The Solar Radiation Balance of Scrub Forest and pasture on the Carmel Mountain, Israel: A Comparative Study

Abstract

Differences in the solar radiation balance of scrub forest and pasture vegetation were investigated. Three years of measurement showed small but consistent differences which were, however, at the limits of the radiometer's absolute accuracy. The mean annual short wave reflectivity of the pasture plot was 0.18, varying between a spring maximum of 0.20 and a midsummer minimum of 0.16. The reflectivity of the woody vegetation averaged 0.15, varying from a maximum of 0.16 to a minimum of 0.14 at approximately the same seasons, which coincided with the periods of maximum and minimum vegetative growth. Annual values of net radiation measured above the forest vegetation were 11% greater than above the pasture, differences increasing to 15% on cloudless days. Nighttime differences were much smaller, but of similar sign. Maximum differences in net radiation of 65 cal cm(—2) day(—1) were measured in early May, when differences in vegetation cover and evapotranspiration rate were also at their maximum; minimum differences were found in early winter, when vegetative activity was minimal. Net radiation was highly correlated with global radiation under both cloudless and average conditions; no statistical advantage in prediction accuracy was found by including a heating coefficient. Annual totals of net terrestrial long—wave radiation during daylight hours were 21% greater from the pasture than from the scrub forest plots: during the night the flux from the two vegetation types was similar but much smaller. Maximum daily differences of 60 cal cm(—2) day (—2) occurred in May and showed that differences in long—wave emission from the two plots, rather than differences in short—wave reflectivity, were the cause of differences in the radiation balance. The mean daily radiative surface temperature of the pasture plot was calculated to be 4oC above that of the scrub forest during this period, assuming an equally high emissivity for the two surfaces. The radiant energy available to the scrub forest during the 6 rainless months (April to September) was 8.52 kcal cm(—2) more than that retained by the pasture. During the same period, evapotranspiration measurements indicated that the scrub forest utilized 7.14 Kcal cm(—2) more latent heat of vaporization than did the pasture plot. The latent—to—radiant heat ratios imply diffusive resistances of water vapor transport of 8.3 sec cm(—1) for the pasture plot and 5.4 sec cm(—1) for the scrub forest. It is concluded that differences in the evapotranspiration rate of the two vegetation surfaces are the cause rather than the result of differences in their radiation balance.