Turbidity and suspended sediment dynamics in small catchments in the NepalMiddle Hills

The Himalayan environment has, until recently, been perceived to be in a cr itical state of environmental decline, resulting From rapid population grow th and associated land-use change. Recent research, however, has emphasized the difficulty of developing an objective appraisal of the state of the en vironment in a region where empirical data are scarce and unstructured and where an understanding of the spatial and temporal dynamics of natural envi ronmental processes remains highly uncertain. This paper presents results f rom an intensive three-year project designed to help address the regional e mpirical deficit, establish detailed baseline environmental data and to gai n an insight into storm period and seasonal suspended sediment dynamics. Th e instrumentation, calibration and analysis of high-frequency infrared turb idimetric records from a number of small subcatchments in the Nepal Middle Hills are reported. Storm period and seasonal Variation in turbidity and su spended sediment are examined and hysteresis patterns explored and explaine d. A variety of methods to estimate seasonal suspended sediment yield in a mixed land-use catchment are examined, and found to vary by up to a factor of five. Despite the inherent uncertainty, all estimates of catchment sedim ent yield are found to be high with respect to erosion plot studies from th e local area, and this suggests the importance of riparian and channel eros ion as major sediment sources. a finding consistent with other regional stu dies. Copyright (C) 2000 John Wiley & Sons, Ltd.