Components of the Surface Radiation Balance of Subarctic Wetland Terrain Units during the Snow-Free Season

Abstract

The magnitude and behavior of net radiation, albedo, and surface radiative temperatures for three coastal wetland sites on southern James Bay[Canada] were monitored during the snow-free season. The three terrains were a relatively dry coastal backshore, an inner marsh, and an alder/willow forest. Surface wetness was found to be the most important factor prior to vegetation growth. During this period the drier site had an albedo of 0.18, while albedo for the wet marsh and the forest averaged 0.11. Albedos at the wetter sites increased rapidly after the initiation of vegetation growth and during the period of grand growth. The increase in albedo is shown to be linearly related to changes in leaf area. After the vegetation cover was fully established, albedos at all sites were similar. Net radiation in the wetland was primarily determined by albedo and incoming solar radiation. Site to site differences were small in all periods. Although mean daily radiative temperatures were similar at all sites before growth, the backshore had a larger daily temperature range than the wetter sites. Radiative temperatures at the forest site averaged 2 to 3.degree.C less than at the other sites after leafing had occurred. Wind direction had a significant influence on the radiative temperature regime at all sites. Radiative temperatures during onshore winds were substantially smaller than for offshore winds. The implications of this to the surface heat balance are discussed.