Watershed protection

Hydrological functions are often cited as being among the most important benefits of forest preservation. Forests prevent soil erosion, maintain water supplies, prevent floods, and maintain rainfall patterns. While there is no doubt that forests play an important role in these processes, hydrological benefits from forest preservation are still poorly understood and likely to be highly variable. Their importance has often been overstated. Many of these functions are a consequence of the fact that forests provide cover and root systems that protect soils. Many other vegetation types, including those resulting from converting forests, may provide the same services.

• Precipitation: Although it is plausible that deforestation should affect local precipitation, the magnitude and even the direction of the effects are unknown. Only in 'cloud forests' forests may increase gross precipitation.
• Streamflow: Controlled catchment experiments show that streamflow increases (rather than decreases as is the conventional wisdom) following loss of forest cover, because of decreased evapotranspiration. Forested lands do tend to have greater detention storage, which delays stormflow initiation and reduces flood peaks. This is mainly of importance in upstream river stretches but the effects are buffered downstream.
• Groundwater: Groundwater tables tend to rise following deforestation, because reduced evapotranspiration means more water is available to percolate down. If the groundwater is saline, this can cause salinization.
• Erosion: Erosion tends to be affected more by soil surface cover than by forest cover per se; some non-forest land uses can have very little erosion, while some forest land uses can have relatively high erosion. Forests can help protect slopes against mass wastage thanks to their root systems. In most cases, most sedimentation comes from riparian zones. Maintaining riparian buffer strips can help reduce sedimentation and increase water quality. Forestry activities may lead to increased erosion due to the construction of poorly designed roads, soil compaction and removal of canopy cover

TBI sources

Tobon, C.M. (1999). Monitoring and Modelling Hydrological Fluxes in Support of Nutrient Cycling Studies in Amazonian Rain Forest Ecosystems.

Further reading:

PAGE report-review hydrological functions of forests (WorldBank)