The forested watersheds provide a high proportion of water used for domestic, agricultural, industrial, and environmental areas upstream and downstream. Those who manage land, forests and water resources have a heavy task is to maximize the wide range of benefits that multi provided by forests, without harming the water resources and ecosystem functions. To meet this challenge, there is an urgent need to improve understanding of the interactions between forests or trees and water, sensitize and strengthen capacity in the field of forest hydrology, and to integrate this knowledge and research findings into policy. It is also essential to establish institutional mechanisms for strengthening synergies in order to solve problems related to forests and water, and implement and enforce action programmes at national and regional levels.
In the past, policies relating to water and forests are often based on the assumption that, whatever the hydrological and ecological conditions, forest cover was the best plant to maximize efficiency in water, regulate the flow and ensure a seasonal good water quality. The conservation (or expansion) of forest cover in the catchments areas upstream was therefore regarded as the most effective measure to increase the available water resources for agricultural uses, industrial and domestic, but also to prevent flooding in areas downstream.
The International Year of Freshwater 2003 and the third World Water Forum (Kyoto, Japan, 2003) served as a guide to integrate policies in this biophysical understanding of the interactions between forests and water. The International Expert Meeting on Forests and water, held in Shiga, Japan in November 2002 in preparation for these events, highlighted the need to adopt a more holistic approach taking into account the interactions between Water, forests and other land uses and socioeconomic factors in ecosystems catchment complex. Over the past five years, the Declaration of Shiga has become a key reference for developing a new generation of politicians of Water Affairs and Forestry (see article Zingari and Achouri, in this issue).
This article provides an update on the current state of knowledge on the interactions between forests and water ecosystems in the catchment. It summarizes some key issues being highlighted by discussions between forest hydrologists, other experts and policymakers in the water sector in the years following the Declaration of Shiga, the third World Water Forum and the International Year of Freshwater.
Realimentation: reconstruction of a groundwater
River basin (or water): the complex system of catchments and sub-catchment areas traversed by a river and its major tributaries between the source and the mouth
Liaisons upstream / downstream interactions environmental, socio-economic and cultural exchanges synergistic and conflicts between the upper and lower a watershed. Catchment area: the geographical area drained by a river - a concept applicable to a range of units, the farm bisected by a stream (micro catchment) to the major river or lake. Watershed management: human intervention to ensure sustainable resource use catchment
Upstream and downstream, a more holistic approach is needed to study the interactions between water, forests and other land uses and socioeconomic factors Recently, forest hydrology focused on three topics of particular interest for policy formulation, namely: comparative advantages and disadvantages of forest cover to maximize the production of water downstream, the role of forests in upstream maintaining the flow of water during the dry season, and preservation of water quality. This section summarizes the findings in these three areas (after Hamilton, 2005).
Previously, those who formulated policies went often assumed that there were more trees, plus there was water, but this assumption is challenged by current research in forest hydrology. The forest ecosystem actually consumes large quantities of water. The tops of trees reduce flows underground and the flow of rivers by interception of precipitation and evaporation and transpiration from leaves, as natural forests and forests established by humans consume more water than most other types of vegetation (including agricultural crops and fodder), it is indisputable that the removal (even partial) of forests increases the production of water downstream.
It was sometimes suggested that the forest cover which consumes a lot of water, particularly in semi-arid areas, to prevent or mitigate the drought. However, the benefits of this approach should be weighed against the disadvantages arising, namely the loss of many other forest goods and services (the fight against erosion, improving water quality, carbon sequestration, recreation and aesthetic appeal, timber, firewood and other forest products and biodiversity). Deforestation is prohibited in areas prone to salinity, where it would go up salts in the soil surface and in the mountain forests of fog where the trees, areas of vegetation epiphyte, twigs, branches, stems and bushes act as "net" in capturing the "horizontal rain" from fog or clouds.
|
