Hydrology in Humid Tropical Asia

Abstract

The humid tropics can be defined in different ways. The definition adopted by this Col1oquium specifies that the mean temperature of the coldest month is above 18℃ and the duration of wet season exceeds 4.5 months (Chang and Lau 1983). A wet month has more than 100mm of rainfal1, and when the rainfall is between 60 and 100mm half a wet month is credited. The humid tropics has three subtypes: (1) wet has 9.5 to 12 wet months, (2) subhumid has 7 to 9.5 wet months, and (3) wet-dry has 4.5 to 7 wet months. The total area of the humid tropics is approximately 29.4 million km^2, or 22% of the earth's land area. It is most extensive in the Americas, accounting for 44% of the total, followed by 32% in Africa, 18% in Asia, and 6% in Oceania. The Asian sector includes nearly the entire Southeast Asia south of the Tropics of Cancer (Fig.1). Over the Indian subcontinent only a narrow strip of west coast, a belt of east coast of about 350 km width from southern Bangladesh to the Coromandel coast and Sri Lanka fall within the humid tropics. Much of the interior of Indian subcontinent is too dry during two-thirds of the year. The southern part of Southeast Asia has numerous islands. They are sometimes col1ectively referred to as the ”maritime continent”. The greater part of the maritime continent and the southern part of the Malay Peninsula belong to the wet subtype, which accounts for 42% of the total area of humid tropics in Asia. The eastern part of the continental Southeast Asia, including Vietnam, Kampuchea, and southern Thail and, has the subhumid subtype. The duration of wet season decreases towards the west, as northern Thailand, Burma, and much of India belong to the wet-dry subb type. In South Asia only southern Bangladesh, southwestern Sri Lanka, and Coimbatore in southern India have a wet season of seven months or longer.