Terrigenous runoff in the tropics observed with a Scanning Low Frequency Radiometer

Monsoon rains in the Australian tropics bring flood water and ground-water runoff into the Great Barrier Reef Lagoon. One of the outstanding problems for the management of the World Heritage Great Barrier Reef Marine Park is to understand how the terrestrial runoff affects the corals throughout the reef. This is a sensitive area, not only because of the conservation and tourism value, but also because the coastal wetlands are breeding grounds for commercial fish species, and there is fairly intensive agriculture along the coastal plains. A prototype Scanning Low Frequency Microwave Radiometer (SLFMR) has been flown in the vicinity of the mouth of the Herbert River, and an associated campaign of in situ, CTD profiles and current meter measurements has been carried out to investigate the dynamics of the river plume. The results show that there is a net northwards drift of the surface water in the Lagoon close to the mainland, which diverts the river water towards the north. The river plume depends strongly on the tidal phase it with the tide. Estimates are made of the eddy diffusion coefficient for the mixing of the low salinity estuary water with sea water in the main Lagoon area. Under these conditions the estuary water remains within a few kilometres of the coast and drifts northwards with the near-shore wind-driven current. This paper shows that the present SLFMR, with its spatial resolution of about 1 km and salinity resolution of about 1 psu, is suitable for mapping salinity gradients in these conditions where there is a fairly high influx of fresh water with high gradients in salinity.