Select Committee on International Development Minutes of Evidence

Memorandum submitted by Professor Nigel Arnell, Department of Geography, University of Southampton


    —  Climate change poses a threat to water resources and their management in developing countries. An important component of this threat is the uncertainty over the magnitude of climate change impacts at a location.

    —  The effects of climate change on river flows and recharge in developing countries are largely dependent on changes in rainfall amount and timing.

    —  Changes in the variability of rainfall from year to year—and changes in patterns such as El Niño—may have as important an effect on water resources as changes in mean climate.

    —  Under most climate change scenarios, southern and western Africa and the Middle East are projected to see reduced river flows and recharge in the future. Possible changes in south Asia are less clear: some scenarios suggest reduced runoff (greater drought risk) whilst others show increased runoff (greater flood risk).

    —  Climate change is likely to have a substantial effect on people living in areas that are currently water-stressed, most of which are in developing countries.

    —  Different aspects of water resources will be affected by climate change in different ways. Whilst access to safe water will probably be influenced by climate change, especially in rural areas, access to sanitation is likely to be relatively unaffected.

    —  The actual impacts of climate change will depend not only on the change in climate, but also on changes in how water resources are managed. The adoption of flexible management approaches and policies which can cope with current climatic variability (such as increased access to safe water) will reduce vulnerability to climate change.

    —  Water resources are also influenced by many other pressures, such as population growth, economic development and land use change, which will affect exposure to climate change.

    —  One practical implication is that all water resources development proposals should consider the effects of climate change on the ability of the proposed scheme to deliver its objectives, and whether the proposed scheme will affect the vulnerability of others to climate change.


  1.1  It is a cliché that human society relies upon adequate water resources. Rivers and groundwater provide drinking water, water for agriculture in general and irrigation in particular, and water for industry and for cooling. River flows generate hydropower and enable navigation, and provide a means for the disposal of waste. Many economically-important ecosystems, such as wetlands, are dependent on inflows of water from upstream. Floods bring damage and disruption and threaten lives, and rivers and water bodies can harbour water-borne disease. The sustainable development of societies in developing countries relies on the sustainable management of water resources in general.

  1.2  However, in the mid 1990s approximately a third of the world's population lived in countries deemed by the United Nations to be water stressed, 20 per cent lacked access to safe drinking water and about 50 per cent lacked adequate sanitation[6]. Pressures on water resources over the next few decades will be strongly influenced by economic developments, attitudes towards resource use and population growth—and climate change will add to these pressures. In some areas climate change may improve the water resources position, but in others it is likely to make matters considerably worse.

  1.3  This memorandum summarises the potential impacts of climate change on water resources in developing countries. The information is largely taken from the Third Assessment Report of the Intergovernmental Panel on Climate Change[7] and climate change impacts research funded by the UK Department of the Environment, Food and Rural Affairs (DEFRA)[8].


  2.1  Climate change has three potential effects on river flows and groundwater recharge. First, it may affect the volume of flow or recharge at a particular time of year. Second, it may affect the timing of flows or recharge, either through the year or from year to year. Third, it may affect the quality of river flows or groundwater through changes in the volume and timing of water, and through changes in water temperature.

  2.2  Although climate change will generally result in increased evaporation, largely due to increased temperature, changes in the volume and timing of river flow and recharge are primarily dependent on changes in the amount and timing of rainfall. A map of changes in river flows due to climate change therefore closely follows the map of changes in rainfall (see attached figure), although the percentage changes are generally greater and if the increase in rainfall over a catchment is less than the increase in evaporation, river flows decrease despite the increase in rainfall. The actual effect of a change in climate on river flows and recharge in a catchment depends on the current climate of the catchment and its geology. For example:

    —  a given percentage change in rainfall produces a greater percentage change in river flow or recharge, the more arid the climate: river flows and recharge in dry areas are more sensitive to climate change than river flows in wetter areas;

—  changes in river flows in a catchment with large amounts of storage—in lakes, wetlands or groundwater, for example—will be determined by changes in the cumulative rainfall over a season, whilst changes in river flows in more responsive catchments will be determined by changes in the intensity of storm rainfall.

  2.3  The timing of river flows through the year in areas receiving snowfall—such as regions receiving water from the Himalaya—reflects the melting of snow in spring. In general, higher temperatures mean that more precipitation during winter falls as rain rather than snow, and runs off quickly to the river. Winter flows are therefore increased, and spring snowmelt flows are reduced. Rivers feeding a small number of communities in the developing world are fed from glaciers: higher temperatures will result in increased glacier melt, and therefore higher flows in the short term, but may result in the glacier disappearing.

  2.4  The sensitivity of river flows and recharge to changes in rainfall means that quantitative estimates of the actual effects of climate change in a catchment are uncertain, because climate model projections of changes in rainfall are uncertain. At the regional scale, however, there are some consistencies. Most scenarios result in a decrease in river flows in southern Africa, much of west Africa, the Middle East and parts of South America. Most scenarios result in an increase in river flows in east Africa and south east Asia. There are conflicting projections for south Asia and central Africa.

  2.5  Changes in flood and drought frequency are dependent not only on changes in mean climate, but also on how variability in climate from year to year will change. This is currently not well known, and the indications so far are that changes in flood and drought flows are similar in direction—but greater in relative magnitude—to changes in average flows. It is possible that a particular location could see both an increase in floods and an increase in droughts, if the seasonal pattern of rainfall is significantly exaggerated. Changes in the pattern of variability in climate from year to year—such as an altered El Niño—are likely to have particular effects in tropical and sub-tropical regions.

  2.6  There have been very few detailed studies of the potential effects of climate change on river flows and groundwater recharge in dry or tropical regions generally, and developing countries in particular. It is possible that changes in climate may result in "surprising" changes in hydrological processes and therefore river flows and groundwater recharge in some catchments.


  3.1  Changes in river flows and groundwater recharge will affect the availability of water resources and the water-related hazards of floods and ill-health, but so too will population change, economic development and alterations in the way in which water resources and hazards are managed.

  3.2  A crude, but widely used, indicator of water resources pressure is the ratio of water withdrawals to the amount of available water: if a country is using more than 20 per cent of its available runoff and recharge, then it is likely to be suffering water-related limitations on development. The table below summarises the number of people living in countries withdrawing more than 20 per cent of their resources in the absence of climate change, together with the numbers of people who would experience a reduction in water availability due to climate change and hence an increase in water resources stress. Most of these people live in south Asia, central Asia and southern Africa. The table shows that resource pressures are likely to increase for very large numbers of people, and also that the effects of climate change are likely to be reduced substantially only by aiming for a low CO2 stabilisation target.

Global population at risk of increased water scarcity

Population in countries using more than 20 per cent of their resources, with no climate change (millions)
Population with an increase in stress
Unmitigated emissions
Stabilisation at 750ppmv
Stabilisation at 550ppmv


  Source: Arnell et al. (2002) Climatic Change in press

  Note that other climate change scenarios could give different quantitative results

  3.3  Such global and regional figures provide only one indication of the potential implications of climate change for water resources. In practice, different aspects of water resources in developing countries are likely to be differently affected by climate change. The attached table summarises the implications of climate change for water resources in developing countries: there are unfortunately very few published quantitative case studies which have looked at these impacts. Many of the users of water resources will be more affected by changes in variability over time and extreme conditions than by change in mean climate.

  3.4  It is likely that the impacts of climate change on water resources will be greatest on the poorest parts of the community, who may be least able to recover from flood damage or obtain access to scarcer water resources. There has, however, been no empirical research on this issue.

  3.5  Whilst climate change has the potential to adversely affect water resources, it is likely that in some cases it may lessen some water resources pressures. For example, an increased frequency of flooding in desert wadis would increase flood risk, but could also increase the recharge of local floodplain aquifers.

  3.6  The actual impacts of climate change on water resources in a particular location will depend not just on the change in climate, but also on the procedures used to manage those water resources. In general, unmanaged resources—such as many rural water supplies—are more exposed to climate change than managed resources, because these in principle have some means of intervention. A corollary of this is that as management procedures and infrastructure change, then vulnerability to climate change alters. As a general rule, measures which are taken to lessen exposure to current water-related problems will also reduce the potential impacts of climate change.

  3.7  An increasing scarcity of water in some regions has the potential to increase conflict between different users of water within an area and between upstream and downstream users. The extent to which such conflict develops will depend on other pressures within the region, although it is likely that resource pressures may become more frequent.

  3.8  The primary uncertainties in the potential impacts of climate change on water resources in developing countries are:

    —  uncertainty in the potential change in climate, and hence river flows and groundwater recharge;

    —  uncertainty in the future development of water management policies and practices, which will affect exposure to climate change; and

    —  uncertainty in the evolution of other pressures on water resources, including population growth, economic development and land cover change (which also affects river flows and recharge).


  4.1  The main implications of climate change for water resources are firstly that there is an additional pressure on water resources in many developing countries, and secondly that the conventional assumption, widely used in project planning and assessment, that the past is a good guide to the future is no longer tenable.

  4.2  One practical implication is that all water resources development proposals and strategies should explicitly consider the effects of climate change on the ability of the proposed plan to deliver its objectives, and whether the proposed scheme will affect the vulnerability of others to climate change.

  4.3  The effects of climate change on water resources are uncertain, and it is this uncertainty, rather than climate change per se, that poses the greatest challenge to scheme design and assessment. This uncertainty suggests that flexible schemes are to be prefered over fixed long-term schemes that are difficult to upgrade to reflect altered circumstances.

Water resources in developing countries: impacts of climate change

Aspect of water resourcesDescription Implications of climate change

Municipal water supplyGenerally derived either directly from large rivers or from reservoirs Source reliability depends on changing frequency or duration of low flows in the catchment supplying the source. Increased high flows could lead to increased sedimentation of reservoirs. Changes in the price of urban water due to increased scarcity will affect the poorest most significantly
Rural water supplyGenerally derived from local streams or groundwater Availability of source will depend on changes in local rainfall
Irrigation supplyLarge-scale from reservoirs, small scale from local streams or groundwater. Higher temperatures will increase demand As above, but coupled with possible effects of increased demands
Access to safe watersee municipal/rural water supply As above Quality of available water largely determined by non-climatic factors around source
Effluent disposalInto small water courses or large rivers Dilution and assimilation capacity influenced by the amount of water in the receiving waters, as well as water temperature (higher temperatures generally lead to degradation of water quality)
Access to sanitationRelated to effluent disposal Unlikely to be affected by climate change
Hydropower generationGenerally from large schemes supported by reservoirs or lakes Change in performance dependent on changes in the timing of inflows to reservoirs and lakes and the frequency and duration of droughts
Riverine and lake navigationCommercial activities focused on large rivers and lakes River navigation affected by frequency and duration of high and low flows; lake navigation influenced by lake levels which affect port facilities
Water-borne diseaseLargely, but not exclusively, associated with standing water bodies, which may be local Changes in local rainfall will influence occurrence of standing water bodies
Flood managementDistinction between flooding on major rivers and localised flash flooding Flooding on major rivers affected by changes in long-duration rainfall upstream; flash flood frequency affected by changes in local storm rainfall frequency
Environmental conservationWetlands are dependent on river flows and/or recharge, and provide valuable services Changes in wetlands dependent on changes in incoming river flows or local recharge

Professor Nigel Arnell, Department of Geography and Tyndall Centre for Climate Change Research,
University of Southampton
January 2002

6   UNEP (1999) Global Environment Outlook 2000. Earthscan: London. Back

7   Arnell, N W & Liu, C (2001) Hydrology and water resources. in: J McCarthy et al. (Editors), Climate Change 2001: Impacts, Adaptations and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. pp 191-233. Back

8   Arnell, N W (1999) Climate change and global water resources. Global Environmental Change 9, S31-S49. and Arnell, N W et al. (2002) The consequences of CO2 stabilisation for the impacts of climate change. Climatic Change. in press. Back

previous page contents next page

House of Commons home page Parliament home page House of Lords home page search page enquiries index

© Parliamentary copyright 2002
Prepared 23 July 2002