A 2024 study in the Journal of Hydrology: Regional Studies exposed how turning grazing land into pine plantations can slash streamflow in New Zealand’s Waikato catchments.
Using the Soil and Water Assessment Tool, the researchers simulated water movement in three hill‑country catchments, two of which had undergone pasture‑to‑pine conversion.
In Mangaotama, 57% of the 17,000‑ha area was planted with radiata pine, producing a modeled 29.7% reduction in mean annual flow, while Kiripaka, with 9% afforestation, saw only a 4.7% drop.
The decline stems mainly from increased evapotranspiration and lowered surface runoff, with groundwater flow also falling in the heavily planted Mangaotama catchment.
SWAT, however, over‑estimates immediate effects, indicating a model lag of about seven years before the observed gradual decline, and mispredicts post‑change increases in two control areas.
These results flag a potential conflict between timber production and water security, prompting the Ministry of Environment to schedule a policy review ahead of its 2025 annual meeting.