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

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 non-heatwave 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 degrees C was observed in unshaded areas, followed by a downstream cooling of 3.5 degrees C in shaded reaches. Water temperature directly responded to air temperature and cloudiness. For an air temperature change of 2 degrees C we modelled a water temperature change of 1.3 degrees C for unshaded reaches, but lower changes for intensively shaded reaches. Similar daily variations at shaded reaches were up to 4 degrees C lower than unshaded ones. This study gives clear evidence that for a medium-sized pre-alpine 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.