Regional, multi-decadal analysis reveals that stream temperature increases faster than air temperature

Abstract

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.

An unexpected negative feedback between climate change and eutrophication: higher temperatures increase denitrification and buffer nitrogen loads in the Po River (Northern Italy)

“…These temperature increases resulted to be faster than the average increases observed in other large European and American rivers in temperate zones during similar periods [59,60]. However, the present outcomes agree with previous studies indicating a major contribution to warming from the hottest period of the annual cycle with stronger positive trends for late spring-summer months and a significant advance of spring warming [61][62][63][64][65].…”

“…Despite long-term increases in river water temperatures being correlated to increases in air temperatures, surprisingly, the warming trend of the Po River water was stronger than the atmosphere, when the latter is supposed to contribute to the warming of the former. These unusual data may be ascribed to the joint effect of rising air temperature and reduced outflow on river temperature trends [65].…”

Trends in Air and Water Temperatures at the Confluence of the Sava and Danube Rivers in Belgrade, Serbia

“…Water temperature serves as a vital indicator of river health and vulnerability [28,29]. Seyedhashemi et al detailed daily discharge and water temperature reconstructions from 1963 to 2019 along with projections spanning from 1976 to 2100, for 52,278 reaches across the Loire River basin in France (117,000 km 2 ) [30],. The water temperature of rivers increased across nearly all reaches and seasons from 1963 to 2019, with the most significant increases observed in spring and summer, with median increases of 0.38 ºC and 0.44 ºC per decade, respectively.…”