Water vapor above canopy

Figure 8. The high frequency nature of the vertical velocity (W), water vapor (q ), and CO2 densities (C ) at 2 meters above a soybean canopy during a 3 minute period. The illustration also shows instantaneous water vapor (W q ) and carbon dioxide (W C ) fluxes and the mean quantities for the 15 minute period from which these traces were taken. Data courtesy of Center for Agricultural Meteorology and Climatology, University of Nebraska, Lincoln, Nebraska, and Environmental Sciences Division, Lawrence Livermore National Laboratory, Livermore, California.

During the daytime, a transpiring and photosynthesizing plant community as a whole can have a net vertical flux density of CO2 (/coz) downward toward it and a net vertical flux density of water vapor (71W) upward away from it into the turbulent air above the canopy. These flux densities are expressed per unit area of the ground or, equivalently, per unit area of the (horizontal) plant canopy. Each of the flux densities depends on the appropriate gradient. The vertical flux density of water vapor, for example, depends on the rate of change of water vapor concentration in the turbulent air, c, with respect to distance, z, above the vegetation ... 

The flux density of water vapor just above the canopy, which includes transpiration from the leaves plus evaporation from the soil, is often termed evapotranspiration. For fairly dense vegetation and a moist soil, evapotrans-piration is appreciable, usually amounting to 60 to 90% of the flux density of water vapor from an exposed water surface (such as a lake) at the ambient air temperature. The daily evapotranspiration from a forest is often equivalent to a layer of water 3 to 5 mm thick, which averages 2 to 3 mmol m-2 s-1 over a day. At noon on a sunny day with a moderate wind, Jw above a plant canopy can be 7 mmol m-2 s-1. Using Equation 9.4 (Ac v = 7wr ) and our value for of 30 s m-1, we note that over the first 30 m of the turbulent air... 

Figure 9-5. Possible variation of water vapor and CO2 concentrations within a 2-m-tall com (maize, Zea mays) crop at noon on a sunny day. At the top of the canopy, the wind speed is 2 m s-1. In the turbulent air 30 m above the vegetation, is 0.27 mol m-3 and - - tun°l mol-1 [for...

In contrast to the concentration of water vapor, which continuously decreases with increasing distance above the ground, on a sunny day the CO2 concentration generally achieves a minimum somewhere within a plant community (Fig. 9-5). This occurs because both the turbulent air above the canopy and also the soil can serve as sources of C02. During the day, C02 diffuses toward lower concentrations from the soil upward into the vegetation and from the overlying turbulent air downward into the plant community. 

B. Suppose that /Jcch for the soil is 200 H20/CO2, that 90% of the water vapor passing out of the canopy comes from the leaves, and that the net photosynthetic rate for the forest (using CO2 from above the canopy as well as from the soil) corresponds to 20 kg of carbohydrate hectare-1 hour-1 (use conversion factor in Table 8-2). If Jm from the soil is 0.6 mmol m-2 s-1, what are the. /Co2 s up from the soil and down into the canopy ... 

It is of passing interest to note that the advective-convective process described above continues above the canopy and as such is fairly typical of events in the surface boundary layer over open ocean, where the causative factor can also include the buoyancy of water vapor as well as the surface heating and direct thermal effects. 

Direct measurement of transpiration is difficult for all but relatively small plants that can be grown in weighing lysimeters or enclosed in chambers in which the flux of water can be ealculated from the humidity increase in the enclosure. Common approaches used to quantify transpiration in the field include precipitation minus runoff on gauged watersheds, energy balance equations (Allen et al., 1998), eddy covariance (water vapor gradients above and below canopy) (Baldocchi, 2003), hydrologic models, and sap flow measurements (Vose et al., 2003). 

---