INTRODUCTION

The continent of Asia covers an area of 43.5 million km2, one-third of the Earth's land surface, and is home to more than half of the world's population. The vast extent of the continent – more than 10 000 km west to east – with its high mountain ranges and extensive lowlands, together with irregularities in atmospheric circulation patterns, cause uneven distributions of heat and moisture. There are permafrost zones, large river systems with areas of water surplus, and hot dry deserts.

Over most of Asia, particularly in the west and south, the distribution of precipitation is uneven, and in some areas the annual water deficit attains 1500–2000 mm. Asia is the world's major user of freshwater, and water resources are generally well developed; more than two-thirds of the world's irrigated land is located here. Total surface water resources amount to 14 400 km3/year, and water availability per capita is 13.1 m3/day, or half of the world average. Shiklomanov and Markova (1987) estimated that in 1990 water consumption for economic uses in Asia would be 2400 km3, or 59% of the world total. This represents 17% of annual streamflow, an amount that could increase to 20–25% in the near future. The development of agriculture, the main source of economic prosperity, will be limited by the availability of water. Reliable assessments of possible changes in climate due to global warming, and the effects of these changes on hydrological regimes and water resources, are therefore vital for economic planning in many Asian countries.

The influence of climate on water resources is also important for Australia, the smallest continent by area and by population.