The Southeast Asian tropics extends from 23.5^o North (Tropic of Cancer) to 11^o South, and 90-140^o East, and includes the countries of Brunei Darussalam, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand, Timor-Leste and Vietnam. Southern China, notably the regions of Hong Kong, Guangdong, Guangxi, Hainan and Taiwan extend below the Tropic of Cancer at 106-122o East and are, therefore, climatologically part of the region. The region is also known as the ‘Maritime Continent’ because of its high proportion of sea, and the ‘Warm Pool’ because of its elevated sea-surface temperatures and consequent impact on evaporation and water vapour input to the upper troposphere (Newell and Gould-Stewart, 1981; Chen and Houze, 1997b). The high water vapour input to the upper troposphere makes SE Asia important for the global climate system (Neale and Slingo, 2003) and hence the global water cycle.

The region exhibits climatic phenomena having important periodicities ranging from hours to decades (Nesbitt and Zipser, 2003; Franks and Kuczera, 2002). The strength and timing of these cycles and trends vary significantly over the latitudinal gradients from the equatorial zone to the Tropic of Cancer in the North and Lesser Sunda Islands (Nusa Tenggara) region to the South (Kripalani and Kulkarni, 1997). The spatial and temporal variability in the climatic cycles strongly affects the patterns in hydrology across the region. As a result, there are marked variations in the likelihood of flooding and droughts, the intensity of erosion and nutrient cycling, and the demand for irrigation water and water supplies. Further, the magnitudes of land-use change impacts on hydrology are also either - masked (Chappell and Tych, 2003) or regulated (Chappell et al., 2004b) by the region’s climate dynamics. This chapter aims to review influential and recent studies from SE Asia that show how hydrology is regulated by climate.