Hydrological changes in the Upper Murrumbidgee catchment and case study catchments
Date
2021
Authors
Wang, Yuqi
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The aim of my thesis is to assess the long-term variability of rainfall and to investigate the interactions between rainfall and streamflow and groundwater in tributaries in the Upper Murrumbidgee Catchment (UMC) with limited data sets, for developing and improving catchments management plans.
In this study, the long-term rainfall and streamflow variabilities are analysed, which is followed by the drought assessments to quantify the Millennium drought impacts on streamflow and the interaction between rainfall and streamflow. Over the UMC, rainfall and runoff have the high positive correlation which indicates the less disturbance in the UMC. During the 10-year period of Millennium Drought, the rainfall and runoff have been significantly lower than the long-term mean. The major contribution of annual rainfall declines during the period of 1997-2006 is the decrease of autumn and winter rainfall. And the 10-year period of drought is associated with the autumn rainfall decrease. The hydrologic changes in three sub catchments over UMC are consistent with the trend of climate. Lower rainfall in cool period (autumn and winter) leads to the lower runoff. It indicates the sub catchments over UMC is natural behaviour rather than other disruptions.
To facilitate better understanding and management of hydrologic process, two monthly water balance models are modified to simulate runoff over Australia and applied for the assessments of bushfire impact over the UMC. Two modified models (MXG and Mabcd) are robust to calibrate and validate streamflow over Australia with lower Mean Absolute Error and higher R square. The results of two modified monthly water balance indicate the robustness of the modified models than the original models in terms of streamflow prediction. The model application on the 2003 bushfire assessments indicates a significant increase in water yield in the first 3 years after the 2003 bushfire. After 3 years, the water yield did not respond quickly to variations in autumn rainfall. Therefore, disturbance-related changes in vegetation have likely contributed to recent reductions in water yield within the MDB. It is indicated that further yield reductions are likely in rivers for the UMC affected by the 2003 bushfire during the next two decades.
Meanwhile, the groundwater estimation is applied in this study to understand the groundwater change and interaction between surface water and groundwater. The groundwater storage estimation examined groundwater storage trend in 3 sub-catchments over the UMC over the last 40 years from the observed daily streamflow. The method is based on that groundwater storage normally is the major controller of base floe in a natural hydrological system. Therefore, groundwater storage can be measured if the streamflow data are available. By analysing the baseflow, it showed that the drainage time scale K was 37 days on average over the UMC. Estimated of groundwater storage was compared with the measured groundwater table in Orroral Valley indicated the similar groundwater trends.
In conclusion, this study chooses the Upper Murrumbidgee Catchment as the study area to investigate the interaction between rainfall, streamflow and groundwater. Results of statistical analysis and modelling performance, as well as the groundwater storage estimation show that the interaction between streamflow and groundwater are dominated by the change of climate variability. More investigation should be considered to uncover the dynamic of climatic and hydrologic process in this area.
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