ABSTRACT
This chapter summarizes the global water resources issues. Variations in river flows through the year and variations from year to year are more significant for water availability and management than the long-term average annual volume of flows. It has become a cliche that water resources are essential to life, and that problems with water resources are likely to be the greatest environmental challenge facing human society during the twenty-first century. Pressures on water resources are increasing because of population and economic changes, changes in catchment land use, changes in human use of water, and the effects of global climate change. Human activities have affected hydrological regimes at the local scale for several thousand years, but during the late twentieth century human activities began to have clear and observable effects on the volume, timing and quality of river flows over very large geographic areas.
