Agroforestry: hydrological impacts

Forests and agroforestry (AF), in which trees are incorporated onto cropland, have been widely promoted as viable solutions to overcome the loss of ecosystem functions associated with conversion of natural landscapes to human use. However, trees in forests and on farms may have both beneficial and detrimental hydrological impacts at farm and watershed levels. In this chapter, we focus on opportunities and challenges of using AF in the semi-arid tropics and explain how to: 1) modify water productivity and make better use of unproductive water, 2) enhance the hydrological impacts of simultaneous and sequential AF systems, 3) improve water infiltration and reduce evaporation, runoff and nutrient leaching, and 4) minimise below-ground competition between trees and crops. We also consider the role of trees in improving watershed functions and the importance of hydraulic lift in dryland environments. We show that there are substantial opportunities to increase water productivity in the semi-arid tropics because trees can access unproductive water which crops cannot acquire. Nevertheless, competition for water in AF remains a serious challenge where rainfall is too low to recharge soil below the crop rooting zone unless tree roots have access to the water table. To avoid competition, it may be necessary to root- or shoot-prune trees, use species with ‘reverse phenology’, or adopt sequential planting of trees and crops to permit spatial complementarity in resource capture.