SELECTED CHALLENGES IN RUNOFF GENERATION RESEARCH IN FORESTS FROM THEHILLSLOPE TO HEADWATER DRAINAGE-BASIN SCALE

When faced with practical forest land management issues such as the im pacts of logging or forest conversion to other land uses, planners ide ally require a comprehensive understanding of within drainage basin hy drological processes to determine the most vulnerable areas to increas ed storm runoff and erosion. Land managers in particular need to know the source areas and magnitude of inputs to the storm hydrograph, in t erms of water quantity, sediment and solute transport; and the routing of such hydrographs from headwater to larger drainage basins. The lat ter includes an overall assessment at various scales of the impacts of forest disturbance and conversion on the water balance. This paper wi ll focus on runoff generation in terms of identifying the various path ways and source areas. Such aspects will be linked with the need for a more comprehensive effort towards the field testing of so-called 'phy sically based' models of runoff generation. Some of the controversial issues arising from the difficulties in reconciling results from hydro chemical investigations with complementary hydrometric studies will be highlighted. Subsequently, attention will be given to topographic-wet ness models, which have promising applications in forestland managemen t. In addition, alternative simple models for application at the catch ment scale will be assessed. The latter is in recognition that at smal ler scales, heterogeneity both in time and space of soil hydraulic pro perties demand a greater number of parameters in modelling. Such consi derations can even prove an obstacle in terms of the confident applica tion of 'physically bases models.