Forest conversion and watershed functions in the humid tropics

Rapid conversion of forests raises concerns over the maintenance of watershed functions that influence the livelihoods of people downstream. However, the perception that only natural forests are able to maintain these functions is an over-simplification and we hypothesize that some farmer-developed agroforestry mosaics may be as effective in protecting watershed functions as the original forest cover. If the latter hypothesis is true, a substantial share of current conflicts between state forest managers and local population in `watershed protection areas' can be resolved to mutual benefit. Research to test this hypothesis is part of an effort to develop predictive tools for landscape evaluation, as well as a process of resource use negotiations among multiple stakeholders. This research is carried out in watersheds in Indonesia, Thailand and the Philippines. In Indonesia, the Sumber-Jaya area in the upper Tulang Bawang river water shed was selected. To test the agroforestry hypothesis we need to analyze the various watershed functions, based on the quantity, quality and timing of waterflow. Overland flow of water and sediment can be intercepted by a wide range of vegetative filters, and thus a non-forest landscape with strategically paced filters can maintain acceptable waterflows. Controls exist at the level of vegetation (interception of rainfall and mist), soil surface (infiltration, surface roughness causing filter functions) and the soil profile with its vertical and lateral subsurface flows of water. Modelling tools that can describe these processes are available, as illustrated by the sensitivity of the WaNulCAS model to the time pattern of rainfall (for a given monthly total). An integral evaluation of the (lack of) filter functions in a landscape and the dynamics of sedimentation zones is needed to come to grips with the elusive concept of `watershed functions' at landscape scale.

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