The refinement of isotopic data layers for three catchment-scale studies in the Southland, Horizons, and Gisborne regions: activities between May 2017 and May 2018

The National Institute for Water and Atmospheric Research established, and is leading, the 'National Hydrological Project (NHP), with GNS Science (GNS) and Landcare Research as sub-contracted partners. This report details the GNS activities in the NHP task "design and conceptualisation of catchment-wide isotopic data layers, including data-acquisition of isotope field samples, in selected case studies" and aims to refine isotopic data layers, for stable isotopes and tritium, to make them applicable for studies that estimate groundwater - surface water interaction at the catchment-scale. We describe the main activities carried out between May 2017 and May 2018: (1) the sampling design in three selected case study areas (Southland, Horizons and Gisborne regions) and associated collected water samples for NHP are described (with the first analyses results expected in the course of the next financial year 2018-19); (2) outline of the theory and rationale behind the sampling design; and (3) a review of existing tritium and stable isotope measurements, obtained outside the NHP, from both national and regional case study scale sampling programmes. The theory and review strengthen the rationale of the sampling design, and combined they lead to recommendations for further work, the most important of which is to align other streams of existing research to overcome the huge data sparsity resulting from using only data collected though the NHP, and to make the important step towards the national scale. (auth)