Abstract
Understanding stream thermal heterogeneity patterns is crucial to assess and manage river resilience in light of climate change. The dual acquisition of high-resolution thermal infrared (TIR) and red–green–blue-band (RGB) imagery from unmanned aerial vehicles (UAVs) allows for the identification and characterization of thermally differentiated patches (e.g., cold-water patches—CWPs). However, a lack of harmonized CWP classification metrics (patch size and temperature thresholds) makes comparisons across studies almost impossible. Based on an existing dual UAV imagery dataset (River Ovens, Australia), we present a semi-automatic supervised approach to classify key riverscape habitats and associated thermal properties at a pixel-scale accuracy, based on spectral properties. We selected five morphologically representative reaches to (i) illustrate and test our combined classification and thermal heterogeneity assessment method, (ii) assess the changes in CWP numbers and distribution with different metric definitions, and (iii) model how climatic predictions will affect thermal habitat suitability and connectivity of a cold-adapted fish species. Our method was successfully tested, showing mean thermal differences between shaded and sun-exposed fluvial mesohabitats of up to 0.62 °C. CWP metric definitions substantially changed the number and distance between identified CWPs, and they were strongly dependent on reach morphology. Warmer scenarios illustrated a decrease in suitable fish habitats, but reach-scale morphological complexity helped sustain such habitats. Overall, this study demonstrates the importance of method and metric definitions to enable spatio-temporal comparisons between stream thermal heterogeneity studies.
Highlights
Introduction published maps and institutional affilThe role of stream temperature as a key aspect of rivers’ ecological functioning has received increasing attention in the last two decades [1,2,3]
We considered that the unmanned aerial vehicles (UAVs)-based thermal infrared (TIR) caption of surface water temperatures was representative of the measurements across the whole water depth due to the following reasons: (i) our surveys were carried out during the lowest flow period, water was still running through the system so hydraulic mixing was ensured, and (ii) several spot temperature measurements in selected deep pools illustrated almost identical temperatures across the water column [60], which supported our assumption of no thermal stratification
The distribution assessment of cold-water patches (CWPs) revealed that this variability and metric sensitivity could be strongly linked to different types of reach morphology, with braided rivers being able to support bigger and cooler
Summary
Introduction published maps and institutional affilThe role of stream temperature as a key aspect of rivers’ ecological functioning has received increasing attention in the last two decades [1,2,3]. Water temperature is a crucial player in all trophic levels and stages of biological organization in rivers. Water temperature governs the biochemical processes and conditioning of all freshwater life stages [4,5,6,7,8], highlighting the importance of understanding stream thermal heterogeneity. The predicted increase in water temperatures as a result of climate change [9]. Will likely grow the relevance of integrating the assessment and monitoring of stream temperature variability in future river management, in identifying thermally resilient areas to changing climate [10,11,12,13,14]. Stream thermal heterogeneity is governed by a complex mosaic of interconnected habitats in rivers [15,16,17,18]. Topographic, and hydrological controls drive stream temperatures at the air–water interface [1,2,3], multi-spatial scale factors such as iniations
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