The Value of Long-term Gauged Watershed Experiments in Forest Hydrology: lessons from the South African Experience.

English Abstract:

Much of what we know about the hydrology effect of forestry has come from gauged watershed experiments conducted over the last 80 years. These experiments have been enormously useful, but there are many critics who consider the paired-catchment experiment to be an out-dated technology. Their argument is based on many valid points: catchment experiments are costly, take a long time, do not provide a detailed understanding of the individual hydrological processes (“catchments are a black box”), are empirical and results are difficult to transfer (size or extrapolation issues). This paper argues that despite the problems there is a need for long-term watershed studies, and uses the South African experience to illustrate the case.

• Gauged watersheds are the ultimate integrators of all hydrological processes. The catchment experiments therefore provide the definite answer to the study of changes in factors that influence the hydrology at a particular location. 
• Catchment experiments are the proper setting for hydrological process studies, as the catchment provides the boundaries and limits within which process studies can be best tested and extended.
• Long term data from gauged watersheds provide a firm baseline for modelling studies, which are being used more and more, but are much more valuable if set within the bounds provided by a gauged watershed.
• As hydrological questions change, so the paired-catchment experiments can be adjusted to answer new questions.

• Long-term catchment experiments provide an historic reference which is needed now, and will be needed in the future, to investigate questions relating to changing climate, land use effects, and other questions which are yet to arise.