Hydrological investigations of forest disturbance and land cover impacts in South-East Asia: a review

Investigations of land management impacts on hydrology are well developed i n South-East Asia, having been greatly extended by national organizations i n the last two decades. Regional collaborative efforts, such as the ASEAN-U S watershed programme, have helped develop skills and long-running monitori ng programmes. Work in different countries is significant for particular as pects: the powerful effects of both cyclones and landsliding in Taiwan, the significance of lahars in Java, of small-scale agriculture in Thailand and plantation establishment in Malaysia. Different aid programmes have contri buted specialist knowledge such as British work on reservoir sedimentation, Dutch, Swedish and British work on softwood plantations and US work in hil l-tribe agriculture. Much has been achieved through individual university r esearch projects, including PhD and MSc theses. The net result is that for most countries there is now good information on changes in the rainfall-run -off relationship due to forest disturbance or conversion, some information on the impacts on sediment delivery and erosion of hillslopes, but relativ ely little about the dynamics and magnitude of nutrient losses. Improvement s have been made in the ability to model the consequences of forest convers ion and of selective logging and exciting prospects exist for the developme nt of better predictions of transfer of water from the hillslopes to the st ream channels using techniques such as multilevel modelling. Understanding of the processes involved has advanced through the detailed monitoring made possible at permanent field stations such as that at Danum Valley, Sabah.