General summary Asia
Compilation of information on water resources shows large methodological discrepancies between countries. This survey distinguishes between internal renewable water resources (IRWR) and total renewable water resources (TRWR). IRWR is that part of a countrys water resources which is generated from endogenous precipitation. It is computed by summing surface water flow and groundwater recharge and subtracting their common part. The computation of TRWR is made by summing IRWR and external flow. It is a measure of the maximum theoretical amount of water available to a country without any considerations of a technical, economic or environmental nature. The methodology used in the survey also distinguishes between natural and actual external flow: natural flow is the average annual amount of water which would flow at a given point in a river without any human influence, while actual flow takes into account the reduction of flow due to upstream withdrawal and volumes of water reserved through treaties.
The large range of climates encountered in the region generates a variety of hydrological regimes. The region is host to some of the most humid climates (with annual precipitation above 10 m in places) giving rise to major rivers, while in other parts it has a very arid climate, with closed hydrologic systems. As a result, the region shows a very uneven distribution of its water resources and of its water use conditions. In the humid areas, water management concerns have mostly been dominated by considerations related to flood control. This is the case in the Mekong, Brahmaputra and Ganges basins. In the arid areas, such as in central China, where water is scarce, hydrological studies have been oriented much more towards water resources assessment.
The hydrology of the region is dominated by the typical monsoon climate which induces large inter-seasonal variations of river flows. In this situation, average annual values of river flows are a poor indicator of the amount of water resources available for use. In the absence of flow regulation, most of the water flows during a short season when it is usually less needed. A fair estimate of water resources available for use to a country should include figures of dry season low flow. However, such information is available only for a very limited number of countries: in Bangladesh, the surface flow of the driest month represents only 18 percent of the annual average; in Indonesia, it is 17 percent. In India, the flow distribution of selected rivers in the monsoon period represents 75-95 percent of the total annual flow. In north China, 70-80 percent of the annual runoff is concentrated in the rainy season. As a first approximation, one could then state that the amount of water readily available for use is between 10 and 20 percent of the total renewable water resources in the absence of storage.
The information collected from the countries of the region does not make it possible to distinguish between actual and natural flow of the major rivers, i.e. the impact of irrigation and other water withdrawal on the runoff. In this survey, figures were systematically considered as natural flow. This option may lead to a slight underestimation of natural flow in some cases. At least in one case, the Ganges River, the withdrawals in the upstream country (India), are known to affect significantly the volumes of water available to the downstream country (Bangladesh). This has led to the recent signature of a treaty between the two countries on agreed procedures for the management of the river flow. In view of the hydrological regime of the rivers in the region, it can be assumed that runoff in the countries of southeast Asia and the islands is not significantly affected by withdrawals, while the difference between natural and actual flow may be much more important in the arid regions (mostly China).
In terms of shared water resources, the region is characterized, on the one hand by a series of insular countries between which no exchange is possible, and on the other hand by a zone in which shared river basins play a critical role and make the computation of water resources relatively complex (southeast Asia). In several cases, large inconsistencies were noted when comparing the flow at border recorded by neighbouring countries, e.g. the runoff of the main rivers flowing from China to India. The maps of Figures 4 and 5 show the main exchanges between the countries of the sub-region as assessed by this study.
Overall, the region is relatively well endowed with water resources: for a total area representing 15 percent of the worlds land surface, it receives 22 percent of its precipitation and produces 28 percent of its water resources. However, as the region is home to 53 percent of the worlds population, the amount of water resources per inhabitant is only slightly above half the worlds average. Table 2 shows the distribution of water resources among the five sub-regions. The relative aridity of the countries of Eastern Asia is shown by the precipitation which is between two and five times less than the average of the other sub-regions. In terms of water resources per person, a standard indicator of water availability, the groups of the Indian subcontinent, Eastern Asia and the Far East show the lowest figures while Southeast Asia and the Islands have much more water resources per person than the world average.
The figure of 2000 m3/inhabitant/year is usually used as an indicator of water scarcity: India and China are reaching this limit, while the Republic of Korea is already below it, at 1538 m3/inhabitant/year and the Maldives have a chronic water scarcity, with 114 m3/inhabitant/year. Furthermore, in the case of India, 34 percent of the water flows from neighbouring countries, the countrys internal water resources being only 1 334 m3/inhabitant/year.