Can product water footprints indicate the hydrological impact of primary production? – A case study of New Zealand kiwifruit

SummaryWater footprints have been discussed as indicators for the influence of primary products on water scarcity and water quality. We assessed the impact of New Zealand kiwifruit on the scarcity and quality of freshwater resources and evaluated how the green-, blue- and grey-water footprints represented this impact.Water scarcity relates to the freshwater stored in soil and groundwater over a yearly timeframe. We found a negligible net change in soil water, as the freshwater in the soil is replenished every year by rain. The dynamics of freshwater in soil is indicated by the green-water footprint. Kiwifruit production has no impact on freshwater scarcity in soils, and we suggest, therefore, discarding the green-water footprint in this and similar studies.The groundwater recharge below kiwifruit orchards showed a large regional variation. A net depletion of groundwater resources occurs only in two kiwifruit growing regions, and only when the orchards are over-irrigated. The blue-water footprint indicates the status of the freshwater resources stored in the groundwater. We compared two different concepts. Our hydrologically based concept (Approach 1) quantifies the net change in the resources, whereas the Water Footprint Network (Approach 2) only accounts for the consumption. The blue-water footprint calculated by Approach 1 could explain 97% and by Approach 2 only 63% of the regional variation of net groundwater recharge below kiwifruit orchards. The values of the blue-water footprints are, on a regional average, about −500 L and 100 L per tray of kiwifruit with Approaches 1 and 2, respectively. According to Approach 1, a tray of kiwifruit delivers a net groundwater recharge of 500 L per tray, whereas according to Approach 2 the production of a tray of kiwifruit consumes 100 L of groundwater. Only Approach 1 contains all the hydrological processes making up the water balance that relates to groundwater.We assessed the impact of regional kiwifruit production on water quality by focusing on nitrate. The grey-water footprint indicates pollutant loading rates. On average, across all regions, the nitrate concentrations in the drainage water were well below the New Zealand drinking water standard of 11.3 mg/L NO3–N, but above the median concentration of all New Zealand groundwaters of 1.3 mg/L NO3–N. The annual nitrogen loads ranged across the regions from 3 to 15 kg NO3–N/ha. We found that the absolute values of the grey-water footprints are very sensitive to the choice of background and maximum admissible concentration value. We recommend that this information must always be given in addition to the grey-water footprint value.Overall, we conclude that the blue-water footprints derived by the hydrologically rational Approach 1, along with the grey-water footprinting metric, are a first step towards the quantification of the impact of agricultural production on the scarcity and quality of freshwater resources. These metrics would enable one to compare, on a product by product basis, the impact of production from different regions or countries. Some additional discussion would still be needed to enable evaluation of the inter-product comparison.

► We modelled the impact of NZ kiwifruit on the scarcity and quality of freshwater.
► We evaluated how this impact can be represented by water footprints.
► We found that kiwifruit production in NZ has little impact on both scarcity and quality of freshwater.
► Only the blue water footprint calculated with our Approach 1 indicated the impact of kiwifruit freshwater scarcity well.
► The grey water footprint addresses water quality and is sensitive to the background and maximum admissible concentration.