STREAMFLOW AND SEDIMENT TRANSPORT FROM MOUNTAIN WATERSHEDS OF THE CHAO-PHRAYA BASIN, NORTHERN THAILAND - A RECONNAISSANCE STUDY

In the mountain watersheds of northern Thailand, the calculated runoff efficiency (that is the ratio between input as precipitation and output as surface runoff) is extremely low for a mountainous region, with an average of 20-25 percent. In those catchment basins for which data are available, the volume of suspended sediment is low, averaging approximately 100 t/km2. Furthermore, time-series analyses of the existing data bases show no evidence that streamflow or sediment regimes have changed significantly since the 1950s. There is no empirical substantiation for the hypothesis that land-use practices have altered the hydrologic regime of these basins, or have contributed to an increase in the sediment load of the rivers. At the present time, the most pressing need for the government of Thailand is to organize the existing data bases, and publish annual statistics. This would increase the efficiency with which planning and management of water resources could be undertaken.