Select Committee on Environment, Food and Rural Affairs Written Evidence

Annex 3



  Awareness of climate change and its implications for water security in the UK is amongst the highest in the world. There remains considerable uncertainty about the rate and magnitude of climate change, particularly for precipitation change, which is vital for water resources. The water supply industry and its regulators have developed a series of scenario-based methodologies for responding to climate change, although these do need more development and have yet to be seriously tested in practice. Irrigators have generally not yet addressed the implications of climate change.

  The actual impacts of climate change on supply reliability depend not only on the change in runoff and recharge, but also on the way water supplies are managed and regulated, so the same change can have different impacts in different catchments. The expected impacts on irrigation will be increased demand as well as lower and less reliable supplies, with possible major implications for the production of high-value potatoes and vegetables in eastern England. Impacts on flood risk depend not only on climate change but also on how exposure to flood changes over the coming decades. Of the three broad methods for managing flood risk (reducing the physical hazard, reducing exposure to flood loss and reducing vulnerability), reducing exposure to flood loss is least sensitive to the effects of climate change, because it depends on policy and planning decision making.

  In many instances, climate change is likely to exacerbate existing pressures or vulnerability in water resources management. This means that areas already experiencing flooding events during winter and supply deficits during summer and dry years may be affected more often and to a greater degree. Many other driving forces will operate on the water sector during this century and the effects of climate change will be strongly mediated by direct and indirect human activities and development trends. Water supply for new housing development in the southeast is a good example of this.


  1.  The main implication of climate change for water resources management is that it is no longer feasible to rely on the conventional assumption that past records are a good guide to future availability. Unfortunately we cannot predict how climate change will affect future water resources because of the substantial uncertainties associated with its assessment, so a scenario approach must be adopted considering a range of plausible futures. This is well recognised in the water supply industry and by the regulators, and scenarios were used in the construction of water resource plans for the third and fourth Asset Management Plans (AMP3 and 4).

  Significant uncertainty surrounds the details of possible future climate changes; some of this uncertainty can be reduced by further scientific research, some is inherent in predicting future climate. The particular aspects of potential climate change that are of most importance for UK water security are:

  (a)  Changes in evapotranspiration. The potential for evapotranspiration will definitely increase with increasing temperature, with negative impacts for water supply/drought.

  (b)  Changes in rainfall. A number of characteristics are important:

  (b.i)  Average rainfall. Changes to the annual average are uncertain—for example, UKCIP98 gave an increase, while UKCIP02 gave a decrease. This controls whether the main concern will be water supply or flood management.

  (b.ii)  Seasonality. It is more certain that there will be increased seasonality, with drier summers and wetter winters (the uncertainty in the smaller annual average change results from sensitivity to whether the summer drying exceeds the winter wetting, or vice versa). Increased seasonality in the direction expected has negative impacts generally, and also allows the possibility that both drought and floods could become more frequent (within their seasons).

  (b.iii)  Geographic distribution. Somewhat uncertain, given that the resolution of the geographic features of the UK is near to the limit of what current climate models can simulate with rigorously tested reliability, but available scenarios indicate an increased heterogeneity, with enhanced gradients in precipitation between the S or SE of the UK and the N or NW.

  (b.iv)  Variability on various time scales. Science is still inadequate to provide the required information about possible changes in variability, partly in terms of the development of capable models, but more particularly in terms of evaluating and analysing the outputs of those models. Key (all inter-related) factors are whether the variability increases or decreases at daily to inter-annual time scales; whether there is a concentration of rain into fewer (or more) days of rain, with more intense rainfall on those days; whether extreme events (on various time scales) become more or less frequent; and whether the sequencing of rainfall changes, altering the occurrence of long periods of dry or wet, or sequences of dry or wet months and years.

  Increased variability is important because it can lead to increases in both droughts and floods. Changes in intensity could alter the balance between runoff and groundwater recharge. There are various other effects. Scientific uncertainty is high, though indications are that sequences, for example, of dry months/seasons are unlikely to change over and above the changes caused directly by changed average rainfall (because the UK relies less on recycling of water by evaporation than, for example, continental interiors, and hence feedbacks between previous and subsequent months are smaller). But changes in dry and wet extremes, over and above the changes caused directly by changed average rainfall, are a real possibility.


  2.  The effects of climate change on the adequacy of water supplies depends partly on changes in river flows and groundwater recharge, but also on how the supplies are managed and the current balance between supply and demand. For example, the effect on supplies to a town (within a specific resource zone) of a given change in river flows depends on whether supplies are taken directly from a river or from a reservoir, and the volume of abstractions relative to river flows. A particular change in flows or recharge will therefore have a different effect on the security of supplies in different catchments or supply zones. Therefore, some water companies currently have a healthy supply-demand balance (eg, in the north), whereas others have a water deficit during dry years (eg, in the southeast). This differentiation is due to a range of factors such as population growth, leakage, rainfall patterns, etc. It does emphasise that there is a geographic dimension, in terms of water resource zones, to the adequacy of present water supplies.

  3.  Under the UKCIP02 scenarios, it is well established that river flows are likely to decrease during spring, summer and autumn across much of the UK, and show only slight increases in winter. Groundwater recharge is unlikely to increase substantially despite increases in winter rainfall, because the recharge season will be reduced. The actual amounts of change, however, are very uncertain—largely due to uncertainties in how rainfall may change in the future—and different climate change scenarios yield larger increases in winter runoff and smaller changes in summer runoff. In many areas the rate of water extraction from aquifers far outstrips the rate of natural recharge and is unsustainable.


  4.  The guidance on climate change provided to water-supply companies by the regulators for AMP4 is arguably the most explicit anywhere in the world: there is a clear methodology for estimating the broad-scale effects of climate change on supply yields, and also on demand, as well as guidance on what to do if the impacts are estimated to be felt by defined time horizons.

  5.  Underneath this broad framework, however, the water industry and its regulators are facing a number of practical methodological problems with coping with climate change. Central to these is the assessment of the likelihood of different impacts of climate change. The industry uses a risk-based approach to estimate "headroom" allowances for its estimated reliable yields, taking into account a range of uncertainties, but climate change uncertainty is not yet included in a very realistic way: only the range between UKCIP scenarios is considered, and it is known that the UKCIP scenarios do not span the full range of possible climate-change effects. The industry recognises this limitation, and has commissioned research with the Tyndall Centre to incorporate climate-change uncertainty into the headroom methodology in a more sophisticated manner. The industry is able to cope with this information, and indeed in many senses is pushing science forward.

  6.  Whilst OFWAT accepts in principle the need for investment to maintain supplies in the face of climate change, this principle has yet to be tested in practice. Such a test will be the first challenge for the new methods for incorporating uncertain climate change into operational water planning. In some instances there may be a mismatch of planning horizons between OFWAT, the economic regulator (five years) and the EA (30 years). This can create difficulties for water companies which, for example, have to make big investments over long timescales and lead-in times (as for a reservoir, 20 years) which are likely to be affected by climate change.

  7.  Irrigators are arguably more exposed to climate-change impacts on water resources because most rely on a single source; individual irrigators do not have the same opportunities as water companies to switch during droughts between surface and groundwater, or between abstraction points. Many catchments are already considered over-licensed or even over-abstracted, and many licences are under threat even without climate change. Further reductions in surface flow and reliability would have major impacts.

  8.  Agricultural abstractors will be particularly affected by decisions on how environmental flows are protected under climate change. The Agency's present methodology sets Hands Off Flows (HOFs) to protect the minimum hydrograph, with the balance available for abstraction. If HOFs are simply held constant whilst flows decline, the abstractors will be squeezed disproportionately. The necessary debate over how and when environmental flows should be changed with climate change has not yet occurred.

  9.  Simultaneously, climate change is likely to increase irrigation water demand. Warmer summers will increase demand for salad crops, and all irrigated crops will transpire more water. Higher atmospheric carbon dioxide concentrations may offset some of the increase, and also help produce higher yields (and hence use water more efficiently), Overall, increases in water demand of around 30% by the 2020s and 30% by the 2050s have been forecast, over and above changes (mostly increases) due to socio-economic change, but there are major uncertainties involved.

  10.  Given higher demand and scarcer supplies, irrigators will have to adapt. Sustainable water resources and adaptation options in the rural sector (agriculture and leisure) are being studied in a current Tyndall Centre project. Winter storage reservoirs, rainfall harvesting, better equipment and better scheduling will all have a role. Irrigation will be further concentrated on the highest value crops (irrigation of low value crops has already stopped). Some crops may have to be grown further north and west. Water management policies that leave flexibility to abstractors will allow more adaptation options; the present moves towards licence trading are welcome in this context.


  11.  Climate change is likely to increase flood risk, both inland and along the coast, although the change in flood frequencies, particularly inland, is highly uncertain and will vary from catchment to catchment. The impact of climate change will also depend on the rate of change of exposure to the flood risk.

  12.  In general terms, there are three main ways of managing flooding. The first is to construct physical flood defences (dykes and embankments, for example), to prevent water from the sea or river flooding land, and this has been the traditional approach to flood management in the UK. Climate change affects this approach by altering the standard of service provided by existing defences and making it difficult to design future defences (although Defra allows a defined allowance to be considered). The second approach is to reduce exposure to flood loss by preventing development in flood-prone areas, making sure that individual properties are flood-proofed, and by implementing flood-warning schemes. Future flood losses will be very dependent on how successful these measures are at reducing exposure. Measures to reduce exposure are relatively insensitive to the precise characteristics of climate change. The third group of measures manage the flood risk by reducing vulnerability to loss. In the UK (almost uniquely) this is largely done through the provision of flood insurance, although poorer flood victims are much more likely to be uninsured than wealthier victims. Climate change affects the financial viability of measures to reduce vulnerability by changing the frequency and magnitude of losses.

  13.  Measures to reduce current and future exposure to flooding are therefore likely to be the least sensitive to the effects of climate change.


  14.  Climate change affects in-stream and riverine ecosystems primarily through changes in river flow regimes (the volume and timing of flow through the year) and water temperature. In practice it will be impossible to offset the effects of increases in water temperature, but it may be possible to introduce river management regimes which seek to maintain current flow regimes and hence ecosystems. However, this is potentially unsustainable in the long term, and would magnify impacts on abstractors in the medium term. A more appropriate policy would therefore be to seek to encourage ecosystems and species to migrate along the river corridor and to ensure that actions taken to meet other impacts of climate change—such as increased abstraction—do not exaggerate the effects of climate change on the water environment. Further research into practical application of these adaptation options will be required as ecosystems change.

  Prepared by Professor Nigel Arnell, Tyndall Centre and University of Southampton, Dr Keith Weatherhead, Tyndall Centre and Cranfield University, Mr Suraje Dessai, Tyndall Centre, University of East Anglia, Dr Tim Osborn, Climatic Research Unit, University of East Anglia, and Dr Declan Conway, Tyndall Centre and School of Development Studies, University of East Anglia.

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Prepared 16 September 2004