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Heterogeneity of Temperature in Rivers and Streams

Periodic Reporting for period 1 - HoTRiverS (Heterogeneity of Temperature in Rivers and Streams)

Reporting period: 2017-01-02 to 2019-01-01

River temperature is of great importance to the survival of a range of iconic fish species (eg. salmon, trout) which are highly intolerant of temperature extremes. However, the growing influence of climate change means that river organisms are increasingly threatened summer temperature extremes. There is widespread concern that high temperature events could drive fish mortality and population decline across Europe. Despite this worrying trend, some rivers contain alternating patterns of cool and warm water (known as river temperature heterogeneity) which may enhance their resilience to climate change. These rivers will be increasingly important in sustaining aquatic biodiversity in the future. However, little is currently known about how river temperature heterogeneity varies through space or time. There is therefore a pressing need for more information with a view to both furthering scientific understanding of rivers and helping to preserve critically-threatened river ecosystems for future generations.

The overall objective of the HoTRiverS (Heterogeneity of Temperature in Rivers and Streams) project was to gain a better understanding of the processes driving temperature heterogeneity in UK rivers and to quantify potential variability under future climatic or land-use change. This was achieved through a multi-disciplinary approach combining field data collection, drone-based aerial thermal imaging and computer-based water temperature modelling. We used these data to develop detailed computer models capable of accurately simulating spatial and temporal patterns in water temperature in key UK river locations. These models simulate river temperature through computing the precise transfers of mass (ie. water) and energy (ie. heat) that drive river temperature heterogeneity; as a result, they are able to shed light on the physical processes responsible for temperature heterogeneity in our chosen study locations.
Early in the project, we identified five rivers across the UK with contrasting climatological, hydrological and geological (and hence, river temperature) characteristics. A representative section of each of these rivers was subsequently instrumented with a range of sensors in order to characterise river temperature patterns and heat transfers to and from the river. Following installation of these instruments, we used a drone equipped with a thermal infrared (TIR) camera to map surface water temperature patterns in each of the rivers. Comparison of the river temperature data obtained from the drone against ‘real’ temperature data recorded by temperature sensors installed in the rivers indicated that the drone was not sufficiently accurate to allow for large-scale characterisation of water temperature heterogeneity. However, the drone-based data was nonetheless useful for mapping small ‘point’ cool water inputs that engendered small-scale changes in river temperature.

The next phase of the project consisted of mapping the environmental context of each river location. We combined GIS mapping with field data from the river corridor and drone-based topographic surveys to produce detailed databases of each river (necessary for the subsequent implementation of river temperature models). The drone-based topographic surveys (carried out in collaboration with the project partner, Marine Scotland Science) resulted in the development of a new methodology for assessing the impact of tree shading on river temperature patterns (detailed below); this new technique has the potential to substantially aid the management of river temperature extremes through the improved identification and modelling of tree shading effects.

These data collection phases of the project were followed by the development of river temperature models for each site. We conducted an exhaustive meta-analysis regarding the advantages and disadvantages of the various river temperature models available to identify the most appropriate one to use, subsequently publishing our findings in a scientific journal (Earth-Science Reviews). The results of this analysis will be particularly beneficial to river managers who are unsure as to the most appropriate approaches for modelling river temperature patterns. Our chosen river temperature model was subsequently supplied with data from the data collection phase of the project, and used to simulate temperatures in the study rivers; results of these analyses clearly show the role of a variety of different processes (eg. tree shading, evaporative heat loss, groundwater inflow) in generating patterns of stream temperature heterogeneity. We have not yet completed model implementation at all sites, so the final objective of the project (an assessment of the impacts of climate/land-use change on future river temperature heterogeneity) is still ongoing. However, work to achieve this aim is currently underway.

Results of these objectives were presented at a range of international conferences, including the EnviroDrones workshop (Darmouth College, USA; May 2017), HydroEco 2017 (Birmingham, UK; June 2017), Americal Geosciences Union fall meeting (New Orleans, USA; December 2017) and European Geosciences Union annual meeting (Vienna, Austria; April 2018).
Although potentially a setback, the unexpectedly poor performance of the drone-based TIR imagery actually resulted in an international collaboration to understand reasons behind the failure of miniaturised TIR cameras to produce accurate river temperature data; this collaboration has resulted in a journal manuscript (currently under review) which has deepened scientific understanding of the limitations of drone-based TIR imagery. The results of this analysis will be of use to river scientists and managers looking to use TIR to map river temperature patterns and has implications for the development of new TIR sensors and image processing techniques.

The project also resulted in the development of a novel methodology for quantifying the role of tree shading in reducing river temperature extremes. River scientists and managers across Europe are currently involved in the development of strategies for mitigating the impacts of climate change on river environments. Indeed, many of these strategies currently suggest the plantation of bankside tree cover for reducing solar warming of the river during the warmest part of the day. Results of our new methodology have the potential to enhance modelling efforts to simulate the exact temperature reduction that might be achieved through planting trees, and also to help devise ‘optimum’ tree plantation strategies detailing the locations where the plantation of bankside tree cover will have the greatest impact river temperature. This new method has resulted in two journal manuscripts, one of which is currently under review and one of which is in an advanced state of preparation.

Although work on the modelling of river temperature heterogeneity under future climate/land-use scenarios is still ongoing, it is expected that these analyses will result in a deepened understanding of how and where certain river environments may be more or less resilient to climate change; we hope that the results of these analyses will enable further improvements in the management of climate change-impacted rivers with a view to ensuring the continued enjoyment of these valuable environments by future generations.