TOPOGRAPHY AND RADIATION EXCHANGE OF A MOUNTAINOUS WATERSHED

This report deals with the radiation exchange of a complex terrain. A relatively simple network for computing topographic parameters, global radiation, and net radiation of a mountainous terrain was developed a nd applied to a forested Appalachian watershed encompassing 39.2 ha (W atershed 4, U.S. Forest Service at Parsons, West Virginia, 39 degrees 20'N, 79 degrees 40'W). Monthly sums of global radiation and net radia tion of the watershed were computed for the period of 40 years (1951-9 0). Topographic map of the watershed in scale 1:2500 and data on globa l radiation at a horizontal surface in the study area (1955-77), month ly average air temperature, moisture, and cloudiness were used in the analysis. The topographic analysis showed that for the 432 terrain seg ments, covering the whole area of the watershed, the average daily pos sible sunshine duration varied from 9.3 to 11.9 h, with an average of 11.0 h, which is about 92% of the possible sunshine duration at a hori zontal surface at the study area. The sky view factor VF (i.e., that f raction of the sky that can be seen from the center of gravity of a te rrain segment) was obtained for all terrain segments and applied in co mputing their diffuse radiation. The average value of VF of the waters hed was 0.91, ranging from 0.65 to 0.98. The conversion factor for dir ect radiation R(b) (i.e., the ratio of direct radiation flux at a slop e to that at a horizontal plane in absence of the atmosphere) in the w atershed varied from 0.39 to 1.41 (average 1.11). The seasonal variati ons of sunshine duration and conversion factor at slopes with differen t azimuths were also analyzed. The prediction of radiation exchange sh owed that for the watershed, the average yearly sums of global radiati on and net radiation were about 4.5 and 2.2 GJ m(-2) yr(-1), respectiv ely. On the yearly average, the upper east-facing slopes received 20%- 30% more direct radiation than a horizontal surface, but the north-fac ing slopes and the lower sites in the watershed received 10%-20% less. The seasonal changes of R(b) values at slopes with different azimuths in the watershed were described. The relationships between solar move ment and average values of R(b), shortwave radiation absorbed, and net radiation at different segment groups were developed. The estimated r elative errors of predicted global radiation and net radiation in the watershed were within about 10% for the average yearly sums.