EVAPORATION AND CANOPY CHARACTERISTICS OF CONIFEROUS FORESTS AND GRASSLANDS

Canopy-scale evaporation rate (E) and derived surface and aerodynamic conductances for the transfer of water vapour (g(s) and g(a), respecti vely) are reviewed for coniferous forests and grasslands. Despite the extremes of canopy structure, the two vegetation types have similar ma ximum hourly evaporation rates (E(max)) and maximum surface conductanc es (g(smax)) (medians = 0.46 mm h-1 and 22 mm s-1). However, on a dail y basis, median E(max) of coniferous forest (4.0 mm d-1) is significan tly lower than that of grassland (4.6 mm d-1). Additionally, a represe ntative value Of ga for coniferous forest (200 mm s-1) is an order of magnitude more than the corresponding value for grassland (25 mm s-1). The proportional sensitivity of E, calculated by the Penman-Monteith equation, to changes in g(s) is > 0.7 for coniferous forest, but as lo w as 0.3 for grassland. The proportional sensitivity of E to changes i n g(a) is generally +/- 0. 1 5 or less. Boundary-line relationships be tween g(s) and light and air saturation deficit (D) vary considerably. Attainment of g(smax) occurs at a much lower irradiance for coniferou s forest than for grassland (15 versus about 45% of full sunlight). Re lationships between g(s) and D measured above the canopy appear to be fairly uniform for coniferous forest, but are variable for grassland. More uniform relationships may be found for surfaces with relatively s mall g(a), like grassland, by using D at the evaporating surface (D0) as the independent variable rather than D at a reference point above t he surface. An analytical expression is given for determining D0 from measurable quantities. Evaporation rate also depends on the availabili ty of water in the root zone. Below a critical value of soil water sto rage, the ratio of evaporation rate to the available energy tends to d ecrease sharply and linearly with decreasing soil water content. At th e lowest value of soil water content, this ratio declines by up to a f actor of 4 from the non-soil-water-limiting plateau. Knowledge about f unctional rooting depth of different plant species remains rather limi ted. Ignorance of this important variable makes it generally difficult to obtain accurate estimates of seasonal evaporation from terrestrial ecosystems.