SURFACE-ENERGY BUDGET DYNAMICS OF SHORT-ROTATION WILLOW FOREST

Knowledge of how energy budget components vary with time, vegetation type and stage of development and field size is important if we are to increase our understanding of the energy budget on a regional scale. The aim of this study was to quantify the seasonal and diurnal variation of energy budget components of a 2.6 ha short-rotation stand. Measurements were made using a thermometer interchange system for gradient and Bowen ratio estimations. Energy storage in soil, air and biomass was determined from temperature and humidity measurements. The partitioning of available energy between sensible and latent heat fluxes changed drastically at the beginning of the season. From the first half of May until the second half of June the maximum (noon) latent heat flux increased by a factor of 3, the total storage decreased by a factor of 2 and the sensible heat flux decreased by a factor of 4.5, while net radiation was unchanged. The vapour pressure deficit was similar during these periods but the leaf area index increased from about zero to three. On a mean monthly basis, the sensible heat flux was negative (directed towards the surface) from June to October, i.e., during most of the season. Heat was supplied to the atmosphere only at the beginning of the season for this type of short-rotation stand. Heat storage in air and biomass was significant on an hourly basis, especially in mornings and evenings when it could be of the same order as the net radiation. It was concluded that the development stage of the short-rotation stand had a large influence on how the energy was distributed between the convective fluxes. It was also concluded that storage in air and biomass had to be accounted for if precise estimates of energy balance on a shorter (hourly) time scale were required.