Thailand's Pollution Control Department reports divide the country into five main geographical regions: north, northeast, central, south, and east.[63] Thailand has a total of 25 river basins[64] 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.[65] 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."[66] Non-potable water was a result of increasing untreated domestic sewage, industrial waste water, and solid hazardous wastes.[66] This is a critical environmental problem for Thailand.[1] Like air pollution, water pollution is most serious in the populous central region, with high levels of industrial and domestic waste water.