Modelling canopy and litter interception in commercial forest plantations in South Africa using the Variable Storage Gash model and idealised drying curves

摘要

There remains a gap in the knowledge of both canopy and litter interceptionprocesses in forest hydrology and limitations in the models used torepresent them. In South Africa, interception is typically considered toconstitute only a small portion of the total evaporation and in some modelsis disregarded. Interception is a threshold process, as a certain amount ofwater is required before successive processes can take place. Therefore anerror or false assumption introduced in modelling interception willautomatically introduce errors in the calibration of subsequentmodels/processes. Field experiments to assess these processes, viz. canopy andlitter interception were established for the three main commercial forestrygenera in South Africa, namely Pinus, Acacia and Eucalyptus, which are described in a companion paper.Drawing on both field and laboratory data, the "Variable Storage Gash"model for canopy interception and an idealised drying curve litterinterception model were developed to represent these processes for SouthAfrican conditions. The Variable Storage Gash model was compared withthe original Gash model and it was found that it performed better than the original model in forests with highstorage capacities yet was similar to the original model in stands with alow storage capacity. Thus, the models developed here were shown to adequatelyrepresent the interception processes and provide a way forward for morerepresentative water resources planning modelling. It was found that canopyand litter interception can account for as much as 26.6% and 13.4% ofgross precipitation, respectively, and are therefore important hydrologicalprocesses to consider in forested catchments in South Africa. Despite thelimitation of both the Variable Storage Gash model and the idealiseddrying curve litter interception model being reliant on empiricalrelationships, their application highlights the importance of consideringcanopy and litter interception in water resources management and planning.