New insights into forest hydrology with Forest Flows Programme – radiata pine catchments’ water use and release

Authors: Dean F. Meason, James Griffiths, Vanessa McWilliams and Priscilla Corbett-Lad
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 2025, Issue N.Z.J.For. 70(1) 2025, pp 25-32, May 2025
Publisher: New Zealand Institute of Forestry

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Abstract: Planted forests are increasingly seen as a competitor for freshwater resources, and regulations are being proposed to limit forestry operations in existing forests and restrict new plantings. This could have a large impact on the New Zealand forestry sector. Knowledge about how much water Pinus radiata (D. Don) planted forests use and how much water is supplied is based on studies from over 45 years ago. These studies relied heavily on a water balance ‘black box approach’ and are not suitable for answering today’s questions. The Ministry of Business, Innovation & Employment (MBIE) Endeavour Forest Flows Programme (2019-2024) employed a fundamentally new approach using integrated terrestrial and remote sensing measurements to directly quantify tree water use, the amount of water stored and released in planted forest catchments, and the underlying drivers. This paper is the first in a series that summarises the main results of the programme. The five primary sites encompassed a rainfall gradient ranging from 800 mm to 3,000 mm per year. The infiltration rate of rainfall events was high across the five sites, averaging 74%. Canopy interception ranged from 9% to 27%. Ashley Forest, the site with the lowest annual rainfall, also had the lowest canopy interception. Tree water use, canopy interception and evapotranspiration ranged from 6% to 37% of total annual rainfall, averaging 25%. Subsurface water flow was an important pathway for water release from planted forest catchments and it ranged from 31% to 71%, averaging 47%. Subsurface flow was substantially larger than surface flow for the three catchments with annual rainfall <1,600 mm. Previous studies did not measure subsurface flow and missed quantifying a critical pathway. These data-driven results dispel some commonly-held beliefs about water use of P. radiata forests and provide the information required for meeting New Zealand’s freshwater management challenges.

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