Potential solar radiation at the H.J. Andrews Experimental Forest

Solar radiation is ultimately the driver of virtually all ecological and atmospheric systems. This studyuses a model to compute the potential clear-sky radiation receipt on the slopes of the H. J. AndrewsExperimental Forest Long-Term Ecological Research site in the foothills of the southern Cascademountains of central Oregon. A comprehensive review of available modeling methods for solar radiationin complex terrain is provided. The method developed by Williams is selected and applied to the forestarea for the times of the solstices and equinox. It is also applied at rn, month times in January,February, April, and May in order to completely characterize the seasonal change of potential radiationat the location. The method uses an 82 x 111 point grid with a 120 m spacing interval. Resulting mapsreveal areas of the Forest with extremely steep gradients of potential radiation. These steep gradientshave higher absolute values in summer compared to winter. The south-facing slopes which have thehighest potential radiation values tend to be at the highest elevations. There are places which receive nodirect radiation as far into the year as February. Standard deviation values of potential radiation acrossthe Andrews show the maximum spatial variability to occur in February. There is a decrease in the ratioof diffuse to direct plus diffuse potential radiation from 0.66 at Dec. 21 to 0.23 at June 21. It seems thatLookout Creek approximately divides the Andrews Forest into an area of relatively high potentialradiation to the north of the Creek and relatively lower potential radiation values to the south of theCreek. The results are also discussed in relation to spatial distributions of the values of other biophysicalvariables available on the Andrews Geographic Information System. Potential radiation values seem to beassociated with the spatial distributions shown on the data layers of debris flows and predominant treespecies zones. A comprehensive series of appendices documents the procedures used so they that they canbe employed to other parts of the forests of the Pacific Northwest and in other areas of complex terrain.Digital versions of the input data, program codes, output results, and other relevant material are alsoprovided