Forests working as rainwater harvesting systems
We conclude that cases of forestry and of a landscape mosaic with trees can be seen as ‘rainwater harvesting interventions’, where the forests and trees provides numerous provisional, regulatory, aesthetic and supporting ecosystem services for sustaining livelihoods and producing economic benefit. The notion of forests being ‘water towers’ is a misconception, as forests and trees actually consume water in generating the ecosystem services. However, this ‘lost’ water creates other benefits in terms of human welfare via the goods and services provided by the forest ecosystems. Depending on local conditions, forest areas can act as sponges, ensuring stable base-flows in downstream river systems, as well as increasing water infiltration into the soil, which can recharge shallow groundwater sources.
However, the cases of water partitioning in semi-arid miombo woodlands and West African parklands cannot be generalized to locations with different species and management strategies. The lack of empirical evidence of linkages between trees, landscapes and rainfall complicates the issue of possible tradeoffs or mutual benefits to be derived from trees, or in terms of ecosystem services and landscape water flows (green and blue water partitioning of rainfall). As Scott et al. (2005) express, possibly in most cases, productive forests might use more water than they contribute to groundwater recharge. On the other hand, with increasing demands for high levels of production of both wood and food, the alternative, with continued deforestation and continued deterioration of forests, parklands and their soils, is hardly a viable alternative.
The ‘rainwater harvesting’ effect of trees and forests is turned into valuable goods and services and is also linked to the impact on the soil surface and the actual consumption of water. Trees generate litter, which improves the organic matter content in soils – a key component to increased water infiltration. Secondly, trees reduce rainfall impacts on soil surfaces that control soil erosion and sediment transport. Although there is limited empirical data on water balances and forests, the well-known benefits of forest ecosystem services can offer a positive regeneration of degraded and water stressed landscapes. Improved provisioning of goods and services as wood, fodder, fruit, medicines, sometimes water flows as well as habitats for diverse flora and fauna are all components that are enhancing the livelihoods of smallholder farmers. Additional benefits such as water purification, build-up of fertile soil systems, and reduced flooding and sediment Sahelina parkland Mali Enfors transport are all complementary benefits for a local community. However, extensive land-use changes from forests to plantations or to decreased forest cover should always be weighed within a comprehensive impact assessment of both environmental and social-economic issues, including the landscape water balance.
Malmer, A., Ilstedt, A. & Barron, J. (2010). Forests working as rainwater harvesting systems. In Barron, R. (ed.) Rainwater harvesting: A lifeline for human will-being. United Nations Environment Programme, Stockholm Environment Institute.
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