Forestry and water yield - current knowledge and further work

Authors: T. Davie, B. Fahey
Publication: New Zealand Journal of Forestry, Volume N.Z.J.For. 2004, Issue N.Z.J.For. 49(4) 2005, pp 3-8, Jan 2005
Publisher: New Zealand Institute of Forestry

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Abstract: This article summarises the current state of knowledge with respect to forestry and water yield. The primary mechanism by which tall vegetation affects the water balance is through evaporation of intercepted rainfall, thereby reducing the amount of water available for runoff and streamflow. Generally trees have a high capability for interception due to a large leaf area and high aerodynamic roughness above the canopy. In experimental studies around New Zealand reductions in annual water yield of between 30-80% have been measured following afforestation of pasture. These figures are lower where afforestation has replaced scrub species.
The effect of afforestation on peak flows is considerable, particularly for small flood events although there is some evidence that storms with long return periods may also be substantially reduced following afforestation. There is considerable debate whether these effects can be seen at a large catchment scale.
The effect of afforestation on low flows is less well studied. Low flows are reduced following afforestation but it appears that in some cases low flows are affected to a lesser extent than annual yield. Public policy on forestry and water yield varies between regions. For example Tasman District Council and Environment Canterbury have land-use restrictions based on water yield arguments while the Otago Regional Council does not.

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