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, respectively) are reviewed for coniferous forests and grasslands. Despite the extremes of canopy structure, the two vegetation types have similar maximum hourly evaporation rates (E _max) and maximum surface conductances (g_smax) (medians = 0.46 mm h^-1 and 22 mm s^-1). However, on a daily basis, median E _max of coniferous forest (4.0 mm d^-1) is significantly lower than that of grassland (4.6 mm d^-1). Additionally, a representative value of g_a 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 low as 0.3 for grassland. The proportional sensitivity of E to changes in g_a is generally ±0.15 or less.Boundary-line relationships between g_s and light and air saturation deficit (D) vary considerably. Attainment of g_smax occurs at a much lower irradiance for coniferous forest than for grassland (15 versus about 45% of full sunlight). Relationships 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 small g_a, like grassland, by using D at the evaporating surface (D₀) as the independent variable rather than D at a reference point above the surface. An analytical expression is given for determining D₀ from measurable quantities. Evaporation rate also depends on the availability of water in the root zone.Below a critical value of soil water storage, the ratio of evaporation rate to the available energy tends to decrease sharply and linearly with decreasing soil water content. At the lowest value of soil water content, this ratio declines by up to a factor of 4 from the non-soil-water-limiting plateau. Knowledge about functional rooting depth of different plant species remains rather limited. Ignorance of this important variable makes it generally difficult to obtain accurate estimates of seasonal evaporation from terrestrial ecosystems.