The importance of shade

River temperature is ultimately driven by solar radiation. The relative significance of solar radiation is governed by catchment conditions. For example, solar radiation will warm smaller volumes of water quicker than larger volumes. Shading a river channel will also reduce the heat gained from solar radiation. Consequently, shading rivers by establishing or protecting trees near river banks (i.e. riparian vegetation) is an important management strategy to limit rising water temperatures. This is supported by LUTEN findings. We have found that the gradient of air-water temperature regressions (see catchment controls for more info) is significantly correlated to the amount of cumulative upstream bank-side tree cover (see Johnson et al. 2013 and Wilby et al. 2014).

Looking upstream at site D20

Uncertainties in shade provision
Because river water is moving downstream, large cumulative areas of shade are required to substantially affect water temperature. The movement of water also means that the temperature at a site is partially determined by the conditions upstream. Consequently, there will be reaches of some rivers where shade will be highly beneficial but there will also be parts of rivers where the provision of shade will have negligible effect on water temperatures. It is likely riparian shading will be most important where:

  • water temperature dynamics are dominated by solar forcing, which might exclude, for example, areas of groundwater dominance,
  • rivers have substantial, dense surface drainage networks,
  • rivers have minimal topographic shade from the wider landscape,
  • water volumes are relatively low because large water volumes will not be as responsive to solar radiation due to thermal inertia and inheritance of heat from upstream.

Site D23 (Dovedale) in January 2014
Site D23
Site D23 (Dovedale) in June 2012

Landscape mapping

In order to untangle where tree planting will  be most beneficial we have constructed a landscape model of the LUTEN catchments, which can be used to estimate the the shade and solar radiation falling across the landscape. The model consists of land-use and geological maps overlain onto a Digital Elevation Model (DEM) (see right top). The river network and key catchment features have also been mapped. This includes accurately mapped tree cover from aerial photographs of the catchment (see right bottom). This information is being used to calculate the distribution of solar radiation the river receives and to quantify the relative significance of landscape and vegetative shade.

The above slideshow shows maps generated with our landscape model in arcGIS. Larger images can be seen at our gallery by following this link.