the past century. Subsidence has caused severe maintenance problems with the urban infrastructure, building, and transport systems. It has also aggravated pollution problems of the aquifers, particularly in critical areas for their recharge. Monitoring of the water in the aquifers has shown deterioration in its quality due to overexploitation of groundwater, and high bacteria counts have been observed in some wells (Mazari et al., 2000). The protection of critical recharge areas of the aquifers is also a critical problem in Mexico City. The rapid expansion of illegal settlements in those areas jeopardizes the recharge of the aquifers.
The last component of water problems in Mexico City is wastewater. The city has a complex sewage collection system where wastewater and rainstorm water are mixed. The capacity of the system is 57 m3/s, 42.8 m3/s for sewage and 14.2 m3/s for rainstorm water. The effluent is shipped to the Tula basin about 50 km north of the city. Twenty-seven treatment plants treat only 7 percent of the total sewage generated in Mexico City.
Box Figure 5.7: Subsidence in Mexico City.
The increasing volume of sewage generated by Mexico City during the last decades has compromised the capacity of the system to evacuate rainstorm water, increasing the risk of flooding. The subsidence of Mexico City has also created problems for the operation of the system. The slope of some of the major drains has been reversed, requiring the construction and operation of pumps to evacuate wastewater. The Mexican federal government and the local government in Mexico City initiated significant maintenance and repairs to the wastewater system to prevent floods during the rainy seasons in 2007 and 2008. Major additional works are still being considered for the near future.
Water problems in Mexico City represent a major challenge for present and future urban growth. Climate change will aggravate those problems. Some of the studies of potential climate change scenarios show an increase in precipitation and temperatures in the city by 2025 and 2050 (Gay et al., 2007).
Box Figure 5.8: Alfalfa, a fodder crop grown with wastewater from Mexico City in the State of Hidalgo. Health problems can arise from the use of untreated wastewater for crop production.
The challenge to secure water supply will increase in light of the expected increase in the demand, particularly during the dry season when temperatures are expected to increase. The risk of flooding, a chronic problem in the city, will also increase under a climate change scenario. Those impacts will not only create significant consequences for the water sector, but also for the energy sector and the health of the population. Mexico City needs a new strategy to address those problems. It will be critical to create integrated and multidimensional strategies recognizing the interactions among the different components of water in the city, as mentioned above. The federal and local governments have addressed each of those elements in isolation from the other, creating fragmented actions that have had limited success in solving a complex problem.
Energy
With warming, urban water demands and uses are likely to increase in many (perhaps most) cities. Future increases in the demand and use of water expected under climate change are likely to result in increased demands for energy. The supply, treatment, and distribution of water supplies in urban areas require operation of pumps and other mechanical devices with attendant heavy energy use. Most sewage treatment plants operating in urban areas are mechanically operated and the collection, recycling, and outflow of sewage also frequently require the operation of pumps. If sewage flow is projected to increase under a given climate change scenario, an increase in the demand for energy must also be anticipated for both operation and capacity expansions.
Health
Urban water systems have close ties to many of the public health challenges associated with climate change.Water-borne diseases are a major health hazard in poor countries and emerging economies due to deficiencies in the supply of drinking water in their urban areas. Climate change can exacerbate those hazards by increasing gaps between drinking-water demands and supplies, and can stress sewage disposal systems and options beyond current conditions. Climate change may also aggravate public health challenges in urban settings by increasing the geographic ranges of some diseases and disease vectors (so that water facilities that did not sustain disease and vectors in the past may do so in the future); by increasing the opportunities for their propagation and development (e.g., by increasing reservoirs of standing water or promoting longer vector lives or more vector generations); and by generally reducing overall public hygiene and resistance to disease (as water supplies are challenged or limited).
Governance
The governance of a vital natural resource such as water is challenging in both poor and rich countries.Climate change will stress further the political negotiations regulating access to water in formal and informal urban water markets even beyond often acrimonious historical levels. Whether privately or publicly owned, the governing structures of ownership, use, and sale of water resources may require redefinition if they are to be adaptable enough to accommodate growing and interacting pressures from rapid urbanization and climate change.
Land use
Urban demands for water often encourage or require land use changes in other areas that provide water supplies, storage, and conveyance corridors with the potential for severe negative social, economic, and environmental consequences. Increased urban water demands under climate change may create additional pressures to import water and to introduce land use changes in areas beyond the city. Water availability can dictate or limit land uses within urban areas, and consequently climate changes may redefine acceptable land uses within urban areas. Perhaps even more importantly in view of the rapid growth of cities expected in the twenty-first century, the forms, extensions, and types (particularly density) of future growth in and around cities will similarly depend on available options for provision of water and wastewater treatment and disposal, and how those options will be impacted by climate change.
Transportation
Water-borne transportation systems that are vital in many cities may be affected by changing climates, sea level rise, and changing stream flow timing and amounts.
