NIGHTTIME EVAPORATION FROM A SHORT-ROTATION WILLOW STAND

The aim of the present study was to quantify and analyse the evaporati on during night-time in a willow (Salix viminalis L.) short-rotation s tand. Evaporation measurements were made throughout the 24 h period on 76 days during the 1988 growing season using the energy-balance/Bowen -ratio method. Canopy conductance was estimated using a rearranged for m of the Penman combination equation. The mean hourly evaporation rate during daytime was 0.18 mm h-1 and the corresponding value during nig ht-time was 0.013 mm h-1, about 7% of the daytime rate. The mean cumul ative night-time evaporation was 0.12 mm per night or 4.3% of the mean daytime evaporation. The highest night-time evaporation occurred in S eptember and October with 0.21 mm and 0.36 mm per night, respectively, some 30-35% of the daytime evaporation during these periods. Total ni ght-time evaporation for the growing season was estimated as 21 mm. Ni ght-time evaporation was controlled mainly by vapour pressure deficit and ventilation whereas net radiation had only a minor influence. The night-time canopy resistance was estimated for a shorter period when t he canopy was completely closed (leaf area index around 5) and the soi l evaporation could be assumed negligible. The canopy resistance range d between 10 and 220 s m-1 depending on vapour pressure deficit. The d ependence on vapour pressure deficit during night-time was similar to the dependence during daytime but with a much larger sensitivity durin g the dark period.