Comparative analysis of albedo and surface energy balance of a grassland site and an adjacent Scots pine forest

The climate conditions in the atmospheric boundary layer are influenced by the underlying land-use type because of its impacts on surface energy balance. Furthermore, the performance of atmospheric models depends crucially on a realistic representation of surface processes, e.g. the partitioning of available energy into individual energy-balance components. The present study investigates the similarities and differences in the energy balance of grassland and forest. Data from the period March 1992 to September 1996 were gathered at 2 adjacent sites in the southern Upper Rhine Plain in southwest Germany: a Scots pine Pinus sylvestris L. forest site at Hartheim, and an adjacent grassland site at Bremgarten. The experimental sites constituted a segment of the REKLIP (Regio-Klima-Projekt) network of energy-balance stations. Synoptic conditions were very similar for both land-use types owing to the short distance between the 2 sites. Differences in available energy (AE) between both land-use types were mainly caused by their different albedo. On average, AE of the Scots pine forest was higher than that for grassland by 24%. Normalised by AE, the mean sensible heat flux H accounted for 38% of the AE for grassland and 47% of the AE for the Scots pine forest, while the mean latent heat flux LE reached 62% for grassland and only 53% for the Scots pine forest. Compared to the mean Bowen ratio of 0.61 for grassland, the relatively high mean value of 0.89 for the Scots pine forest resulted from the strong stomatal control of the pine trees due to the warm and dry site conditions. In the case of a sufficient water supply, forest AE was converted in equal proportions to LE and H, while for grassland, 60 and 40% of AE were converted to LE and H, respectively. With a low water supply, more AE was converted into H for both land-use types.