Stream turbidity responses to storm events in a pristine rainforest watershed on the Coral Coast of southern Fiji

A significant power function demonstrates that turbidity (T) is a suitable proxy for total suspended solids (TSS) for turbidity measurements above 5 NTU, with TSS=0.930T1.111 (r=0.98, P<0.001). Over the study period, 10 individual storms 11.2-120.1 mm in size produced a 'significant turbidity response' (STR) in the Votua Creek. Rainfall parameters (totals and intensities) showed positive linear relationships (r=0.72-0.94) with stream turbidity parameters (mean, maximum, duration), whilst relationships of similar strength (r=0.76-0.98) were also derived between stream flow depth and turbidity. This implies that for small rainforest watersheds in Fiji, rainfall parameters offer no substantial disadvantage over flow as predictors of stream sediment responses to major storms. Event-based analysis revealed that negative (anticlockwise) hysteresis is a typical flow-turbidity pattern for STR events. Negative hysteresis is produced when secondary episodes of renewed (heavy) rainfall occur after maximum intensity, in the later phase of storm events. Tropical Cyclone Mick in December 2009 generated the largest flood and the greatest turbidity response (Tmax=1021 NTU, Tmean=207 NTU). This concurs with earlier work confirming that tropical cyclones are the most important events for sediment transport in Fiji stream networks.