Unmanned Aerial Vehicle (UAV)-Based Thermal Infra-Red (TIR) and Optical Imagery Reveals Multi-Spatial Scale Controls of Cold-Water Areas Over a Groundwater-Dominated Riverscape

Abstract

The forecast of warmer weather, and reduced precipitation and streamflow under climate change makes freshwater biota particularly vulnerable to being exposed to temperature extremes. Given the importance of temperature to regulate vital physiological processes, the availability of discrete cold-water patches (CWPs) in rivers to act as potential thermal refugia is critical to support freshwater ecosystem function. Being able to predict their spatial distribution at riverscape scales is the first step to understanding the capacity to maintain thermal refuges and to inform future river management strategies. Novel Unmanned Aerial Vehicle (UAV)-based Thermal Infra-Red (TIR) imagery technologies provide an opportunity to assess riverscape stream temperature. On the example of a 50 km linear length of the groundwaterdominated Upper Ovens River (Australia), this study presents a methodology addressing critical challenges in UAV-based TIR and optical data acquisition, processing, and interpretation. Our methodological approach generated 49 georeferenced highresolution TIR and optical orthomosaicked imagery sets. The imagery sets allowed us to identify river-length longitudinal patterns of temperature and to detect, characterize, and classify 260 CWPs. Both stream and CWPs temperatures increased but presented considerable variability with downstream distance. CWPs were non-uniformly distributed along the riverscape, with emergent hyporheic water types dominating, followed by deep pools, shading, side channels, and tributaries. We found associations between CWPs and key physical controls including land use, riparian vegetation, longitudinal and lateral CWP location, and CWP area size, illustrating processes acting at multiple spatial scales. This study provides a basis for future works on the thermal associations with

Applications of Unmanned Aerial Systems (UASs) in Hydrology: A Review

“…Thirdly, shallow groundwater temperature can be similar to the mean annual surface temperature, weakening the TIR images contrast. Casas-Mulet et al [161] generated high-resolution TIR and RGB orthomosaics to characterize cold water patches associated with groundwater inputs over a 95 km-long river section using simultaneous UAS flights. This method allowed to identify riverscape spatial patterns of temperature and to detect and classify these patches.…”

Environmental threats and conservation implications for Atlantic salmon and brown trout during their critical freshwater phases of spawning, egg development and juvenile emergence

“…The most likely scenarios for the main distribution areas of Atlantic salmon and brown trout include higher temperatures, wetter winters, dryer summers and more extreme events of flooding and drought (ICES, 2017b; IPCC, 2007, 2014) affecting all components of the freshwater ecosystem (Wilby et al, 2006). Models and predictions of the consequences of climate change have focussed on the adaptability of Atlantic salmon and brown trout to rising temperatures during different life stages (Casas‐Mulet et al, 2020; Sternecker et al, 2014). The likelihood for future losses of salmonid populations is considered high, especially in their lower latitude distribution range.…”

Assessing Stream Thermal Heterogeneity and Cold-Water Patches from UAV-Based Imagery: A Matter of Classification Methods and Metrics

“…UAVs have been extensively used to detect thermally differentiated patches in rivers, with a particular focus on biologically relevant cold-water patches (CWPs). Here, we refer to CWPs as discrete areas of cooler water in relation to the average water temperature in the mainstream channel [24]. CWPs are most relevant in summer as potential refugia during critically warm periods because cooler waters provide an oxygen-richer environment that may help the survival of mobile aquatic organisms such as fish [25][26][27].…”