Integrating blue and green water flows for water resources management and planning

The “Green Water” approach, where flows of water vapour in the form of transpiration, interception and evaporation from the soil and vegetation is considered green water and runoff and groundwater recharge is considered blue water, has been an extremely useful illustrative concept in many situations where the role of land use in water resources management needs to be highlighted. The approach has been the subject of much interest in recent years, particularly in semi-arid and arid regions where Green Water Flows dominate the hydrological cycle. However, it is clear that there are limits to the concept in informing water resources management and planning. In this paper, these limits are explored through case studies of commercial afforestation and runoff harvesting in the SADC region. Issues highlighted include the degree of simplification of the hydrological cycle in many green water focused studies, appropriate spatial and temporal scales for the consideration of low flows and the uncertainty regarding the storage of water in the soil profile and the generation of flows from saturated and unsaturated soil water. It is concluded that rather than focusing on green or blue water flows, it is the hydrological linkages between these and their representation in water resources management and planning that needs most attention.