Paired catchments observations on the water yield of mature eucalypt and immature radiata pine plantations in Victoria, Australia

Abstract

Results from a paired catchment project measuring the water balance of native eucalypt forest and the change caused by conversion of one catchment to radiata pine are presented. The results are from 1975 to 1987 and from 1997 to 2006. The project measured the water yield from three small, steep, contiguous, forested catchments carrying similar native eucalypt vegetation in south-eastern Australia. One of the catchments was converted to radiata pine in 1980. Data from the untreated catchment showed that annual water yield was substantially a function of annual rainfall, and that annual rainfalls below 900 mm generated little runoff. The conversion to radiata pine led to an increased water yield of up to 300 mm per annum immediately after clearing. This response has declined but in general the runoff from the pine catchment is still higher than that of the eucalypt catchment except for “drought” years. Combination of these results with other Australian results gave a simple model of radiata pine water use (relative to the native eucalypt vegetation) as a function of age and annual rainfall. Older trees and low-rainfall years are associated with a higher water use by the radiata pine than the native forest, but conversely younger trees and higher rainfalls give lower water use. Extrapolation of the results to grassland showed that reforestation of grassland sites would lead to diminished yields. The relationships generated may be useful for estimation of the net change in streamflow caused by afforestation with radiata pine in larger catchments.