Generating Two-Monthly Surface Water Images for the Murray-Darling Basin

Mapping surface water extent is an important step in estimating water volume within a catchment, which is needed for managing flood events, as well as water supply for human consumption, agriculture, and the environment.Satellite-based remote sensing technologies provide an affordable means of capturing surface water extent with reasonable spatial and temporal coverage suited to the purpose of water monitoring.A new multi-index method (MIM) has been developed for mapping surface water across the Murray-Darling Basin (MDB) based on Landsat surface reflectance data available in Digital Earth Australia.More than thirty years of two-monthly images of surface water extent across the whole MDB have been produced using this method, along with Water Observations from Space (WOfS) to fill in any gaps associated with cloud cover due to the different cloud masks used (this combined product is referred to as MIM_WOfS_max).This product is currently being combined with a DEM to produce estimates of water depth across the basin and can be used with hydrology models for assessing connectivity between waterbodies.This product can also be used to assess long-term trends of surface water extent across the basin, as well as used for seasonal or bi-monthly analysis.The methods used to produce the two-monthly WOfS and MIM products across the MDB are different and were based on utilizing the available cloud masks attached to the native products, and computational efficiency for generating the products.A comparison of the MIM and WOfS method shows that MIM identifies major perennial rivers and wetlands better than WOfS, as well as in some of the floodplain areas, while WOfS can identify more surface water in general areas where cloud cover still exists in the MIM product.A small misregistration (up to 30 metres in some areas) between the MIM and WOfS products was also found -possibly due to the different native spatial projections of the datasets.The MIM_WOfS_max product is used for identifying long-term and seasonal trends across the MDB.Over the 33-year time period (1988 to 2020) a reduction in surface water extent of 0.15% was found for the whole basin.The long-term trends of four sites were also investigated: two irrigated regions and two wetlands.They also showed a long-term reduction in surface water extent.The southern sites (one irrigated agriculture and one wetland) had regular annual flooding which gradually reduces in size through time, while the northern sites showed irregular flood patterns -with respect to the flood's timing and magnitude.Details of the method used and challenges associated with producing the bi-monthly images of surface water across the MDB are provided in the paper, as well as further discussions on some of the long-term and seasonal trends that can be derived from this product for the entire MDB and selected regions within.