International river basins cover 47% of the earth’s surface. Many countries are dependent on neighbouring states, where the origin of rivers and influx to lakes is located. If an upstream country interferes with the natural river system, e.g. trough building of dams, the discharge in the lower part of the basin decreases which may cause water stress in downstream countries. This repeatedly led to political crises or even wars between neighbouring states. Worldwide more than 260 international river basins exist. Main hot-spot regions like the Nile (Egypt – Sudan) or the Euphrates and Tigris (Turkey - Syria – Iraq) are well known. Many other countries are dependent on water inflow from other countries to satisfy their human and ecosystem water requirements, because the internally generated renewable water is insufficient. The per capita water available differs extremely between countries, depending on the population and the inflowing and internally produced water. With the global water model WaterGAP, the world-wide water availability and water use for the current situation has been simulated.

This paper provides information for more than 150 countries about per capita water inflow to and outflow from neighbouring states. Further, the per capita internally renewable water availability from endogenous precipitation has been computed. In addition to water quantity, water quality aspects have been considered as well. Used water that returns to the water cycle (return flow) is potentially polluted. The return flow proportion is used to make a qualitative estimate of the quality of water outflow to another country. All results presented give an interesting overview of the world wide international transboundary water situation, and of the dependencies on water quantity and quality from upstream countries.