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

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 radiation.

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

### **3.5.2 Long-wave radiation**

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