Suspended solids discharge from a small forested basin in the humid tropics

Suspended solids (SS) discharge from a tropical rain forest was observed at the Bukit Tarek Experimental Watershed in Peninsular Malaysia in order to elucidate mechanisms of SS production and transport. Peaks of water discharge and electrical conductivity (EC) lagged further behind rainfall peaks than did dissolved oxygen (DO), indicating that the discharge in the early stage of a storm is mainly formed by rain water with high DO. Stream water showing a high value of EC originating from subsurface water formed the main storm flow and lagged behind the rainfall. SS concentrations rose to a peak quickly and, like DO, the peak preceded that of water discharge. A clockwise hysteresis loop in the relationship between SS and water discharge exists, and the magnitude of hysteresis loop is in proportion to storm size. The values of SS concentration correlate positively with the values of rainfall intensity on logarithmic axes. The time intervals between peaks of rainfall and SS concentrations are assumed to be a delivery term expressing distance from sediment source to measuring point. Immediate transport of SS from the source to the sampling site, the short time gap between the peaks of rainfall and SS concentrations, and the high rate of infiltration on the hillslope suggest that the sources of high SS concentrations are located close to the stream. The calculated source area is located at a gentler part of the stream, where wet riparian areas exist. The strong relationship between water discharge and SS concentration during the small storm proves that the source areas of SS and water were the same. In contrast, the source area of SS disappeared when rainfall ceased, whereas the source area of water discharge was still expanding in the larger storm. These phenomena may produce clockwise loops in the SS concentration–water discharge relationship. Copyright © 2004 John Wiley & Sons, Ltd.