Reply on RC1

Hanieh Seyedhashemi

doi:10.5194/hess-2021-450-ac1

Abstract

Stream temperature appears to be increasing globally, but its rate remains poorly constrained due to a paucity of long-term data and difficulty in parsing effects of hydroclimate and landscape variability. Here, we address these issues using the physically-based thermal model T-NET (Temperature-NETwork) coupled with the EROS semi-distributed hydrological model to reconstruct past daily stream temperature and streamflow at the scale of the entire Loire River basin in France (105 km2 with 52278 reaches). Stream temperature increased for almost all reaches in all seasons (mean = +0.38 °C/decade) over the 1963–2019 period. Increases were greatest in spring and summer with a median increase of +0.38 °C (range = +0.11– +0.76 °C) and +0.44 °C (+0.08– +1.02 °C) per decade, respectively. Rates of stream temperature increases were greater than for air temperature across seasons for 50–86 % of reaches. Spring and summer increases were typically the greatest in the southern headwaters (up to +1 °C/decade) and in the largest rivers (Strahler order > 5). Importantly, air temperature and streamflow exerted joint influence on stream temperature trends, where the greatest stream temperature increases were accompanied by similar trends in air temperature (up to +0.71 °C/decade) and the greatest decreases in streamflow (up to −16 %/decade). Indeed, for the majority of reaches, positive stream temperature anomalies exhibited synchrony with positive anomalies in air temperature and negative anomalies in streamflow, highlighting the dual control exerted by these hydroclimatic drivers. Moreover, spring and summer stream temperature, air temperature, and streamflow time series exhibited common change-points occurring in the late 1980s, suggesting a temporal coherence between changes in the hydroclimatic drivers and a rapid stream temperature response. Critically, riparian vegetation shading mitigated stream temperature increases by up to 16 % in smaller streams (i.e., < 30 km from the source). Our results provide strong support for basin-wide increases in stream temperature due to joint effects of rising air temperature and reduced streamflow. We suggest that some of these climate change-induced effects can be mitigated through the restoration and maintenance of riparian forests, and call for continued high-resolution monitoring of stream temperature at large scales.

Highlights

Stream temperature is a critical water quality parameter affecting the distribution of aquatic communities (Poole and Berman, 2001; Ducharne, 2008), but its future under global change remains uncertain
Large rivers exhibited a small Tw underestimation, with a median bias ranging from -0.29 °C to +0.15 °C, and the overall biases were still quite small across seasons (IQR=0.4–0.7 °C). 225 Trends in observed and modeled Q were well correlated for all seasons (Fig. S3), with the highest correlation across stations found in spring (r = 0.69, p < 0.05) and the lowest correlation found in summer (r = 0.17, p = 0.26)
The rate of warming for stream temperature was in the majority of cases higher than the rate of atmospheric warming, suggesting a decoupling of thermal trajectories linked to decreasing Q, especially in the southern headwaters

Summary

Introduction

Stream temperature is a critical water quality parameter affecting the distribution of aquatic communities (Poole and Berman, 2001; Ducharne, 2008), but its future under global change remains uncertain. The paucity of long-term time series of Tw (Webb and Walling, 1996; Nelson and Palmer, 2007; Webb et al, 2008; Arora et al, 2016) has impaired the larger scale assessment of such trends, especially in light of confounding factors like hydrological changes and land use change. Many landscape and basin characteristics (e.g., stream discharge [Q], streambed morphology, karst resurgences, topography, and vegetation cover) contribute to the response of Tw to climate change over time and space (Stefan and Preud’homme, 1993; Webb and Walling, 40 1996; Webb et al, 2008; Hannah and Garner, 2015). Riparian vegetation can obstruct solar radiation, which is the dominant heat flux at air-water surface (Caissie, 2006; Hannah et al, 2004), and decrease Tw response to Ta (Loicq et al, 2018; Johnson, 2004). Intensification of the water cycle (Huntington, 2006), with more frequent and severe droughts (Mantua et al, 2010; Giuntoli et al, 2013; Prudhomme et al, 2014), as well as more intense and sudden floods (Blöschl et al, 2019) may decouple Ta-Tw trends, exacerbating Tw increases that will most likely 50 be evident during low summer flows when thermal capacity and flow velocity are at their minima (Webb, 1996; Webb et al, 2008)

Results

Discussion

Conclusion