Management of sedimentation in tropical watersheds

The sedimentation of reservoirs is a serious problem throughout the tropics , yet most attempts to control sedimentation in large river basins have not been very successful, Reliable information on erosion rates and sources of sediments has been lacking. in regions where geologically unstable terrain combines with high rainfall, natural erosion rates might be so high that t he effects of human activity are limited, Estimates of natural erosion in t hese situations often have been poor because of the episodic nature of most erosion during large storms and because mass-wasting may supply much of th e sediment. The predominance of mass-wasting in some watersheds can result in an unexpectedly high ratio of bedload to suspended load, shifting sedime ntation to "live" rather than "dead" storage within reservoirs. Furthermore the inappropriate use of the Universal Soil Loss Equation to assess the ef fectiveness of erosion control measures has led to inaccurate estimates of the sediment reduction benefits that could accrue to watershed treatment ef forts. Although reducing erosion from cultivated areas is desirable for oth er reasons, efforts aimed at reducing reservoir sedimentation by controllin g agricultural sources of erosion may have limited benefits ii the principa l sources are of natural origin or are associated with construction of the darns and reservoirs and with rural roads and trails. Finally, the most app ropriate locations for watershed rehabilitation depend on the magnitude of temporary storage of colluvium and alluvium within the river basin: Where s torage volume is large and residence time of sediment very long, reducing a gricultural erosion may have limited impacts on sedimentation within the ex pected life of a reservoir. Systematic development and analysis of sediment budgets for representative watersheds is needed to address these limitations and thereby improve both the planning of river basin development schemes and the allocation of resou rces towards reducing sedimentation. When sedimentation of reservoirs is th e key issue, sediment budgets must focus especially on channel transport ra tes and sediment delivery from hillsides. Sediment budgets are especially c ritical for tropical areas where project funds and technical help are limit ed, Once sediment budgets are available, watershed managers will be able to direct erosion control programs towards locations where they will be most effective.