The effects of climate and geomorphology on bankfull conditions in subtropical Australian streams

Southeast Queensland (SEQ), Australia. Bankfull conditions are used to derive stream properties, such as bankfull discharge and interval of recurrence, that are essential for hydrologic modelling and channel restoration design. Bankfull conditions are well studied in temperature regions in the northern hemisphere, but elsewhere, data are sparse and bankfull applications are underdeveloped. We hypothesised that subtropical regions in the southern hemisphere, such as parts of eastern Australia, are different hydrologically to well-studied areas due to more variability in hydroclimate and different geophysical settings. We assessed how bankfull properties such as bankfull discharge and recurrence interval are affected by hydroclimatic and geomorphic contexts in subtropical areas in the southern hemisphere. We developed models to predict bankfull discharge based on catchment size while accounting for climatic and geomorphology. Additionally, we evaluated two analytical methods of bankfull stage identification compared to a qualitative, visual assessment. In general, we found that hydroclimate exerted little influence on bankfull properties in (SEQ). We also found that geomorphology significantly relates to recurrence intervals but was not an explanatory factor for bankfull discharge. Modelling of bankfull discharge demonstrated that including rainfall as a predictive variable did not substantially improve estimations, which climatic regionalization improved predictions in some areas, but not in others. Furthermore, one analytical method of bankfull identification, the bench index method, produced similar bankfull stage estimates to the qualitative bankfull identification. Overall, these results suggest bankfull discharge in rivers and streams of subtropical regions in the southern hemisphere is expressive of similar hydrologic characteristics as rivers and streams in temperate climates even with different geomorphological conditions. Our study also demonstrates the bench index method could provide an efficient and effective surrogate for identifying bankfull conditions. • Hydroclimate and geomorphology impacts were assessed in subtropical rivers and streams. • Annual recurrence interval of bankfull discharge were different for different geomorphological categories. • The addition of rainfall as an explanatory variable in bankfull discharge models did not improve their predictive power. • Bench index method of bankfull discharge identification produces similar bankfull estimates to qualitative identification.