EXPORT OF NUTRIENTS AND SUSPENDED SEDIMENT DURING A CYCLONE-MEDIATED FLOOD EVENT IN THE HERBERT RIVER CATCHMENT, AUSTRALIA

Changes in the river chemistry of the Herbert River (northern Queensla nd) during a flood event that followed Cyclone Sadie in January 1994 a re presented. Parallel data sets collected by AIMS and CSIRO were gene rally well correlated. Around the flood peak, concentrations of dissol ved inorganic nutrients declined to a minimum, whereas particulate nut rient concentrations increased to a maximum (particulate nitrogen, 120 0 mu g N L-1; particulate phosphorus, 225 mu g P L-1). Concentrations of dissolved organic nutrients varied erratically. Concentrations of s ilicate and potassium, pH and electrical conductivity varied inversely with discharge. Good correlations were observed between the concentra tions of particulates and concurrent discharge, with differing relatio nships existing during the rising and falling stages of the flood. It is estimated that this flood event resulted in the export of at least 600 t of N, 65 t of P and 100 000 t of suspended sediments over a peri od of six and a half days, with most transport (85%) occurring within the first two days. Particulate fractions of N (50%) and P (80%) const ituted the bulk of the nutrient flux. This study illustrates the poten tial for high nutrient exports during brief flood events from intensiv ely farmed agricultural land within tropical catchments.