WORLD WATER DEMAND AND SUPPLY : 1990 TO 2025
SCENARIOS AND ISSUES

David Seckler, Upali Amarasinghe, David Molden, Rhadhika de Silva and Randolph Barker

International Water Management Institute
PO BOX 2075 Colombo Sri Lanka
Tel 94-1-867404 Fax94-1-866854
E-mail: IIMI@CGNET.COM

It is widely recognized that many countries are entering an era of severe water shortages. The International Water Management Institute (IWMI) has a long-term research program to determine the extent and depth of this problem, its consequences to individual countries and what can be done about it. This study is the first step in that program. We hope that water resource experts from around the world will help us by contributing their comments on this paper and sharing their knowledge and data with the research program.

The study began as what we thought would be a rather straightforward exercise of projecting water demand and supply for the major countries in the world over the 1990 to 2025 period. But as the study progressed, we discovered increasingly severe data problems and conceptual and methodological issues in this field. We therefore created a simulation model that is based on a conceptual and methodological structure that we believe is valid and on various estimates and assumptions about key parameters when data is either missing or subject to a high degree of error and misinterpretation.

The model is in a spreadsheet format and is made as simple and transparent as possible so that others can use it to test their own ideas and data (and we would like to see the results!). One of the strengths of this model is that it includes a submodel on the irrigation sector that is much more thorough than any used to date, to our knowledge, in this context. Since irrigation uses over 70 percent of the world’s supplies of developed water, getting this component right is extremely important. The full model, with a guide, can be downloaded on IWMI’s home page (http://www.cgiar.org/iimi).

Most of the discussion in this paper is devoted to explaining why this simulation model is needed and how it works. Once this is done, two alternative scenarios of water supply and demand over the 1990 to 2025 period are produced, and indicators of water scarcity are developed for each country and for the world as whole.

Part I of the paper describes the water balance approach which provides the conceptual framework for this study. The water balance framework is used to derive estimates of water supply and demand for countries. These estimates are adjusted to take explicit account of return flows and water recycling, the importance of which is often neglected in studies of water scarcity.

Part II presents the data for the spreadsheet model of water supply and demand for 118 countries that include 93 percent of the world’s 1990 population. Following a discussion of the 1990 data, two scenarios of world water supply and demand are presented. Both make the same assumptions regarding the domestic and industrial sectors. The difference between the scenarios is due to different assumptions about the "basin irrigation efficiency." This is the efficiency of irrigation at the level of the entire river basin, which includes water recycling within irrigation. The first is a base case, or "business as usual," scenario. The second scenario assumes a high, but not unrealistic, degree of basin irrigation efficiency in the irrigation sector.

It is found that the growth in world requirements for the development of additional water supplies varies between 59 percent in the first scenario to 25 percent in the second scenario. The truth perhaps lies somewhere between. Thus increasing basin irrigation efficiency reduces the need for development of additional water supplies in 2025 by roughly one-half. This is a substantial amount, but development of additional water supplies through small and large dams, conjunctive use of aquifers and, in some countries, desalinization plants will still be needed. Also, these world figures disguise enormous differences among countries (and among regions within countries). Many of the most water-scarce countries already have high basin irrigation efficiencies, so this will not substantially reduce their needs for development of additional water supplies. On the other hand, most of the world’s gain in basin irrigation efficiency would be in countries with a high percentage of paddy (rice) irrigation. It is not clear how much basin irrigation efficiency can be practically increased in paddy irrigation; and paddy irrigation tends to occur in areas with high rainfall where water supply is not a major problem. The fact that South China has a lot of water to be saved through improved irrigation efficiency is small consolation to a farmer in Senegal who hardly has any--or for that matter to a farmer in the arid north of China (unless there are interbasin transfers from south to north). This is why the country data--and, ultimately, the data for regions within countries--is much more important than world data.

Part III presents two basic criteria of water scarcity that together comprise the overall IWMI indicator of water scarcity for countries. Using the high basin efficiency scenario, these criteria are (i) the percent increase in water "withdrawals" over the 1990 to 2025 period and (ii) water withdrawals in 2025 as a percent of the "Annual Water Resources" (AWR) of the country. Because of their enormous populations and water use, combined with extreme variations between wet and dry regions within the countries, India and China are considered separately. The 116 remaining countries are classified into five groups according to these criteria.

Group 1 consists of countries that are water scarce by both criteria. These countries, which have 8 percent of the population of the countries studied, are mainly in West Asia and North Africa. For countries in this group, water scarcity will be a major constraint on food production, human health and environmental quality. Many will have to divert water from irrigation to supply their domestic and industrial needs and will need to import more food.

The countries in the four remaining groups have sufficient water resources (AWR) to satisfy their 2025 requirements. However, variations in seasonal, interannual and regional water supplies may cause underestimation of the severity of their water problems based on average and national water data. A major concern for many of these countries will be developing the large financial, technical and managerial wherewithal needed to develop their water resources.

Group 2 countries, which contain 8 percent of the study population and are mainly in sub-Saharan Africa, must develop more than twice the amount of water they currently use to meet reasonable future requirements.

Group 3 countries, which contain 15 percent of the population and are scattered throughout the developing world, need to increase withdrawals by between 25 percent and 100 percent, with an average of 50 percent.

Group 4 countries, with 17 percent of the population, need to increase withdrawals, but by less than 25 percent.

Group 5 countries, with 11 percent of the population, require no additional withdrawals in 2025 and most will require even less water than in 1990.

We believe that the methodology used in this report may serve as a model for future studies. The analysis reveals serious problems in the international data base, and much work needs to be done before the methodology can be used as a detailed planning tool. However, the work to date highlights the national and regional disparities in water resources and provides a basis from which we can begin to assess the future supply and demand for this vital natural resource.

World Map of scarcity to be inserted here on lanscape format.