Part of understanding habitat conditions on a landscape involves looking at basic inputs.  For aquatic habitats, the amount of sunlight is known to have effect on the community.  The amount of sunlight that reaches a water body in a day depends on the orientation of the stream, the surrounding vegetation, and the time of the year (stream Irradiation).  Taking advantage of the scalability of the GRASS GIS software, a first attempt was made at modeling annual solar irradiation for the aquatic environments in the State of North Carolina using a 60 ft resolution top of canopy raster as the base input elevation surface.

The 365 statewide daily model inputs were filtered by canopy types derived from the 2001 SE GAP vegetation data on a monthly basis and applied to a raster mask of aquatic areas derived from a rasterized version of the 1:24,000 scale detailed hydrology layer from the North Carolina Center for Geographic information and Analysis (NC-CGIA.)  This gave a final output in Watt-hours/square meter/year for every grid cell in the aquatic habitat footprint  for North Carolina.

A comparison of the difference between calculating the the solar irradiation inputs to streams from the bare earth grid and canopy filtering method showed that the bare earth calculation gave 184% more solar irradiation to aquatic habitats.

The annual aquatic habitat solar irradiation data was then summed for subbasins derived from known locations of Dwarf wedgemussel (Alasmidonta heterodon) and subbasins derived from sampling points where no mussels were found in the Tar and Neuse River basins in eastern North Carolina from data provided by the North Carolina Natural Heritage program and the North Carolina Wildlife Resources Commission.  Solar irradiation was summed for the entire basin , 500m and 1000m meters upstream from the sampling point locations. 

While the results of the mussel no-mussel subbasin irradiation comparison were not conclusive (Mussel/No Mussel graph), further tuning of the comparison might reveal more positive results. The full publication for this work is in Transactions in GIS, 2012, 16(2): 161–176 , available online at http://onlinelibrary.wiley.com/doi/10.1111/j.1467-9671.2012.01313.x/abstract

Thanks to the South Atlantic LCC and the USFWS Raleigh Field Office for the opportunity to explore this subject.