Turbidity and suspended sediment dynamics in small catchments in the Nepal Middle Hills

The Himalayan environment has, until recently, been perceived to be in a critical state of environmental decline, resulting From rapid population growth and associated land-use change. Recent research, however, has emphasized the difficulty of developing an objective appraisal of the state of the environment in a region where empirical data are scarce and unstructured and where an understanding of the spatial and temporal dynamics of natural environmental processes remains highly uncertain. This paper presents results from an intensive three-year project designed to help address the regional empirical deficit, establish detailed baseline environmental data and to gain an insight into storm period and seasonal suspended sediment dynamics. The instrumentation, calibration and analysis of high-frequency infrared turbidimetric records from a number of small subcatchments in the Nepal Middle Hills are reported. Storm period and seasonal Variation in turbidity and suspended sediment are examined and hysteresis patterns explored and explained. 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 sediment yield are found to be high with respect to erosion plot studies from the local area, and this suggests the importance of riparian and channel erosion as major sediment sources. a finding consistent with other regional studies. Copyright (C) 2000 John Wiley & Sons, Ltd.