THE RESPONSE IN WATER YIELD TO THE THINNING OF PINUS-RADIATA, PINUS-PATULA AND EUCALYPTUS-GRANDIS PLANTATIONS

The paired catchment method was used to test for the effects of thinni ng on the water yield in three afforested catchments in South Africa, namely, Biesievlei, Jonkershoek, 98% afforested with Pinus radiata (th ree thinnings); Westfalia catchment D, 100% afforested with Eucalyptus grandis (two thinnings); and Cathedral Peak CII that was 74% afforest ed with Pinus patula (one thinning). During and after two separate thi nnings, each of which removed roughly one third of the stems in a matu ring P. radiata plantation in the Biesievlei catchment, annual streamf low increased by between 10 and 71% (19-99 mm). These increases persis ted for three and two years after the thinning, respectively. A final thinning in the same catchment removed only 22% of stems at an age of 28 yr. The following years (1977 and 1978) were wetter than average, a nd reductions in annual streamflow of 26 and 55% were recorded in thes e two years. At Westfalia catchment D and Cathedral Peak CII, the hydr ological trends were entirely dominated by the rapidly declining strea mflow caused by the developing E. grandis and P. patula plantations re spectively. Any savings in water use that may have resulted from the t hinning of these plantations were insufficient to affect the downward trend in annual streamflow. Thinnings may have had a minor effect of d elaying or reducing the desiccation of these catchments but such effec ts could not be assessed due to natural variability and the limited re solution of the paired catchment method. The trends in total water yie ld from the catchments were generally mirrored in the dry season strea mflow, and there were no strong indications that thinning effects are linked to a particular season. (C) 1997 Elsevier Science B.V.