Nighttime, wet canopy evaporation rates and the water balance of an evergreen mixed forest

Abstract

The average nighttime evaporation rate of water intercepted on an evergreen mixed forest canopy is estimated as 0.37 mm/h from a regression of interception loss on gross rainfall for 35 storms occurring between 1900 and 0700 LT. The mean wet canopy evaporation rate estimated by the same method for both daytime and nighttime rainfall in 1975-1976 was also 0.37 mm/h. The similarity of daytime and nighttime evaporation rates indicates that evaporation from the wet canopy is driven by advected energy not by radiation. The diurnal rainfall distribution and nighttime evaporation rate indicate that net interception loss was 81-84% of gross interception, considerably higher than net losses estimated in other studies where nighttime losses were not considered separately from daytime losses. Interception losses at night were 40-50% of total gross interception, 50-60% of the total net interception, and ,--30% of total evaporation.

Evaporation from a tropical rain forest, Luquillo Experimental Forest, eastern Puerto Rico

“…Also, for similar amounts of rainfall above this "threshold" of 2500-2700 mm yr -•, both absolute and relative values of E i tend to be somewhat higher for "type 2" locations (equatorial continental edge sites) than for "type 3" locations (midcontinental sites). In turn, values for "type 1" locations (maritime outer tropical sites) exceed those for "type 2" locations (Figures 10b and 10d) (Figure 10d), confirming the importance of intercepted rainfall in both humid tropical and temperate maritime settings [e.g., Pearce et al, 1980;Calder, 1990]. Such findings lend further credibility to the high values presently found for E i and ET in Puerto Rico.…”

Evaporation of Intercepted Rainfall

“…An additional consequence for aerodynamically rough vegetation, such as forests, is that the aerodynamic (righthand) term of equation (2) is often much larger than the radiation term (Calder, 1990). Under these circumstances, it is the supply of advected energy, via the cooling of the above air mass that is able to support the high rates of evaporation from wet forests (Stewart, 1977;Pearce et al, 1980;Calder, 1990). High evaporation rates are possible even at night when net radiation is close to zero (Pearce et al, 1980;Calder, 1990).…”

“…Under these circumstances, it is the supply of advected energy, via the cooling of the above air mass that is able to support the high rates of evaporation from wet forests (Stewart, 1977;Pearce et al, 1980;Calder, 1990). High evaporation rates are possible even at night when net radiation is close to zero (Pearce et al, 1980;Calder, 1990). Dry-air advection, heat added to the air or removal of water from it may be responsible for the maintenance of a significant air vapor pressure deficit during rainfall events (McNaughton and Jarvis, 1983).…”

L'interception des précipitations par les peuplements forestiers

“…Pearce, Rowe et Stewart (1980) ont enfin montré que, pendant la nuit, l'évaporation de l'interception est aussi rapide que le jour, ce qui confirme encore le fait que l'apport d'énergie advective joue un grand rôle dans ces phénomènes. La nuit, les phénomènes transpiratoires sont réduits en raison d'une fermeture des stomates, liée à l'absence de lumière.…”