Is rainfall intensity significant in the rainfall-runoff process within tropical rainforests of northeast Queensland? The Hewlett regression analyses revisited

Following the statistical analyses of long-term rainfall-runoff records from research basins in humid temperate latitudes, Hewlett and co-workers extended the global challenge to disprove their findings that rainfall intensity was non-significant. This paper responds to Hewlett's challenge as no preceding analyses have involved forested basins in a tropical cyclone-prone area. Based on a 7 year rainfall-runoff record, quickflow (QF), peak flow (QP) and quickflow response ratios (QRR) were regressed as dependent variables against rainfall parameters (intensity, P-i, amount, P), storm duration, D and antecedent flow, I. These data sets were categorised into total streamflow (Q) classes and stratified into three seasons, (monsoon, post-monsoon and dry) for forested and cleared catchments. Where rainfall variable collinearity met acceptable levels, the addition of Pi to regression models including P, D, I contributed up to 9% and 66% of the respective variations in quickflow and peak flow. For the highest Q storm classes (monsoon), P-i alone accounted for up to 67% and 91% of the variation in Q(F) and Q(P) respectively and was the dominant influence on QP for all seasons. The very high rainfall intensities experienced in the monsoon season is a causal factor why these results differ from those of other research drainage basins. Surprisingly, P-i continued to have a significant influence on Q(F) for dry season classes when less-intense rainfall occurs. Further the results were similar for both catchments across all seasons. P was the dominant independent variable affecting Q(F) above a threshold Q of 50 mm (monsoon), as rainfall contributes directly to saturation overland flow and return flow under saturated conditions. Further although QRR increased with increasing Q for each season, the regression results for that parameter were poor possibly due to the non-linearity of the rainfall-runoff relationship. Copyright (C) 2010 John Wiley & Sons, Ltd.