Seeing the landscape for the trees: Metrics to guide riparian shade management in river catchments

Abstract

Rising water temperature (Tw) due to anthropogenic climate change may have serious consequences for river ecosystems. Conservation and/or expansion of riparian shade could counter warming and buy time for ecosystems to adapt. However, sensitivity of river reaches to direct solar radiation is highly heterogeneous in space and time, so benefits of shading are also expected to be site specific. We use a network of high-resolution temperature measurements from two upland rivers in the UK, in conjunction with topographic shade modeling, to assess the relative significance of landscape and riparian shade to the thermal behavior of river reaches. Trees occupy 7% of the study catchments (comparable with the UK national average) yet shade covers 52% of the area and is concentrated along river corridors. Riparian shade is most beneficial for managing Tw at distances 5-20 km downstream from the source of the rivers where discharge is modest, flow is dominated by near-surface hydrological pathways, there is a wide floodplain with little landscape shade, and where cumulative solar exposure times are sufficient to affect Tw. For the rivers studied, we find that approximately 0.5 km of complete shade is necessary to off-set Tw by 18C during July (the month with peak Tw) at a headwater site; whereas 1.1 km of shade is required 25 km downstream. Further research is needed to assess the integrated effect of future changes in air temperature, sunshine duration, direct solar radiation, and downward diffuse radiation on Tw to help tree planting schemes achieve intended outcomes.

Influence of landscape and hydrological factors on stream–air temperature relationships at regional scale

“…On small and medium streams, it is necessary to preserve and/or favour the presence of riparian vegetation to moderate TS (Fabris, Malcolm, Buddendorf, & Soulsby, 2018). The effects will be most pronounced, in comparison with large streams, because of their smaller width, but investments have to be made strategically (Isaak et al, 2017;Johnson & Wilby, 2015). From a watershed management perspective, stream shading would be less effective in streams where…”

Human Impact on Spatial Water Temperature Variability in Lowland Rivers: a Case Study from Central Poland

“…The most common human impacts on the river thermal continuum are defi nitely dam and weir operations [20], infl ows from wastewater treatment [21] and power plants [22], and land use changes [23]. The thermal continuum may also be disrupted by natural factors, such as tributary infl ows and groundwater inputs [24], as well as different degrees of shading [25]. As detailed below, a complex set of human alternations was identifi ed to create a longitudinal heterogeneity of water temperature in the current study.…”

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?

“…Although vegetation can have important effects on stream temperature, there will be river sections which will not be affected by the addition (or removal) of vegetation due to upstream or lateral, surface or subsurface advection of heat or topographic shade (Johnson and Wilby, 2015). Groundwater influence was not measured and no groundwater influence was assumed in the model.…”

“…The climate input used only one possible emission scenario simulated by one regional climate model. The percentage contributions of surface, subsurface, groundwater and/or snowmelt still have to be analysed in more detail (Johnson and Wilby, 2015). Apart from rising air temperatures and discharge changes, anthropogenic influences like discharges from waste water treatment plants and cooling water can influence stream temperatures in a negative way and are therefore presently illegal in Austria (WRG, 1959).…”

“…One result or trend may however not be transferred from one river to another. Particular statements about the riparian vegetation's potential to mitigate the influence of climate change are only reliably valid for a given type of stream with its unique combination of morphologic and hydrologic parameters, local climate (Sinokrot and Stefan, 1993;Johnson, 2003;Steel and Fullerton, 2017) and regional climate change (Johnson and Wilby, 2015). Air temperature was normally used as a surrogate for stream temperature and energy flux variations in different river sections were not considered.…”