Ecohydrology in space and time: functioning and changes in the streamflow generation of catchments

Abstract

Surface freshwater yield is a service provided by catchments, which cycle water intake by partitioning precipitation into evapotranspiration and streamflow. Streamflow generation is experiencing changes globally due to climate-and human-induced changes currently taking place in catchments. However, the direct attribution of streamflow changes to specific catchment modification processes is challenging because catchment functioning results from multiple interactions among distinct drivers (i.e., climate, soils, topography and vegetation). These drivers have coevolved until ecohydrological equilibrium is achieved between the water and energy fluxes.Therefore, the coevolution of catchment drivers and their spatial heterogeneity makes their functioning and response to changes unique and poses a challenge to expanding our ecohydrological knowledge. Addressing these problems is crucial to enabling sustainable water resource management and water supply for society and ecosystems.This thesis explores an extensive dataset of catchments situated along a climatic gradient in eastern Australia to understand the spatial and temporal variation in their ecohydrological functioning and to potentially identify responses to climate-and human-induced changes. To address this aim, the thesis has four major objectives: 1) to investigate how streamflow similarity varies according to the annual water and energy balances and to determine to what degree biophysical drivers of runoff explain the observed spatial streamflow variability;2) to determine the changes in the drivers of key streamflow characteristics across different regions and scales;3) to examine long-term ecohydrological changes in the water and energy balances of catchments and separate out the climate-and human-induced components; and 4) to investigate trends in baseflow and separate the contribution of precipitation, potential evapotranspiration and elevated atmospheric CO2 feedbacks with vegetation.Three hundred and fifty five catchments were analysed spanning a tropical to Mediterranean climatic gradient in the Australian east coast over multiple periods of time. An extensive dataset was compiled and analysed, composed of: daily streamflow and precipitation time series, monthly satellite-based time-series of vegetation, model-based soil moisture and temperature, and spatial data of land-cover, topography, soil properties, physiography, bioregions and human population density. To address the first objective, catchments were classified using streamflow signatures and modelling was used to relate the spectrum of water and energy balances (Budyko framework) to the Trancoso, R. (2016) PhD Thesis, The University of Queensland Ecohydrology in space and time ii drivers of runoff generation (Dunne diagram). To address the second objective a robust statistical modelling framework was applied to understand changes in streamflow drivers with regions and scales. In the third objective, the temporal displacement into the Budyko framework and its decomposition into climate a...