Tracing hydrological connectivity

Abstract

Understanding how different structural and functional elements of a catchment connect together to generate spatially and temporally variable fluxes of water, sediment and nutrients is fundamental for gaining a holistic comprehension of catchments’ hydrological behaviour. However, advancements in hydrological connectivity research still suffer from technical limitations inherent to the (i) investigation of connectivity at large scales (e.g. catchment scale), (ii) identification of intra-catchment heterogeneous landscape structures and connectivity pathways, and (iii) characterisation of the hydrological processes occurring at small scale around transition zones (e.g. the riparian zone). In this thesis, some of these challenging aspects are addressed by investigating hydrological connectivity at different spatio-temporal scales through two innovative, multidisciplinary approaches – namely terrestrial diatoms (unicellular, eukaryotic algae) and ground-based thermal infrared imagery (TIR). This thesis is wired around five studies that have explored the potential for these innovative avenues to overcome the prevailing status quo in hydrological connectivity research. Chapter 2 focuses on terrestrial diatoms’ ecological behaviour and sensitivity to environmental factors. Understanding the physiographic controls on terrestrial diatom communities is fundamental for advancing their employment as a tracer of hydrological connectivity at the catchment scale. Chapter 3 explores the potential for ground-based TIR imagery to provide instantaneous mapping of stream water mixing and mixing dynamics at confluence-scale. Cross-sectional variability in surface temperature observed through TIR imagery is found to reflect the in−stream variability of temperature and chemistry. Chapter 4 reports a technical study on the application of ground-based TIR imagery for mapping surface-saturation dynamics in the riparian zone. The results of a repeated mapping of surface saturation through ground-based TIR imagery are presented in Chapter 5. In this chapter, the seasonal dynamics of surface saturation in seven different riparian areas are characterised. The development of surface-saturation in the different areas is found to be influenced by heterogeneous catchment features such as riparian areas elevation, local riparian morphology (i.e. width of the riparian areas) and the presence or absence of groundwater exfiltration points. These heterogeneous catchment features also influence the shape of the surface-saturation vs. outlet baseflow relationship. In Chapter 6, the previously explored riparian surface-saturation dynamics are related to streamflow contributions from the corresponding stream reaches. The observed relationships provide information on the degree of hillslope-riparian-stream connectivity of different portions of the catchment. Overall, the studies presented in this thesis show how novel, multidisciplinary approaches and techniques can provide new information on hydrological connectivity across different spatial and temporal scales of investigation. An improved understanding of how connectivity is established within the catchment will eventually lead to a better management of our hydrological resources, increasingly put under pressure by climate change and anthropogenic activities.