Continental differences in the variability of annual runoff-update and reassessment

Continental differences in the variability of annual runoff were reassessed using an expanded precipitation database and an improved methodology for allocating precipitation and runoff stations to Köppen climate zones. Application of the new Köppen zone allocation methodology resulted in changes in Köppen zone for 44% of runoff stations. Statistical analysis of the data, stratified by continent and Köppen climate zone, confirmed the presence of continental differences in the variability of annual runoff and the conclusions of previous research. Variability of annual runoff for temperate Australia, arid southern Africa and temperate southern Africa was found to be higher than that for other continents across the same climate zones. In this analysis a more consistent hemispheric difference in the variability of annual runoff was observed, with variability being higher in the Southern hemisphere. Previously suggested causes of the continental differences in the variability of annual runoff were also confirmed in this re-analysis. Statistically significant continental differences in the variability of annual precipitation were observed and the influence of large-scale circulation systems on precipitation variability is more apparent in this analysis than in previous work. Continental differences in the variability of annual precipitation could not account for all of the continental differences in the variability of annual runoff. The distribution of evergreen and deciduous vegetation in temperate regions remains a potential cause of the runoff variability differences. Additional potential causes identified are: continental differences in the percentage of forested catchment area, and continental differences in mean annual daily temperature range.