HYDROLOGIC PATHWAYS AND STORMFLOW HYDROCHEMISTRY AT SOUTH CREEK, NORTHEAST QUEENSLAND

Earlier investigations at South Creek in northeastern Queensland estab lished the importance of overland flow as a hydrologic pathway in this tropical rainforest environment. Since this pathway is 'fast', transm itting presumably 'new' water, its importance should be reflected in t he stormflow chemistry of South Creek: the greater the volumetric cont ribution to the stormflow hydrograph, the more similarity between the chemical composition of streamwater and of overland flow is to be expe cted. Water samples were taken during two storm events in an ephemeral gully (gully A), an intermittent gully (gully B) and at the South Cre ek catchment outlet; additional spot checks were made in several poorl y defined rills. The chemical composition of 'old' water was determine d from 45 baseflow samples collected throughout February. The two even ts differed considerably in their magnitudes, intensities and antecede nt moisture conditions. In both events, the stormflow chemistry in Sou th Creek was characterized by a sharp decrease in Ca, Mg, Na, Si, Cl, EC, ANC, alkalinity and total inorganic carbon. pH remained nearly con stant with discharge, whereas K increased sharply, as did sulfate in a n ill-defined manner. In event 1, this South Creek stormflow pattern w as closely matched by the pattern in gully A, implying a dominant cont ribution of 'new' water. This match was confirmed by the spot samples from rills. Gully B behaved like South Creek itself, but with a dampen ed 'new' water signal, indicating less overland flow generation in its subcatchment. In event 2, which occurred five days later, the initial 'new' water signal in gully A was rapidly overwhelmed by a different signal which is attributed to rapid drainage from a perched water tabl e. This study shows that stormflow in this rainforest catchment consis ts predominantly of 'new' water which reaches the stream channel via ' fast' pathways. Where the ephemeral gullies delivering overland flow a re incised deeply enough to intersect a perched water table, a delayed , 'old' water-like signal may be transmitted.