Watercourse hydrology: An interesting dilemma for the Gwydir and Warrego-Darling LTIM projects

The Gwydir and Warrego (Western Floodplain) River watercourses consist of a mosaic of small, incompetent channels, broader flow pathways and shallow wetland depressions. Both systems have similarities: they are near-terminal with only larger flood event flowing through them; they are very flat (slopes > 1:1000), small elevation changes (0.1 m) can mean a substantial change in inundation; land management activities can impact flow paths substantially; vegetation communities are largely intact. For the Commonwealth Environmental Water Office Long Term Intervention Monitoring (LTIM) projects, some indicators require calculation of inundation areas and volumes. In these low, flat systems traditional approaches to inundation modelling are not successful. We have developed an approach that incorporates data from multiple sources to create and verify inundation models for both of these systems. In the Gwydir, dense vegetation, standing water and super flat landscape has limited the value of Lidar data for Digital Elevation Model (DEM) creation and inundation modelling. Dense reed mats present during Lidar capture mean that these areas appear to stand above the surrounding landscape and hence shed rather than collect water. In this system, an approach linking existing vegetation mapping to watercourse area and depth along with inundation information from satellite imagery was used to develop an inundation model. On the Western Floodplain of the Warrego, less dense vegetation has meant that Lidar was of value for inundation mapping, verified by water level loggers and satellite imagery. This paper documents our struggles to produce the best possible inundation maps for these systems. We show you what worked and what didn’t.