Export of nutrients and suspended sediment during a cyclone-mediated flood event in the Herbert River catchment, Australia

Abstract

Changes in the river chemistry of the Herbert River (northern Queensland) during a flood event that followed Cyclone Sadie in January 1994 are presented. Parallel data sets collected by AIMS and CSIRO were generally well correlated. Around the flood peak, concentrations of dissolved inorganic nutrients declined to a minimum, whereas particulate nutrient concentrations increased to a maximum (particulate nitrogen, 1200 µg N L^-1; particulate phosphorus, 225 µg P L^-1). Concentrations of dissolved organic nutrients varied erratically. Concentrations of silicate and potassium, pH and electrical conductivity varied inversely with discharge. Good correlations were observed between the concentrations of particulates and concurrent discharge, with differing relationships 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 100000 t of suspended sediments over a period of six and a half days, with most transport (85%) occurring within the first two days. Particulate fractions of N (50%) and P (80%) constituted the bulk of the nutrient flux. This study illustrates the potential for high nutrient exports during brief flood events from intensively farmed agricultural land within tropical catchments.