Water productivity in forestry and agroforestry.

Abstract

AbstractThis chapter reviews the recent findings on the hydrology of forests and agroforestry systems and indicates how modifications in tree-based systems might increase water productivity. In forestry, the focus of research has moved from the hydrological functions of upland forest reserves that are close to settlements to a greater recognition of the roles played by upland communities in the management of water resources. A major source of conflict over water resources is the contrasting perceptions of 'watershed functions' between forest managers and local people, which are often based more on myths of forest functions than on science - for example, the idea that forests increase rainfall. These myths continue to dominate the views of policy makers and institutions and should be revised. The challenge is to gain a better insight into how farmer-developed land use mosaics have modified watershed-protection functions. Priority must be given to the perceptions, experiences and strategies of local communities. Trees on farms have the potential for improving productivity in two ways: (i) trees can increase the amount of water that is used on farm as tree or crop transpiration., and (ii) trees can increase the productivity of the water that is used by increasing biomass of trees or crops produced per unit of water used. Plot-level evidence shows that improvements in water productivity as a consequence of modifications to the microclimate of the crop are likely to be limited. Instead, evidence from semiarid India and Kenya showed that the greater productivity of agroforestry systems is primarily due to the higher amount of water used. Almost half of the total water use occurred during the dry season, when cropping was impossible, and the rest was extracted from soil reserves. This implies a high temporal complementarity between the crop and tree components of the landscape mosaic. Research is needed to examine the impact of the increased water use on the drainage and base flow at the landscape level. Some of the technical approaches that can be used to improve land and water management, the role of trees and its relation to hydrology and the challenges for rational land use decision-making are also described.