Evaporation and canopy characteristics of coniferous forests and grasslands

Abstract

Canopy-scale evaporation rate (E) and derived surface and aerodynamic conductances for the transfer of water vapour (g and g, 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 ) and maximum surface conductances (g) (medians = 0.46 mm h and 22 mm s). However, on a daily basis, median E of coniferous forest (4.0 mm d) is significantly lower than that of grassland (4.6 mm d). Additionally, a representative value of g for coniferous forest (200 mm s) is an order of magnitude more than the corresponding value for grassland (25 mm s). The proportional sensitivity of E, calculated by the Penman-Monteith equation, to changes in g is >0.7 for coniferous forest, but as low as 0.3 for grassland. The proportional sensitivity of E to changes in g is generally ±0.15 or less.Boundary-line relationships between g and light and air saturation deficit (D) vary considerably. Attainment of g occurs at a much lower irradiance for coniferous forest than for grassland (15 versus about 45% of full sunlight). Relationships between g 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, like grassland, by using D at the evaporating surface (D) as the independent variable rather than D at a reference point above the surface. An analytical expression is given for determining D 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.

How plant functional-type, weather, seasonal drought, and soil physical properties alter water and energy fluxes of an oak–grass savanna and an annual grassland

“…Yet direct measurements of the response of canopy evaporation to changes in soil moisture are rare (Kelliher et al, 1993;Hunt et al, 2002) or exist across a limited range of soil moisture conditions due to intermittent rainfall . Furthermore, the functional shape for the response of tree transpiration to soil moisture deficits shows much more diversity ( Lagergren and Lindroth, 2002) than is assumed in ecohydrological models ).…”

“…For example, predictions of potential evaporation rates with radiation-based models, such as the Priestley-Taylor equation (Priestley and Taylor, 1972), severely overestimate evaporation of savanna woodlands (Major, 1988;Lewis et al, 2000). This occurs because a semi-arid climate limits the amount of leaf area that can be sustained by the ecosystem (Eagleson, 1982;Baldocchi and Meyers, 1998;Eamus and Prior, 2001;Eamus, 2003) and drying soils force stomatal closure, which limit transpiration when evaporative demand is greatest (Kelliher et al, 1993;Goulden, 1996;Kiang, 2002).…”

“…Since trees and grass canopies differ in their ability to intercept and absorb photons and transpire (Kelliher et al, 1993;Miranda, 1997), their relative composition can have a profound effect on the surface energy balance of a landscape, which in turn, can have a modifying feedback on the regional climate (Zeng and Neelin, 2000) and its water balance (Joffre and Rambal, 1993;Rodriguez-Iturbe et al, 1999a).…”

Net CO2 and H2O fluxes of terrestrial ecosystems

“…Several micrometeorological techniques, such as the flux-gradient method or the eddy covariance technique, offer the potential to measure net fluxes with high temporal resolution. Subsequent data analyses allow the calculation of net ecosystem CO2 exchange (NEE) Some abiotic factors are well known to constrain ecosystem gas exchange, such as light [Ruimy et al, 1995], water vapor deficit [Kelliher et al, 1993], or nitrogen nutrition [Schulze et al, 1994]. In contrast, the effects of other factors, such as age or leaf area index (LAI) are not well studied, although they are known to influence gas exchange at both leaf and canopy levels [Kelliher et al, 1993 Leuning, 1995].…”

“…Subsequent data analyses allow the calculation of net ecosystem CO2 exchange (NEE) Some abiotic factors are well known to constrain ecosystem gas exchange, such as light [Ruimy et al, 1995], water vapor deficit [Kelliher et al, 1993], or nitrogen nutrition [Schulze et al, 1994]. In contrast, the effects of other factors, such as age or leaf area index (LAI) are not well studied, although they are known to influence gas exchange at both leaf and canopy levels [Kelliher et al, 1993 Leuning, 1995]. Transferring some of these concepts from the leaf to a higher organizational level, Schulze et al [1994] established relationships between Asmax and other properties such as nutrition and leaf stomatal conductance.…”