Impacts of soil erosion in the upper Manupali watershed on irrigated lowlands in the Philippines

With concerns about possible environmental degradation associated with agricultural production, the impacts of soil erosion in the upper Manupali watershed and the consequent sedimentation in the plains on the productivity of the Manupali River Irrigation System's (ManRIS) service area of 4,422 ha were assessed. Land-use changes in the watershed for the past 2 decades caused soil erosion and the consequent increasing trend of canal siltation at ManRIS. This resulted in a significant decline in the productivity and income of the farmers. Rice yields in farms that were heavily affected by siltation had decreased by 27% from 1990 to 1995. Furthermore, in addition to the regular Operation and Maintenance (O and M) costs, the ManRIS management incurred desilting costs in its operation. To cope with the siltation problem, the ManRIS management and farmers made adjustments in the water delivery schedule, cropping pattern and land allocation to various crops. However, it is expected that the siltation in the ManRIS canal network and the consequent decline in crop yield and income will continue in the coming years. Drastic measures are needed to remedy the soil erosion problem in the upper Manupali watershed.