Changes in catchment runoff after harvesting and burning of a Pinus caribaea plantation in Viti Levu, Fiji

Abstract

A catchment study was conducted between January 1990 and April 1992 to quantify the impact of harvesting a mature Pinus caribaea plantation forest on total runoff, baseflow recession, peak discharge, stormflow volume and sediment concentration. As rainfall differed between pre- and post-harvesting periods, a streamflow prediction model (HBV) and statistical methods were used to evaluate observed changes and to test if these were significant. Most runoff occurred as baseflow (64–74% of total runoff). Baseflow increased significantly after harvesting, from 10% to 16% of rainfall. The minimum daily flow increased by 70% after harvesting, in spite of lower rainfall in that period. Stormflow volumes and peak discharges also increased significantly as a result of Horton overland flow generation on new road and log-landing surfaces and a higher wetness of the catchment due to lower evapotranspiration. Stormflow volume doubled from 2.7% to 5.5% of rainfall after harvesting. The combined effect amounted to estimated increases in flows from 252 mm under forest to 580 mm after harvesting for the 324-days period of 1990 (total rainfall of 1613 mm) and from 90 mm to 194 mm for the drier 309-days period in 1991/1992 (total rainfall of 992 mm). Sediment concentrations increased significantly during stormflow events after harvesting, but not under baseflow conditions. Harvesting of the pine plantation therefore increased stormflow, baseflow and total water yield significantly. Stormflow water quality was affected negatively, though, through increased sediment concentrations. These aspects are important for water resources management considerations related to plantation forestry in areas with a distinct dry season.