Agroforestry and its hydrological impacts under global change

There are substantial opportunities to increase water productivity in the semiarid tropics because trees can access otherwise unproductive water which crops cannot acquire. Nevertheless, unless tree roots have access to deep water reserves replenished elsewhere, competition for water in agroforestry remains a serious challenge where the rainfall is too low – relative to the potential rate of evapotranspiration – to recharge soil below the crop rooting zone. Agroforestry in which a patchwork of trees is maintained or incorporated into cropland or pasture, has been widely promoted as a viable solution to overcome the loss of ecosystem functions associated with the conversion of natural landscapes to human use. However, trees in forests and on farms have multiple hydrological impacts at both farm and wider watershed levels that can be seen as beneficial or detrimental from various perspectives. With a focus on the opportunities and challenges of using agroforestry in the semiarid tropics, this article examines how to: (1) make better use of currently unproductive water; (2) enhance the positive hydrological impacts of simultaneous and sequential agroforestry systems; (3) improve water infiltration and reduce evaporation, surface runoff, and nutrient leaching; (4) minimize below-ground competition for water between trees and crops; and (5) understand surface and groundwater flows at landscape scale, interacting with a plot-level water balance. The role of trees in improving watershed functions and the importance of root-based hydraulic redistribution in dryland environments are also considered. 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.