Short-term environmental controls of heat and water vapour fluxes above a boreal coniferous forest: model computations compared with measurements by eddy correlation

Abstract

Eddy correlation and stem flow measurements were coupled with detailed microclimate and soil measurements made in a boreal Scots pine forest in the late growing season of 1998 to determine sensible and latent heat fluxes from the soil and the canopy separately. A ‘resistance/energy’ model is constructed and parametrized in order to reproduce the dynamics of water and heat exchange between the soil, the canopy and the atmosphere as a part of a larger forest ecosystem model (FinnFor; Kellomäki and Väisänen, 1997). Unique features of the present model are that (1) energy flux equations are expressed in terms of conceptual resistances and their solutions are obtained by closing two surface energy budget equations defined separately for canopy and soil surface; (2) the forest canopy is divided into shaded and sunlit fractions in the radiation transfer submodel and the canopy resistance submodels; (3) a numerical integrating solutions are derived separately for net radiation absorption in the canopy, bulk canopy resistance and the bulk aerodynamic resistances of the forest; and (4) iterative determinations of canopy water potential based on a classical one-dimensional water flow model enable the model to represent explicitly the interaction between the above-ground and the below-ground water dynamics. The model is validated against 19-day flux measurements. In general, the total system sensible heat flux ( H), total system latent heat flux ( λE), canopy latent heat flux ( λE c), and soil surface heat flux ( G s) computed by the model matched well with the measured data. Based on 1/2 h flux measurements, daily λE varied from 0.50–7.38 MW m −2, H from 0.64–8.3 MW m −2, and λE c from 0.30–6.93 MW m −2. The Bowen ratio ( H/λE) ranged from−4.5 to 9.8, but 82% of the values for the Bowen ratio were within 0.5–2.5. The model computations showed that daily λE c and H c accounted for 21–64% and 43–66% of the daily total system flux, respectively. Daily soil latent heat ( λE s) and soil sensible heat ( H s) fluxes accounted for 0.02–4.5% and 0.05–7.6%, respectively, and the daily energy storage within the canopy ( S c) and G s accounted for 0.1–7.2% and 0.8–5.6%, respectively. Plotting of 1/2 h flux data against a single environmental factor indicated that a 68% change in λE c and a 72% change in H c can be explained by a change in canopy radiation absorption ( R nc ) at the 5% probability level. The high correlation between the canopy fluxes and R nc could be related to the moderate weather conditions and high soil water content during the selected days, whereas λE s, H s, S c and G s give no significant correlation with R n . As expected, λE c was strongly dependent on canopy resistance ( r cs), but less impact on aerodynamic resistances during most of the measuring time. The proportion of energy partitioning in H and λE exhibited a clear diurnal trend and was mainly controlled by the system total resistance and the vapour pressure deficit, but less related to changes in soil water content.