Abstract
AbstractRising 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 1°C 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.
Highlights
Water temperature (Tw) is critical to the survival, growth, and development of poikilothermic fauna such as fish and invertebrates [Thackeray et al, 2010; Martins et al, 2011; Dallas and Rivers-Moore, 2012; Everall et al, 2015]
Even so, shading high solar angles is disproportionately significant for the river network because of the proximity of trees to the channel, resulting in 40% of the river network receiving less than 50% of the solar radiation that would be received if trees were absent
Increased riparian shade is not a panacea for climate change and there will be large areas of river that would be little affected by the addition of vegetation because of advected heat from upstream, landscape shade, groundwater, reservoir releases, and/or snowmelt inputs
Summary
Water temperature (Tw) is critical to the survival, growth, and development of poikilothermic fauna such as fish and invertebrates [Thackeray et al, 2010; Martins et al, 2011; Dallas and Rivers-Moore, 2012; Everall et al, 2015]. Changes to catchment land use and hydrology, including the removal of riparian vegetation that shades channels from solar radiation, can elevate Tw [Rutherford et al, 2004; Richardson and Beraud, 2014]. Riparian vegetation has many impacts on waterways and, as a result, riparian buffer strips have long been a focus of river management and restoration [e.g., Osborne and Kovacic, 1993; Everall et al, 2012; Wilby et al, 2010; Sweeney and Newbold, 2014]. The thermal impact of riparian vegetation is widely recognized by the forestry sector in North America [see Moore et al, 2005] and is attracting attention in the UK as a possible climate adaptation option [e.g., Nisbet et al, 2011; Environment Agency, 2012]. Daily and seasonal solar energy receipts depend on orbital variations and latitude
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