Simulation of Gauged and Ungauged Streamflow of Coastal Catchments across Australia

We estimated the mean annual discharge to Australian coastal regions through the application of the GR4J daily model to the gauged and ungauged catchments. The estimated mean annual streamflow varied significantly from one drainage division to another (Table 5). The mean annual discharge from all drainage divisions, including the gauged and ungauged catchments, was 419,950 gigalitres (GL), with an ungagged catchment contribution of 232,200 GL, representing 55% of the total. Our findings compare well with the National Land and Water Resources Audit [66] estimate of 387,184 GL. The Murray–Darling Basin, Australia’s food bowl, has only about 1% catchment area ungauged (Table 3). It is also a very highly managed system, represents 55% of Australia’s water use (http://www.bom.gov.au/water/nwa/2020/mdb/regiondescription/geographicinformation.shtml, accessed on 12 November 2023), and only a small proportion of the streamflow reaches the ocean. Estimated discharge from the ungauged catchments was not significant (Table 5). The mean annual streamflow from the NEC drainage division was estimated at 58,470 GL, with an ungauged area contribution of 22,800 GL. This finding compares well with the CSIRO [67] runoff estimates. Estimated streamflow from ungauged areas of SAG and SWC were not significant compared to that of gauged areas. There are no gauging stations, and very limited rainfall gauges exist within SWP. In this drainage division, stream networks are also not well-formed; therefore, estimates of streamflow are very preliminary. The estimated average annual streamflow from gauged and ungauged areas of SWC was 3480 GL (Table 5), which compares fairly with gauged streamflow estimates of 4700 GL [68]. The difference could be due to different periods of data and recent below-average rainfall in Western Australia [69]. The ungauged areas in the PG and NWP were 16% and 24%, respectively. However, there is only one streamflow gauging station in NWP (Figure 1). Estimates of mean annual runoff from gauged catchments within these two drainage divisions range from 2% to 9% (Figure 10). Thius compares well with the Pilbara water resources assessment study covering part of these two drainage divisions [70]. The TTS and CC drainage divisions have the largest proportion of ungauged areas—43% and 51%, respectively (Figure 1, Table 3). Mean annual runoff from the TTS drainage division spatially varies significantly from 2% to approximately 45% (Figure 10), which compares well with CSIRO [71] finding of 3–40% of all gauged catchments. In the CC drainage division, mean annual runoff was slightly lower than that of TTS and ranged between 3–60%. However, estimates of runoff from ungauged areas seemed to be high compared to their gauged counterparts, ranging from 15 to 45% (Figure 10), probably due to an oversimplified application of the GR4J model. The mean annual streamflow from CC drainage division of 109,440 GL compared well with the CSIRO [71] of 90,000 GL.