Influence of evaporation from the forest floor on evapotranspiration from the dry canopy

Evaporation from the forest floor (Ef) of a secondary broad-leaved forest was monitored for 1 year at 30 min intervals using a closed chamber system. The diurnal and seasonal variation of Ef and forest structure were analysed for days undisturbed by rainfall. The diurnal change in Ef reached a maximum at about 14:00 and gradually decreased towards midnight along with the vapour pressure deficit (D). Although Ef comprised about 20% of evapotranspiration from the dry canopy (Et), it had only a small influence on diurnal evaporation efficiency (β) characteristics above the canopy because its diurnal range was much smaller than that of Et. Although leaf emergence and leaf fall clearly affected available energy (Ae) beneath the canopy, the influence was not clear with Ef. In contrast, seasonal variation in Ef was strongly correlated with D and Ae above the canopy, and the determination coefficient (R²) changed with the Bowen ratio (B). At night, Et was almost equal to Ef when the friction velocity (u*) ranged between 0·2 and 0·4 m s⁻¹. The ratio of Ef to Et was exponentially correlated with leaf area index (LAI) when the soil was not dry. The ratio of Ef to Et was mainly influenced by LAI and soil moisture, but the existence of understory vegetation did not have a strong influence. Copyright © 2008 John Wiley & Sons, Ltd.