Thailand's Pollution Control Department reports divide the country into five main geographical regions: north, northeast, central, south, and east. Thailand has a total of 25 river basins in the country and Thailand's annual rainfall is around 1,700 mm. Despite the annual southwest monsoon, Thailand is subject to drought, particularly the northeastern region. As of 2002, Thailand had less water available per person than any other country in Asia, and nearly one-third of its water was "unsuitable for human consumption." Non-potable water was a result of increasing untreated domestic sewage, industrial waste water, and solid hazardous wastes. This is a critical environmental problem for Thailand. Like air pollution, water pollution is most serious in the populous central region, with high levels of industrial and domestic waste water.