Relative shade of rivers. The relative shade is based on the amount of solar radiation energy received on a given surface area in a given time (i.e. insolation); the higher the insolation, the less shade.
Riparian shade map is a national data set but do not have full coverage. They have been created for the following WFD (Water Framework Directive) Management catchments in England and Wales: Hampshire Avon, Wye, Tyne, Ribble, Frome, Adur & Ouse, Don, Irwell, Wear, Welland, Tone, Ecclesbourne, Learn, Lower river Lee. EA Regional boundaries: Midlands, West Thames.
The relative shade maps have been derived from incoming radiation rasters, produced using the ArcGIS function "Area Solar Radiation", with the date parameters set to May, June, July, August and September, with hourly intervals (every 14 days). The units of the incoming solar radiation are Watt Hours per square metre (WH/m2). The vegetation surface objects grid (EA LIDAR derived Vegetation Object Map dataset) was used as input to the "Area Solar Radiation" tool. Then, the incoming insolation rasters have then been clipped using Ordnance Survey Mastermap Water Feature polygons, that have themselves been clipped and dissolved using a 25mx25m or 100mx100m grid (depending upon the Area team request at the time). These clipped water polygons have been attributed with the Ordnance Survey reference code and overlaid on the hillshade of the DTM. The incoming radiation values were then classified and given a relative shade value based on the average incoming radiation calculated values calculated for the catchment.
Pre-January 2013 the methodology for processing relative shade was to include all water features in the catchment, including ponds, lakes and reservoirs. This means that in catchments with lots of unshaded reservoirs, such as Welland, the data is slighted skewed so that the rivers appear more shaded, than in those catchments without reservoirs.
Limitations of LIDAR:
The primary issue with LIDAR is that these data were initially collected to help flood inundation mapping and are thus concentrated in low lying areas. As a result many headwaters and areas of higher ground are not covered. Gaps in the LIDAR coverage mean that our derived maps of riparian tree cover will be an underestimate of the true extent of riparian trees. The LIDAR data is largely recorded in winter, when only coniferous tree canopies are present. As the LIDAR hits the tree it records a ‘first return’ and a ‘last return’. The last return of the tree, in winter, normally being part way down the trunk. The combined effect of this method of data collection and the season in which it is collected, means that the height of trees and the extent of tree canopy is probably underestimated. This means that in addition to the problem of incomplete LIDAR coverage, taller vegetation is likely to be underestimated, and therefore also the potential extent of shade on water courses.