Streamflow response toPinusplantation harvesting: Canobolas State forest, southeastern Australia

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Streamflows were measured in two Pinus radiata plantation catchments and one native eucalypt forest catchment in Canobolas State forest from 1999 to 2007. In 2002/2003, clearfall harvesting of 43·2 and 40·3% of two plantation catchments occurred, respectively. Water yields increased by 54 mm (52%), 71 mm (35%) and 50 mm (19%) in the first three years post-harvest in treated catchment A and by 103 mm (118%), 157 mm (82%) and 119 mm (48%) in treated catchment B relative to the native forest control catchment. In the fourth post-harvest water year annual rainfall was only 488 mm, which resulted in negligible run-off in all catchments, regardless of forest cover. In both plantation catchments, monthly streamflows increased significantly (p = 0·01, p < 0·001) due to a significant increase in baseflows (p < 0·001) after harvesting. Monthly stormflows were not significantly affected by harvesting. Flow duration curve analyses indicated a variable response between the two plantation catchments. Treated catchment A was converted from an ephemeral stream flowing 42% of the time pre-harvest to a temporary stream flowing 82% of the time post-harvest. These changes occurred throughout all seasons of the year but were most pronounced during summer and autumn when baseflows were maintained post-harvest but were not observed under native forest or mature pine plantations. By contrast, flow duration increased in treated catchment B from 12% of the time pre-harvest to 38% of the time post-harvest with the greatest changes measured during the winter and spring months when streamflow would normally occur under native forest conditions. These observations have important implications for the development of models of plantation water use to be utilized in water resource planning in Australia.

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