Mourot F, Westerhoff RS, Cameron SG. 2020. Effects of afforestation on groundwater recharge: perspectives for New Zealand in relation to the One Billion Trees Programme. Lower Hutt (NZ): GNS Science. 84 p. (GNS Science report; 2020/21). 84 p. doi:10.21420/5ZCK-DJ09.
Abstract
Global attention on the development of afforestation programmes and the need to evaluate the trade-off between tree carbon sequestration potential and tree water use are increasing. In New Zealand, a large afforestation programme (the One Billion Tree Programme) was launched in 2018. This programme aims to assist the nation’s transition to a low-emission economy while also providing employment, improving erosion control and water quality; and supporting Māori objectives for their land and forests. Most of the New Zealand forest hydrology knowledge is based on studies established 40 years ago, and the learnings from these studies cannot systematically be applied to different environments and at different scales. An important shift of the views on the interactions of forest and water has also occurred since the beginning of this century: in the last decade, the “it depends” theory has evolved from the “no forest, no water” theory and “more trees, less water” theory of the early 2000s.
This study summarises the state-of-the-art international knowledge in forest hydrology and provides relevant concepts on how afforestation programmes could affect the NZ freshwater resources, with a focus on groundwater. Our approach, mainly based on remote-sensed imagery processed in the cloud, allows scalable assessments from local to national levels and utilises a combination of national and global datasets. National forest cover was assessed to be approximately 108,300 km2 in 2000, decreasing to 103,300 km2 in 2018 based on the Global Forest Cover dataset. In comparison, the Land Cover Database data indicated approximately 106,200 km2 in 2001, decreasing to 105,700 km2 in 2018. Differences between these two assessments were mainly caused by the difference in inclusion and exclusion of canopy cover of younger trees in the underlying methods. Hydroclimatic maps were produced to identify water stressed areas, where particular attention to afforestation management is needed to avoid critical reduction in water yield.
Full implementation of the One Billion Tree Programme requires approximately 8,500 km2 of land. This is considerably less than our estimate of 105,700 km2 of land potentially available for afforestation nationwide, which includes 84,600 km2 of high-production grassland.
The afforestation scenarios indicate increased evapotranspiration rates for exotic forests in comparison to original grassland, with nationwide averaged increases of 120–140 mm/yr and 210–230 mm/yr for high-producing and low producing grasslands, respectively. The “more trees, less water” approach leads to a greater water demand of trees in comparison to grass. Our study demonstrates that forest planting management practices, such as selecting low groundwater recharge areas for afforestation, lead to smaller reductions in groundwater recharge. Accordingly, the “it depends” approach suggests that several factors can influence the effects of afforestation and that the assumption that afforestation will reduce water yield may not always be correct.
This study provides insight on the effects of afforestation on groundwater quantity and recommends further research on the adoption of a more integrated approach, considering catchment transboundary processes, to better capture the interactions between forest and water. The water supply side of forests merits particular attention, including mechanisms such as enhanced soil infiltration capacities and rainfall recycling. (auth)
