ADAPTING WATER SUPPLY SYSTEMS

Climate is one of many external drivers which have the potential to significantly influence water supply systems. However, a changing climate is the main driver to affect the availability of raw water resources. Predominately changes in the timing, frequency and intensity of precipitation can have a significant impact on the entire hydrological system. For example, increasing temperature often results in increasing water demand and enhanced water losses due to intensified evaporation. While increasing precipitation can damage water supply systems, due to flooding or increased erosion and water abstraction can be affected due to high turbidity and decreased water quality. Less precipitation or shifting seasonal precipitation events can cause serve water shortages, in particular when water storage facilities are insufficient. To respond to current and future climatic challenges and other external drivers, water resource management plans are developed and constantly upgraded. The traditional approach in water resource planning and management has been based on the assumption of stationarity of the hydrological system. However, the assumption that the past will be the key to the future is no longer valid. The climate and therefore the entire hydrological system is changing, and relying on the traditional planning approach increases the risk of mal-adaptation, water shortages and monetary losses. However, the methods to identify the future changes in water resources due to climate change are very uncertain. This cascade of uncertainty stems from the assumptions made about the state of future society and the greenhouse gas scenarios affiliated to these states. The uncertainty envelop expands further when the greenhouse gas scenarios are used to drive the global climate models, which are then downscaled to the regional models, and finally more uncertainty is added through local impact models. Such uncertain simulations are  Corresponding author: Julia Hall. Phone: +353 (0) 1708 6836. Email: Julia.Hall@nuim.ie. Julia Hall and Conor Murphy 2 problematic when decisions on future adaptation have to be agreed upon to avoid expensive mal-adaptation. This chapter examines climate change as emerging pressure on water supply and the evidence of climate change from climate records. Future impacts of climate change on water resources are highlighted and the challenge of adaptation is reflected on. Being confronted with deep uncertainty, the need for alternative approaches, shifting the paradigm away from the traditional “predict and provide” approach, seeking an optimum adaptation solution, is highlighted. Instead, alternative approaches are needed that allow the development of water supply systems that are robust to the uncertainty framing future changes in water resources. International examples of these approaches are described; including a case study of the Boyne catchment's water supply system in the east of Ireland. The uncertainties involved in generation of future catchment hydrology are discussed and the vulnerability of the case study water supply system to a changing climate is investigated. Where vulnerability exists, sample plausible future adaptation options are then assessed to their robustness to uncertainty, to quantify the water supply system response, to aid decision making. This chapter concludes that water managers will have to engage with alternative methods for climate change adaptation in conjunction with future observational evidence of change. 1. CLIMATE CHANGE AS EMERGING PRESSURE ON WATER SUPPLY Climate plays a central role in water resource management, as it influences the hydrological cycle at all stages. But the climate is changing. Increasing atmospheric temperature, due to increasing atmospheric concentrations of greenhouse gases, can lead to an intensification of the hydrological cycle, as evidence of several studies reviewed by Huntington, (2006) suggests. For example, as consequence of increasing temperature models suggest an approximate global average precipitation increase of 3.4% per degree Kelvin of temperature (Allen and Ingram, 2002), leading to more intense but less frequent precipitation periods. However, effects of a changing climate will not be uniform on the earth and will differ at different locations. For example, climate models suggest a greater warming at high latitudes and less warming in the tropics (Hegerl et al., 2007), while precipitation changes will not occur uniformly around the globe. On a regional scale, some locations will receive more rainfall, whereas others may suffer from extended drought periods. These changes in precipitation will also for example influence surface runoff, with an increase in higher latitudes (i.e. of North America and Eurasia) and a decrease in southern Europe, the Middle East, mid-latitude western North America, and southern Africa (Milly et al., 2005). Historically, pressure was put on water supply systems mainly from increasing water demand; however, with a change in climate, water demand could further increase while water supply may be reduced. Therefore, a changing climate is a key driver of the future availability of raw water resources. Predominately changes in the timing, frequency and intensity of precipitation can have a significant impact on the hydrological system. For example, the shifting of seasonal precipitation patterns or reduced precipitation can lead to reductions in stream flow and groundwater recharge rates, which can affect the quantity and quality of the available water. Increasing temperatures can result in higher evaporation losses and increasing water demand. Whereas increasing precipitation can damage water supply systems Adapting Water Supply Systems in a Changing Climate 3 due to flooding or increased erosion, and water abstraction can be affected due to high turbidity and decreased water quality. Such climate-induced changes can pose a challenge in various ways to any water supply utility. However, the effect of climate change on water resources and supply systems will not only depend on the location and the degree of chances in the hydrological system but also on the water supply system itself. Depending on the main characteristics of water supply systems, the same change in climate can have various effects on different water supply systems. For example, robust systems with plenty of excess headroom are likely to cope with large changes, but where water supply systems are already under pressure and operating at, or close to their design capacity, a small change can have a big impact. Therefore, an assessment of the balance of future water supply and demand of individual water supply systems or even a single water abstraction point is crucial taking future climate and the future development of non-climatic pressures into account. The quantity and/or the quality of the water resources can be affected by climate-independent factors such as population changes, increasing wealth, the political and legislative framework, economic activity, technological and monetary potential, land-use change and urbanisation. Demand increase due to population growth, the emergence of water-intense industry or agricultural practices for example reduces the overall water availability even without the occurrence of climatic change. Whereas, the introduction or rise of water charges (if water is already priced) can result in water saving, or the water loss reduction can increase the amount of water that can be provided to the individual water users. These key climatic and non-climatic factors need to be combined in an assessment to determine future vulnerabilities of a water supply system. There is a consensus that climate change will have a small effect on water supply systems compared to growing demand over near term planning horizons. However, future hydrological simulations suggest that climate change will significantly alter catchment hydrology over medium and long time scales. In response to these anticipated changes; longterm adaptation has to take place locally, at the catchment scale. This need for adaptation is supported by the evidence of climate change from observations. 2. EVIDENCE OF CLIMATE CHANGE FROM OBSERVATIONS Climate is the most central driver of the hydrological cycle. A change in either the climate system or water cycle would induce a change in the other due to their intimate association (Kundzewicz, 2004). The earth’s climate system has changed since the preindustrial period with an accumulation of evidence suggesting an anthropogenic increase of carbon dioxide being the very likely cause (IPCC, 2007). Global average surface temperature has increased, with warming accelerating to 0.13 o C per decade over the past 50 years (IPCC, 2007). The average atmospheric water vapour content has amplified since at least the 1980s over land and ocean and corresponds to long-term increasing trends in precipitation amount over North America, northern Europe and northern and central Asia (IPCC, 2007). Warming of the climate is in theory expected to bring about increases in evaporation and precipitation, leading to the hypotheses that one of the major consequences will be an intensification (or acceleration) of the water cycle (Held and Soden, 2000; Huntington, 2006). Huntington (2006) explains the theoretical basis for this intensification is summarised in the ClausiusJulia Hall and Conor Murphy 4 Clapyeron relation that implies specific humidity would increase approximately exponentially with temperature. Globally, evidence of direct human influences on the hydrological cycle is now apparent (Gedney et al., 2006; Huntington, 2006; IPCC, 2007; Barnett et al., 2008). However, for a number of components of the water cycle, for example groundwater, the lack of data has made it impossible to determine whether their state has changed in the recent past due to climate change. Gedney et al., (2006) suggest that raised carbon dioxide le