Quantifying thermal refugia connectivity by combining temperature modeling, distributed temperature sensing, and thermal infrared imaging

Abstract

Abstract. Watershed-scale stream temperature models are often one-dimensional because they require fewer data and are more computationally efficient than two- or three-dimensional models. However, one-dimensional models assume completely mixed reaches and ignore small-scale spatial temperature variability, which may create temperature barriers or refugia for cold-water aquatic species. Fine spatial- and temporal-resolution stream temperature monitoring provides information to identify river features with increased thermal variability. We used distributed temperature sensing (DTS) to observe small-scale stream temperature variability, measured as a temperature range through space and time, within two 400 m reaches in summer 2015 in Nevada's East Walker and main stem Walker rivers. Thermal infrared (TIR) aerial imagery collected in summer 2012 quantified the spatial temperature variability throughout the Walker Basin. We coupled both types of high-resolution measured data with simulated stream temperatures to corroborate model results and estimate the spatial distribution of thermal refugia for Lahontan cutthroat trout and other cold-water species. Temperature model estimates were within the DTS-measured temperature ranges 21 % and 70 % of the time for the East Walker River and main stem Walker River, respectively, and within TIR-measured temperatures 17 %, 5 %, and 5 % of the time for the East Walker, West Walker, and main stem Walker rivers, respectively. DTS, TIR, and modeled stream temperatures in the main stem Walker River nearly always exceeded the 21 ∘C optimal temperature threshold for adult trout, usually exceeded the 24 ∘C stress threshold, and could exceed the 28 ∘C lethal threshold for Lahontan cutthroat trout. Measured stream temperature ranges bracketed ambient river temperatures by −10.1 to +2.3 ∘C in agricultural return flows, −1.2 to +4 ∘C at diversions, −5.1 to +2 ∘C in beaver dams, and −4.2 to 0 ∘C at seeps. To better understand the role of these river features on thermal refugia during warm time periods, the respective temperature ranges were added to simulated stream temperatures at each of the identified river features. Based on this analysis, the average distance between thermal refugia in this system was 2.8 km. While simulated stream temperatures are often too warm to support Lahontan cutthroat trout and other cold-water species, thermal refugia may exist to improve habitat connectivity and facilitate trout movement between spawning and summer habitats. Overall, high-resolution DTS and TIR measurements quantify temperature ranges of refugia and augment process-based modeling.

Sub-Daily Temperature Heterogeneity in a Side Channel and the Influence on Habitat Suitability of Freshwater Fish

“…This improvement was even larger when the thermal imagery was calibrated with in-situ measurements (76%). These results indicate the added value of thermal imagery to model temperature-based habitat suitability of freshwater fishes, similar as in studies by Vatland et al [22] and Dzara et al [25]. Even without in-situ calibration, uncertainty in POF estimates is greatly reduced by using temperatures measured with thermal imagery.…”

“…Spatial variation in temperatures results from inflow of tributaries [23] and groundwater, shading by riparian vegetation [23], water depth [24], amongst other factors. The high spatiotemporal variability of water temperature limits the usability of point measurements to accurately assess habitat suitability [10,25]. This can partly be solved by modelling, but a temperature model capturing high spatiotemporal details requires a thorough knowledge on complex processes of the water body, which is often lacking [26].…”

Small dams alter thermal regimes of downstream water

“…Exploring this question may be especially relevant because small dams clearly alter stream thermal regimes. Moreover, future global warming could exacerbate these effects as it is expected to increase daily minimum temperatures more than daily maximum temperatures, with a corresponding decrease in the diurnal temperature range and an increase in mean daily temperature (Easterling et al, 1997;Vose et al, 2005).…”

“…Given the complexity and high variability of the river systems encountered in this study (Strahler orders spanning 3-5), it seems essential to us (and see Isaak et al, 2017Isaak et al, , 2018Steel et al, 2017;Dzara et al, 2019) to continue to conduct and expand well targeted stream temperature monitoring. This type of monitoring is requisite before being able to model stream temperature with sufficient spatial and temporal resolution.…”

Potential for Managed Aquifer Recharge to Enhance Fish Habitat in a Regulated River

“…Further identification of groundwater inflows, their hydrogeologic properties, and their use by fish could inform water and fisheries management actions to enhance groundwater springs for fish populations. Understanding the connectivity of thermal refugia will also help managers understand where fish can become trapped or bottlenecks occur for movement [13,68].…”