Soil water storage changes in a small headwater catchment in the central North Island of New Zealand following afforestation with Pinus radiata

The effect of afforestation of pasture with Pinus radiata D. Don on soil water storage was measured in a small headwater catchment (8.7 ha) located in the Central North Island of New Zealand, as part of a larger study of land use effects on streamflow. Soil water content of the pumice soil was measured on an approximately monthly basis at 10 cm intervals to approximately 200 cm depth to the ignimbrite rock layer, using a neutron probe. Measurements occurred between stand ages 2-8 years (1975-1982) during which time the leaf area index of the forest was measured annually. Soil water storage in the profile varied between relatively narrow limits, with annual mean storage ranging from 766 to 878 mm even though annual rainfall and streamflow varied considerably between years. Soil water storage was more variable on a monthly basis, decreasing to approximately 630 mm in the entire profile during the summer drought in 1978 when streamflow almost ceased. The pumice soil at Puruki stored sufficient water in the profile to meet annual evapotranspiration requirements of P. radiata, including during a summer drought when water stored below 100 cm depth was utilised. The root systems of P. radiata trees can extend to below 3 m depth when growing on pumice soils in the Central North Island and plantations will therefore not be water limited unless roofing depth is impeded or annual rainfall decreases substantially in future owing to climate change.