An empirical, comparative model of changes in annual water yield associated with pine plantations in southern Australia

Abstract

SummaryResults from three Australian multiple catchment projects that examine the change in water yield on conversion from native eucalypt forest to radiata pine were combined. The projects were located in southern Australia near Myrtleford (Victoria), Daylesford (Victoria) and Bathurst (NSW). From these data a model of change in water yield relative to the eucalypt forest was derived using age and annual rainfall as independent variables. This model was extended to derive estimates of change in water yield from grassland sites converted to radiata pine by using the model of Zhang et al. (2001) to estimate the difference in water use between native forest and grassland. The results showed (and quantified) that conversion of native forest to radiata pine usually increases water yield but conversion of grassland sites to radiata pine usually decreases yields. Increased water yields are associated with young pines and high rainfalls, while decreased water yields are associated with older pines and low rainfalls. The models were tested using data from radiata pine plantations planted on grassland sites in Tumut (NSW), Kilmore (Victoria) and ‘fynbos’ in South Africa. In general the models performed reasonably well in estimating sequences of changes in flows. Estimates of total water yield were less accurate. Heavy or frequent thinning may be a source of change that may need to be accounted for separately if the details of this are known. The derived models may be useful in estimating the comparative changes of flow associated with the development of multiple blocks of radiata pine plantations on catchments. This can be programmed in a spreadsheet.