Converting grazed pasture to pine plantation can significantly reduce the amount of water flowing through streams, according to a hydrological modelling study of 2 afforested catchments in New Zealand’s Waikato region.
uncertain
Converting → convert → region
Published in Journal of Hydrology: Regional Studies in 2024, the study, titled
Modelling the hydrological impact of afforestation in hill country catchments in New Zealand, used the Soil and Water Assessment Tool (SWAT) to estimate the effects of land-use change.
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study → publish → change
The researchers, Linh Hoang and Andrew Hughes, modelled a 29.7% reduction in mean annual streamflow in Mangaotama, where approximately 57% of the catchment was afforested.
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% → model → catchment
In Kiripaka, where about 9% was afforested, the modelled reduction was 4.7%.
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reduction → afforest → Kiripaka
A third catchment, Whakakai, served as a native-forest control and had no land-use change.
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catchment → serve → change
Increased evapotranspiration and reduced surface runoff were the main mechanisms behind the modelled decline.
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evapotranspiration → increase → decline
However, the size and timing of the changes varied, highlighting the need to consider model uncertainty when assessing the hydrological effects of plantation forestry.
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size → vary → forestry
How researchers modelled pine afforestation in two Waikato catchments
Hoang and Hughes examined 3 hill-country catchments in the Waikato region, but only 2 underwent pasture-to-pine conversion.
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2 → model → conversion
Mangaotama was approximately 57% afforested, while Kiripaka was about 9% afforested
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Kiripaka → afforest → ?
Whakakai, which was covered by native forest, served as a control catchment without land-use change.
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which → cover → change
Whakakai had no land-use change and served only as a native-forest control.
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Whakakai → have → control
The modelled streamflow reductions applied to the 2 afforested catchments, with a larger estimated effect in Mangaotama than in Kiripaka.
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reductions → model → Kiripaka
The researchers used SWAT, a hydrological model that simulates how water moves through a catchment, to compare modelled streamflow under scenarios with and without land-use change.
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water → use → change
They evaluated the model against observed streamflow data from before and after the change.
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They → evaluate → change
According to the paper, SWAT reproduced the magnitude, timing and variation of streamflow at daily and monthly scales, allowing the researchers to investigate how much of the observed change could be associated with afforestation.
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much → accord → afforestation
The best-fit model results indicated a 29.7% reduction in mean annual streamflow in Mangaotama.
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results → indicate → Mangaotama
However, the estimated effect varied across the behavioural parameter sets, which represent different model configurations that met the researchers’ performance criteria.
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that → estimate → criteria
In Kiripaka, the modelled reduction was 4.7%.
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reduction → model → Kiripaka
These differences show that the estimated hydrological effect varied with the extent of afforestation and the parameters used in the model.
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effect → show → model
The researchers also identified limitations in the model’s representation of forest growth.
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researchers → identify → growth
SWAT produced a streamflow reduction immediately after planting, whereas observations showed a more gradual decline over approximately 7 years.
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observations → produce → years
The authors attributed this discrepancy to limitations in the model’s forest-growth representation.
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authors → attribute → representation
The model also overestimated post-change increases in streamflow in the other 2 catchments.
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model → overestimate → catchments
In Whakakai, it predicted an increase of 24%, compared with an observed increase of 11%.
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it → predict → %
In Kiripaka, it predicted an increase of 10.5%, while the observed change was approximately a 1% decline.
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change → predict → %
These differences are important when interpreting the precision of the modelled estimates.
Why pine plantations can reduce streamflow
The 2024 study identified increased evapotranspiration and reduced surface runoff as the main reasons for the modelled decline in streamflow.
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study → interpret → streamflow
Evapotranspiration combines water evaporating from land and vegetation with water absorbed by plants and released into the atmosphere.
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Evapotranspiration → combine → atmosphere
Replacing short pasture with trees changes how rainfall is intercepted, stored and returned to the atmosphere, potentially leaving less water available to reach streams.
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rainfall → replace → streams
The model also indicated a decline in groundwater flow in Mangaotama.
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model → indicate → Mangaotama
Across the behavioural parameter sets, the median reduction was 30%, and more than 75% of the sets showed a decline.
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% → show → decline
In Kiripaka, the median change was approximately −1%, indicating little overall change and considerable uncertainty about the direction of the effect.
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change → indicate → effect
The findings therefore suggest a clearer modelled groundwater decline in the more heavily afforested catchment, rather than a uniform response across the study area.
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findings → suggest → area
An earlier field study provides the observational evidence against which the later model was evaluated.
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model → provide → which
Published in the New Zealand Journal of Marine and Freshwater Research in 2020, “The stream hydrology response of converting a headwater pasture catchment to Pinus radiata plantation” examined 23 years of data from Mangaotama, a steep pastoral catchment in the western Waikato.
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response → publish → Waikato
The researchers reported that approximately 62% of the catchment had been planted with pine.
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% → report → pine
This differs slightly from the approximately 57% afforestation used in the 2024 modelling study.
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This → differ → study
Seven years after planting, annual runoff was 380 mm lower than the researchers predicted it would have been if the catchment had remained in pasture.
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catchment → predict → pasture
The 2024 model estimated a 29.7% reduction in mean annual flow for Mangaotama, broadly consistent with the approximately 32% reduction reported by the earlier field study.
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model → estimate → study
Because the 2020 observations were used to evaluate the later model, this agreement represents validation against field data, not independent confirmation from an unrelated catchment.
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agreement → use → catchment
The 2020 study also reported that peak flows fell by approximately 50%, while low flows appeared to decline by about 25%.
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flows → report → %
…and 15 more, not listed.
The Aporia
