THE CONTRASTING EFFECTS OF WILDFIRE AND CLEARFELLING ON THE HYDROLOGYOF A SMALL CATCHMENT

A wildfire in an afforested research catchment presented the rare oppo rtunity to compare the hydrological effects of wildfire with the effec ts of clearfelling in the same catchment in the Jonkershoek Valley, in the south-western Western Cape Province of South Africa. The timber p lantation, which occupies 57% of the 2 km(2) catchment, had been clear felled and re-planted to Pinus radiata roughly five years before the f ire. The effects of the two treatments on total flow, stormflow and qu ick-flow volumes, peak discharge and storm response ratio were determi ned by means of multiple regression analysis, employing the dummy vari able method to test for the significance of treatments. Both clearfell ing and wildfire caused significant increases in all the stream-flow v ariables analysed. But the clearfelling effect was dominated by large increases in total flow (96% over three years), of which storm-flow an d quick-flow volumes formed only minor parts. After the wildfire, by c ontrast, increases in total flow were small (12%) but the storm flow i ncreases were three- to fourfold in the first year and roughly double in the second year. The wildfire caused fire-induced water repellency in the soils which led to overland flow on mid-slope sites, where soil infiltrability normally far exceeds local rainfall intensities. It is argued that these results support the hypothesis that stream-flow gen eration processes were changed by the wildfire in that overland flow m ade a direct contribution to storm flows, but that clearfelling had no such effect. (C) 1997 by John Wiley & Sons, Ltd.