MIXING MODEL APPROACHES TO ESTIMATE STORM FLOW SOURCES IN AN OVERLANDFLOW-DOMINATED TROPICAL RAIN-FOREST CATCHMENT

Previous hydrometric studies demonstrated the prevalence of overland f low as a hydrological pathway in the tropical rain forest catchment of South Creek, northeast Queensland. The purpose of this study was to c onsider this information in a mixing analysis with the aim of identify ing sources of, and of estimating their contribution to, storm flow du ring two events in February 1993. K and acid-neutralizing capacity (AN C) were used as tracers because they provided the best separation of t he potential sources, saturation overland flow, soil water from depths of 0.3, 0.6, and 1.2 m, and hillslope groundwater in a two-dimensiona l mixing plot. It was necessary to distinguish between saturation over land flow, generated at the soil surface and following unchanneled pat hways, and overland flow in incised pathways. This latter type of over land flow was a mixture of saturation overland flow (event water) with high concentrations of K and a low ANC, soil water (preevent water) w ith low concentrations of K and a low ANC, and groundwater (preevent w ater) with low concentrations of K and a high ANC. The same sources ex plained the streamwater chemistry during the two events with strongly differing rainfall and antecedent moisture conditions. The contributio n of saturation overland flow dominated the storm flow during the firs t, high-intensity, 178-mm event, while the contribution of soil water reached 50% during peak flow of the second, low-intensity, 44-mm event 5 days later. This latter result is remarkably similar to soil water contributions to storm flow in mountainous forested catchments of the southeastern United States. In terms of event and preevent water the s torm flow hydrograph of the high-intensity event is dominated by event water and that of the low-intensity event by preevent water. This stu dy highlights the problems of applying mixing analyses to overland flo w-dominated catchments and soil environments with a poorly developed v ertical chemical zonation and emphasizes the need for independent hydr ometric information for a complete characterization of watershed hydro logy and chemistry.