EROSION, SEDIMENT TRANSPORT AND RESERVOIR SEDIMENTATION IN THE UPPER DRAINAGE BASIN OF THE KONTO RIVER, EAST JAVA, INDONESIA

摘要

This paper presents a diachronic hydrogeomorphological study of a hydrosystem in a tropical and volcanic area. The upper Konto River in East Java (Indonesia) has a medium size catchment (236 km~2) dominated by active volcanoes, which provide high-profit arable soils although very sensitive to erosion. The population densities are then very high, in excess of 1000 per km~2. People thus play a key role in the evolution of the natural environment. The Konto hydrosystem as been disturbed during the last past decades by several factors: (1) A dam built in 1979 in the downstream of the study area; (2) a reforestation program (so-called Konto River Project) in the upper watershed in 1982, followed by (3) a deforestation period since 1998 following the economic and financial crisis in Indonesia (Reformasi). We have then analysed how this watershed reacted to anthropic disturbance, in studying the erosion sources, sediment transport, and sedimentation in the reservoir. The research on potential erosion zones in the watershed has been based on a GIS (ArcView™), which combines thematic vector-based maps of the natural environment (slopes, climate, soil, etc.), and raster-based maps of the landuse, derived through Landsat™ images. A diachronic analysis of the suspended load as well as echosounding in the reservoir since the sixties helped us to assess soil erosion and resulting reservoir sedimentation. The erosion rates in Indonesia range among the highest in the world. Sedimentation rates within the Selorejo reservoir average 0.25 x 10~6 m~3·yr~(-1) during the afforestation period, but is six times larger (1.57 x 10~6 m~3·yr~(-1)) during deforestation period. Sediment rates in the Konto watershed are representative of other small watersheds strongly disturbed by human activities on Java and other tropical environment.