Forests located in watersheds contribute in a number of ways to maintaining local and downstream environmental conditions in a state conducive to agriculture, and protective of human settlements.
Forests help stabilize soil, regulate water flow rates and periodicity, maintain water quality, and, in the unique case of cloud forests, capture additional water suppplies from the atmosphere.
These services cannot be quantified at the global level, nor would such an exercise by useful. Information on watershed conditions is most relevant at the basin, subbasin, and site-specific level but such data are currently scattered.
The following summarizes key findings of the PAGE study regarding the condition and trends of environmental services of watershed forests, as well as the quality and availability of data.
Conditions and trends
- One-third of the world’s major watersheds have lost more than 75 percent of their original forest cover.
- Tropical montane forests, which are often located in the upper reaches of watersheds, are disappearing faster than any other tropical forest type.
- Deforestation is associated with alteration of stream flow quantity, quality and regularity, although links to major floods are more complicated than sometimes portrayed.
- Ground cover vegetation appears to be more important than tree cover in preventing erosion, but erosion rates under shifting cultivation are ten times higher than in natural forest. Erosion rates can be 100 times higher in plantations where weeds and leaf litter are removed.
Information status and needs
- The relationships between forest cover, forest type, and hydrological regimes are still inadequately understood.
- Information is most valuable at the site-specific and river basin levels, which can provide a sound basis for land use planning decisions affecting watersheds and downstream populations.
- Information is needed on the evaporative characteristics of different tree species and soil combinations, background and human-induced rates of soil erosion, and sedimentation rates and flooding incidence.
- Site-specific models are required to predict the impacts of afforestation or deforestation in catchment areas.
Quality and availability of data
PAGE measures and indicators
Data sources and comments
Forest cover in major watersheds (% remaining) Revenga, C., S. Murray, J. Abramowitz, and A. Hammond. 1998. Watersheds of the World: Ecological Value and Vulnerability. Washington D.C.: World Resources Institute and Worldwatch Institute.
Global survey of 145 major and secondary watersheds. FAO, 1993. Estimates of deforestation rates in montane areas.
Vegetation and soil erosion Survey of local studies, mostly in tropical and subtropical region.
