The influence of riparian vegetation shading on water temperature during low flow conditions in a medium sized river

Abstract

-Stream water temperature limits the growth and survival of aquatic organisms; whereby riparian shading plays a key role in inhibiting river warming. This study explains the effects of riparian shading on summer water temperatures at a pre-alpine Austrian river, during heatwave and nonheatwave periods at low flow conditions. A vegetation-shading index was introduced for the quantification of riparian vegetation effects on water temperature. For maximum water temperatures, a downstream warming of 3.9°C was observed in unshaded areas, followed by a downstream cooling of 3.5°C in shaded reaches. Water temperature directly responded to air temperature and cloudiness. For an air temperature change of 2°C we modelled a water temperature change of 1.3°C for unshaded reaches, but lower changes for intensively shaded reaches. Similar daily variations at shaded reaches were up to 4°C lower than unshaded ones. This study gives clear evidence that for a medium-sized prealpine river, restoration practices should consider that discontinuity of riparian vegetation should be less than 6000 m; with more than 40% dense vegetation in order to minimize water temperature increases due to unshaded conditions. Keywords: riparian vegetation / vegetation-shading index / water temperature / river vegetation management / restorationRésumé -L'influence de la végétation rivulaire sur la température de l'eau pendant les conditions de faible débit dans une rivière de taille moyenne. La température de l'eau d'une rivière limite la croissance et la survie des organismes aquatiques ; de ce fait l'ombrage rivulaire joue un rôle clé dans la limitation du réchauffement des rivières. Cette étude analyse les effets de l'ombrage rivulaire sur les températures estivales de l'eau dans une rivière autrichienne préalpine, pendant les périodes de canicule et de non-canicule à faible débit. Un indice d'ombrage de la végétation a été introduit pour la quantification des effets de la végétation rivulaire sur la température de l'eau. Pour les températures maximales de l'eau, un réchauffement en aval de 3,9°C a été observé dans les zones non ombragées, suivi d'un refroidissement en aval de 3,5°C dans les zones ombragées. La température de l'eau a répondu directement à la température de l'air et à la nébulosité. Pour un changement de température de l'air de 2°C, nous avons modélisé un changement de température de l'eau de 1,3°C pour les zones non ombragées, mais des changements plus faibles pour les niveaux intensivement ombragés. Des variations quotidiennes semblables dans des zones ombragées étaient jusqu'à 4°C inférieures à celles non ombragées. Cette étude montre clairement que pour une rivière préalpine de taille moyenne, les pratiques de restauration devraient considérer que la discontinuité de la végétation riveraine doit être

Can riparian vegetation shade mitigate the expected rise in stream temperatures due to climate change during heat waves in a human-impacted pre-alpine river?

“…Daily minimum temperatures increase at an even higher rate. These trends are in agreement with findings about experimental data analysed by Kalny et al (2017).…”

Hydrologic Resilience from Summertime Fog and Recharge: A Case Study for Coho Salmon Recovery Planning

“…Stream temperature is an aggregate of conductive, convective and advective fluxes between water column, stream bed, hyporheic zone, groundwater, seeps of varying origin, shade and substrate type (Webb and Zhang 1997;Johnson 2004;Guenther et al 2014;Fullerton et al 2015). A barrier to solar loading such as riparian vegetation or topographic shading reduces stream temperature (Bond et al 2015;Kalny et al 2017;Wawrzyniak et al 2017). A barrier to solar loading such as riparian vegetation or topographic shading reduces stream temperature (Bond et al 2015;Kalny et al 2017;Wawrzyniak et al 2017).…”

“…Stream temperature impacts from FLCC are currently poorly quantified yet this relationship forms the basis for the temperature analysis in this work. Measurements and models of impact from riparian shade (Rounds 2007;Bond et al 2015;Kalny et al 2017;Wawrzyniak et al 2017;Sullivan and Rounds 2018) suggest stream temperature reductions from a solar loading barrier is significant. Measurements and models of impact from riparian shade (Rounds 2007;Bond et al 2015;Kalny et al 2017;Wawrzyniak et al 2017;Sullivan and Rounds 2018) suggest stream temperature reductions from a solar loading barrier is significant.…”

“…The dominant process influencing stream temperature is solar loading (Brown 1969;Torgersen et al 2001;Mayer 2012;Dugdale et al 2017;Dugdale et al 2018;Loicq et al 2018). A barrier to solar loading such as riparian vegetation or topographic shading reduces stream temperature (Bond et al 2015;Kalny et al 2017;Wawrzyniak et al 2017). The strong correlation between air and stream temperature is commonly used to derive relatively accurate estimates of stream temperature modeled from air temperatures (Caldwell et al 2015).…”

“…Calculations of the impact of reduced SW using watt/min reductions from Iacobellis and Cayan (2013) and Matus and L'Ecuyer (2017) suggest at least a 3°C reduction from FLCC (100 watts = 3.2 Celsius heat units per minute), much less than the direct measurements of 10°C reductions (Walker and Anderson 2016) for roads under clouds. Measurements and models of impact from riparian shade (Rounds 2007;Bond et al 2015;Kalny et al 2017;Wawrzyniak et al 2017;Sullivan and Rounds 2018) suggest stream temperature reductions from a solar loading barrier is significant. Although the precise amount of temperature reduction due to FLCC is lacking, a small amount (0.5°C) of reduction can be important, as discussed earlier with temperature bands of Figure 5.…”