**3.5.3 Photosynthetically active radiation**

In micrometeorology, special attention is paid to photosynthetically active radiation (*RPAR*), which is defined as radiation in the spectral region between 400 and 700 nm (Monteith, 1975). *RPAR* is needed when stomatal resistance is calculated. The calculation of *RPAR* is performed by means of a conversion factor *CPAR* as a function of solar zenith angle (φL) is given by Perelyot (1970).

A knowledge of solar radiation is of interest in studies relating to crop evapotranspiration, forest transpiration and for solar energy applications. The incident global radiation (350- 3000 nm) on the earth's surface (*Rnet*) is a product of incident radiation outside the atmosphere (*Ro*) and the atmospheric transmissivity, which is dependent upon the degree of

If uniform cloudiness is assumed over the day, the mean irradiance (*Rn* M) is computed from the ratio of the daily sum of actual global radiation to the daily sum of global radiation from a clear sky. The effects of cloudiness on radiation within the day can be approximated by

For the radiation-mediated processes of a canopy energy study, it is not sufficient to know the total incoming radiation, but estimates of the direct short wave, diffuse short wave and long wave components are required. Bristow *et al*. (1985) established a relationship to estimate hourly diffuse transmittance (*Tdd*) from hourly total transmittance (*Ttd*). Diffuse radiation can then be obtained by multiplying *Tdd* by potential solar radiation. The difference between total incident radiation and diffuse radiation is the direct beam

Apart from Bristow *et al.* (1986), there are several references for the partition of global radiation available (Weiss and Norman, 1985), but no general agreement on the method to be used to estimate the proportions. There are several methods to calculate the proportions of *Rb* and *Rds* in global radiation, both empirical and theoretical (Liu and Jordan, 1960; Weiss and Norman, 1985; Bristow et al., 1986). Although empirical methods give better results, their validity is limited to a particular place and time. Theoretical methods are preferable because they are more general, but no method is completely satisfactory for all latitudes and seasons (Castro and Fetcher, 1998). Many factors including clouds, aerosols, etc., affect the scattering of radiation in the atmosphere and therefore the proportion of *Rds.*  Consequently, a theoretical method such as that described in this section for the partition of

Long-wave radiation comes from objects with extended radiating surfaces such as clouds, sky, rocks, soil, water, and vegetation or animals. Arbitrary limits of 3 and 100 μm are usually taken to define the long-wave spectrum (Monteith and Unsworth, 1990). Downward long-wave radiation (*Rlw*) can be given as a function of air temperature (*Ta* in 0C) and the

In micrometeorology, special attention is paid to photosynthetically active radiation (*RPAR*), which is defined as radiation in the spectral region between 400 and 700 nm (Monteith, 1975). *RPAR* is needed when stomatal resistance is calculated. The calculation of *RPAR* is performed by means of a conversion factor *CPAR* as a function of solar zenith angle (φL) is

introducing a factor that varies according to cloudiness.

*Rb* and *Rds* in global radiation can be used.

vapour pressure (*ea*(*Ta*) in hPa) (Brutsaert, 1982).

**3.5.3 Photosynthetically active radiation** 

**3.5.2 Long-wave radiation** 

given by Perelyot (1970).

**3.5.1 Separation of solar radiation into direct and diffuse components** 

cloudiness.

radiation.

#### **3.6 Water flow resistance process in the canopy-atmosphere process**

The turbulent transport between canopies and the bulk of the atmosphere depends on the turbulent nature of the planetary boundary layer (Meyers and Paw U, 1986 and 1987). In the transfer of water vapor to and from trees, some exchanges occur by molecular diffusion such as the passage of water through stomata. The flux of diffusing gas (kg m-2 s-1) can then be equated to the concentration difference (kg m-3) over a diffusion resistance (s m-1) as given by Fick's law. In the process of the diffusion of water vapor away from the leaves, the stomatal resistance (*R*t) accounts for diffusion from the evaporation sites within the leaf to the leaf surface, while the leaf boundary layer resistance (*R*a) accounts for diffusion from the surface to the well-mixed surrounding air. Both the stomatal resistance and the leaf boundary layer resistance are highly dependent on the size, shape and surface properties of the leaves, and wind velocity.
