Quantifying the Hydrological Effect of Permitted Water Abstractions across Spatial Scales

Water abstraction from rivers and aquifers has considerable potential to alter flow regimes, thereby influencing the physical, chemical, and ecological well-being of freshwater ecosystems. The economic and social benefits of abstraction need to be balanced against its potentially deleterious consequences for hydrologically-driven ecological functions, ecosystem services, cultural values, and recreation. In New Zealand, recent legislation states that limits for the use of water resources should be set for all waterbodies to manage the potential cumulative impacts of abstraction and reduce allocation of the hydrological resource in over-allocated catchments. These limits must comprise at least a predefined minimum flow (the flow at which all abstraction must cease) and a total allocation (the maximum rate of abstraction summed across upstream abstractions). Over-allocation occurs when the sum of all upstream abstractions exceeds the total allocation. A national database describing consents to abstract water was collated. A replicable, transferable, and objective method was applied to calculate total allocation at the national, catchment, and reach scales across the entire country. Total allocation for each catchment was expressed by mapping Weighted Allocation Impact; an index that integrates magnitude and spread of water resource allocation across entire catchments. Results show that existing consents have caused over-allocation in several catchments, prompting questions about how to reduce abstraction in these locations.