The global demand for goods is placing greater pressure on our available water reserves, with water scarcity an issue that an increasing number of regions already have to cope with. Coupled to this demand are projected climate changes to our rainfall, snowfall and temperature patterns.
The long term effects of such change influence how much water is available for domestic or industrial use. Considering the lifetime of infrastructure means planners and operators must consider these predictions to maintain their operations. One area where climate change impacts are expected to be felt strongly is in changes to the water cycle.
Access to water is a topic that is more frequently in the news, though ‘water resources’ has a specific meaning to different user groups. Whether water is withdrawn for use in domestic, agriculture, manufacturing or electricity production, certain key criteria need to be met. This is true for the extracted water as well as return flow to natural environments. In many locations across the globe over exploitation is posing a threat to water resources, placing certain sectors at risk. Considering the overall availability of freshwater in Europe, water appears to be abundant with total resources reported around 2270 km3/yr; of this only 13% is abstracted.
These figures give the impression that water is sufficient to meet demands; however, in many locations this is not the case. Demand is often greater than availability which leads to problems of water scarcity, lower lake volumes, reduced flow in rivers and drying of wetlands. The detrimental impacts can be seen on freshwater systems but also on water users. Changes in rainfall patterns and rising temperatures – whether due to climate change or human actions – have a strong bearing on our river systems’ ability to meet the demands from different sectors (e.g. agriculture, domestic, industry) both now and in the future.
On a global scale, anthropogenic disturbances are already altering flows, though understanding of the issues is growing which is leading to construction of resilience and policy on these issues.
Hotter atmosphere and low river volumes
Increased air temperatures is one outcome of climate change, but altered rainfall patterns is another. The combination of higher temperatures and potentially reduced river volumes – particularly at times of peak demand – highlights the need to look beyond short term needs. Future forecasts generally agree that warmer temperatures will prevail in the coming century.
This trend is already being observed through a change in the timing of snow melt, which has been recorded to occur earlier in mountain headwaters – up to three months earlier in some cases. Though this means an initial increase in river volume, this reduction in meltwater stores leads to lower flows occurring during the summer months, in turn leading to water stress at certain peak use times. For example, the Rhine is expected to experience more flood and drought events because estimates for the future are for higher discharge from more intense snow melt and higher rainfall in winter, coupled with less summer snow melt and higher evaporation. This means that even though catchments may experience higher flows as a result of increased snow melt, this gain may be negated by higher evaporation rates in the lower reaches of the river. Timing is crucial.
If snow melt occurs earlier then there is less meltwater during the summer months, leading to low flows persisting for longer. Detection and attribution of emerging trends is central to decisions taken by both industry and policy. A recent trend analysis of monthly stream flows across Europe for the period 1962-2004 confirmed what has been observed at the national level of a shift in flow patterns. This trend analysis describes higher flows occurring in the winter months and a decrease in flows during the summer months, with the low flow minimum generally occurring in August.
Consequently, the distinction between annual and seasonal trends needs to be carefully made given the timing when certain sectors require water. For instance during hot spells the demand for water peaks and there may not be enough to meet the demands from all sectors and water users, thereby forcing regulatory decisions by local or state governments. Water temperature is an important physical property of rivers and air temperature is one driver, but so too is river volume. The link between air and water temperature is linear – as one rises so too does the other – but as discharge decreases river temperatures rise at reduced river volume.
The importance of including temperature and discharge was evident by a modelling study of the 2003 heat wave in Europe; models were better able to capture the observed results when both temperature and discharge were considered, as opposed to temperature alone. This is an important point since at these extremes the exceedences of acceptable ecological or cooling thresholds are more likely than under average conditions. Both climate and hydrology are key factors governing river temperatures, which have a direct affect on water quality, ecological status and economic importance to users.
A sensitivity study on river temperature to changes in river volume and air temperature was done for a number of river basins across the globe. A rise in air temperature of 2° C and 4° C results in an annual mean increase in water temperature by 1.5° C and 2.9° C respectively. When a decrease in discharge of 20% was factored in, the temperature increase was intensified to 3.1° C (for +4° C air temperature and -20% in discharge). Conversely, when considering the sensitivity to increased discharge, river temperature increases were less severe, leading to a 2.8° C increase in water temperatures (for +4° C air temperature and +20% in discharge).
The main reason for discharge, or river volume, having this influence is reduced thermal capacity when river levels are low, consequently making the river more sensitive to atmospheric warming.
Impact on power generation industry
Changes in temperature, volume and increased variability of flow will have a profound effect on many industrial processes where water is needed for cooling. The threshold for intake by industry and thermal power plants is set at 23° C. Across Europe, 44% of the water extracted is used for thermal energy production, with most of it sourced from surface waters due to the ease of extraction in large volumes and relative low cost.
Regional variations exist. In Western countries abstraction for electricity is roughly 52% of the total – the largest extractor in Southern countries is agriculture (about 60%); however, in Eastern counties the electricity generating sector is the largest (>50%, European Environment Agency). Very little of the water is actually consumed, however, and more than 90% of the water abstracted for cooling is returned to surface waters.
At present high withdrawal figures are recorded across Europe – Scandinavia being the exception due to their high use of hydropower for electricity production. Electricity production at thermal power plants requires large amounts of water, especially when considering the supply chain from mineral extraction through to delivery of fuel to the plant; but the main water need is to cool the turbines at the plant and the method employed to do this. The type of cooling system a power plant uses determines its water requirements. The two most commonly used are once-through cooling and tower-cooling. In once-through cooling water is used to cool the turbines and discharged directly back to a waterway or pond.
For tower-cooling, turbines are cooled and the hot water goes to a cooling tower, is reused several times and eventually discharged back to the waterways. Two advantages of tower-cooling are that water is returned at a much cooler temperature, thereby impacting less on freshwater systems; secondly, it is recycled within the power plant. Because water is reused, tower-cooling requires less than 3% of water withdrawals compared to once-through cooling per unit energy produced.
Even though tower-cooling requires lower withdrawals it consumes twice as much water per unit of energy compared to once-through cooling, since water is evaporated in the cooling towers. Despite this, tower-cooling is seen as the preferred method due to its lower withdrawal limits. It is important to note that water withdrawals by thermal power plants are used and returned to waterways without a major degradation of water quality – except for thermal pollution by once-through cooling – making the downstream impacts less than, for example, manufacturing or municipal sectors.
For the Rhine (considering Lobith station), the mean number of days the 23° C intake threshold is exceeded is 16 days, under current climate conditions. Factoring in an air temperature increase of +4° C this number rises to 47 days, but including a combined air temperature increase (+4° C) and decreased discharge of 40%, the number of exceedence days rises to 104. These numbers are only rough estimates but give a picture of the challenges facing the industry and the relevance of considering both river volume as well as air temperature changes.
An increase of duration when the 23° C threshold is exceeded has ecological consequences, but is a real economic concern for power plants and the users of their generated electricity. This was witnessed during the 2007 drought in the South Eastern USA, where many thermal generators were shut down due to water related issues of thermal pollution, to receiving waters and low water levels at intake sites.
The use of scenarios opens a way for us to assess the future and combine many important drivers such as environmental, society, policy, population growth or land use change. All of these describe how the future may unfold and against which different strategies can be tested. When estimating future water withdrawals for electricity production, two scenarios were considered: the A2 and B1 scenarios, described by the Intergovernmental Panel on Climate Change (IPCC) 2007 assessment.
The A2 scenario assumes technological changes will focus on security first – for future cooling systems this assumes tower-cooled plants will replace once-through plants within a lifetime of 50 years. The B1 scenario anticipates a focus on policy first and the change from once-through to tower-cooled will occur faster, using a power plant lifetime of 35 years. Both scenarios estimate dramatic decreases in water withdrawals as the industry shifts to tower-cooling. This estimated decrease by both scenarios is despite an increase in electricity production under the A2 scenario, since the extra water withdrawn is dwarfed by the transition to tower-cooling.
For example, in Western Europe under the A2 scenario, in 2050 a decrease in abstractions of 80% is projected, while under the B1 scenario this decrease is 97%. In addition to a change in cooling methods, the B1 scenario also anticipates shifts to more renewable energies such as wind power. Exploring these two scenarios allows us to test the impact technological changes have on the water extractions for certain sectors. Here, changes in cooling methods are focused on but a further reduction of impact on freshwater could be achieved if dry-cooling were used, or non-freshwater sources of water were adopted.
The availability of water in certain regions could be a driver for such changes in cooling system technologies or adopting different policy requirements. There is broad agreement of projected changes to future runoff across Europe and the impact of climate change on future drought in soil water and stream flows. The frequency and severity of droughts is set to increase in the future; indeed, during the last century several studies report higher river temperatures in relation to air temperature, but also changes in river flow. The Danube River has experienced higher river temperature because of low flows in the summer caused by the earlier occurrence of snow melt and decreased summer precipitation.
Added to this are anthropogenic impacts of thermal effluents from power plants, reservoir construction and urbanisation and a complex picture emerges. At the European scale droughts are known to have wide ranging impacts and over the past 30 years the total cost in Europe amounts to 100 billion Euros. The predictions for the next 100 years are for droughts to become more frequent and more severe across the globe. During the 2000s the number of exceptionally hot days (and sustained periods of warm weather) have been higher than previously, with the frequency of hot days nearly tripling for Europe in the past decade.
For the 2003 heat wave German nuclear power stations operated at reduced capacity as the cooling water was above the 23° C threshold needed for cooling. Data suggests that in many areas where nuclear power stations operate, future river water temperatures could be above 23° C for up to three months of the year, limiting energy production. Given the outlook for increased atmospheric temperatures (+2 or +4° C are general policy reported thresholds) as well as changes to precipitation events, the combined effects on river flows and temperature is an important challenge for industry users and regulators alike.
Some of the risks associated with droughts and the climate change predictions can be lowered by a shift towards alternatives such as dry cooling or non-freshwater sources of cooling technologies.
Climate change will have a significant effect on river temperature and water availability is likely to also be more variable across the year. This will mean that planners need to consider different scenarios when determining the balance of use between different sectors, and the site of infrastructure, as well as the needs of industry, to sustain production.
The WATCH (water and global change) project has created datasets capturing events at global and regional scales, providing a new instrument for better assessments of water availability and analysis of extremes such as floods and droughts. What the WATCH project has done is to divide water use for the past, present and future into the different sectors. Knowing what proportion agriculture, households, electricity generation and manufacturing will use, we can potentially manage it better given climate change effects.
In order to ensure long term sustainability and economic growth we all need to support the efforts between the water and energy legislation to align efficient water and energy use for long term sustainability. Regionally coordinated river basin management plans coupled with national renewable energy action plans could serve as conduits to more sustainable water use. Determining past, current and future water resources requires assessments of a complex interaction of physical, social, economic and political factors.
Future analysis must be based on the physical accounting of what is available and what is – and will be – extracted. Water providers and heavy industry will be able to use the WATCH data sets to give a better assessment of water availability than previously – particularly in the regions of the world where data availability is limited. The methodologies, data and models developed by the WATCH project provide a sound basis for future analyses of water availability in all regions of the world.
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Published: 21st Jun 2012 in AWE International