Estimating effective net radiation for a mountainous Watershed

Abstract

Net radiation estimates are frequently required in watershed research, e.g., in calculating evapotranspiration and snowmelt. In mountainous areas, the effective net radiation, i.e., the horizontal projection of the flux through a surface parallel to the slope, is a more accurate measure of the available energy than that measured with a horizontal sensor. In a non-homogeneous area, however, a basin average of effective net radiation is difficult to estimate. The annual curves for net and global solar radiation under clear skies at one point in the Marmot Creek Experimental Watershed in Alberta, Canada, show variations from 55 to 650 lyday-' for net radiation, and from 100 to 760 ly day-' for global radiation. A factor to convert measured net radiation at the point to a basin average of effective net radiation is obtained by comparing these curves with that for effective clear sky global radiation for the basin, and by considering the ratio of net to global radiation over the various types of vegetation in the basin. This conversion factor varies throughout the year with the elevation of the Sun and the basin albedo, ranging from a maximum of 1.27 in December to a minimum of 0.93 in April, and averaging 1.06 for the year. In watershed research there may be some dispute with the statement by Geiger (1965) that "radiation is undoubtedly the most important of all meteorologic elements" because of the importance of precipitation, but there can be no denying that radiation is of prime importance in studies of evapotranspiration and snowmelt. The problem which confronts the researcher is: which element in the radiation field should be used and how should it to be measured or calculated? Net radiation is the most useful element because it integrates all wavelengths of both incoming and out- going energy. The World Meteorological Organization (1961) defines net radiation as "the net radiant flux through a horizontal surface", but in mountainous terrain, the "effective net radiation", hereby defined as "the horizontal projection of the flux through a surface parallel to the slope", is a more meaningful measure of the available energy. The difference between radiation measured with a horizontal sensor and one parallel to the slope may be positive or negative, and is of course greatest when slopes are steep and the sun is low in the sky. The problem has been studied for incoming short-wave radiation by Lee (1963), Ohmura (1968), Rouse and Wilson (1969), Fergu- son et al. (1971), and others. Kondrat'yev (1965) discussed the radiation balance of a slope, and Hay (1971) presented a complex model for computing the mean monthly intensities of the component fluxes of net radiation for drainage basins. This paper presents a simple method of estimating a basin average of effective net radiation for periods as short as one day from net global radiation data measured by horizontal sensors, along with calculated effective global solar radiation.