ENERGY USE EFFICIENCY IN BIOMASS PRODUCTION SYSTEMS - Economics of Sustainable Energy in Agriculture - page 74

In the system, irrigation is required (495 mm/year). Energy
requirements for this irrigation were determined using data from Stanhil
(1981), de Koning et al. (1992) and Nonhebel (1997). It should be noted
that the values used for nitrogen and water use efficiency are high; in
practice, the values are lower.
The energy required for harvesting the crops is yield-level
dependent (higher yield implies more costs for harvesting) and data from
Hall (1993) were used.
For all systems, the energy requirements of the inputs over the
complete life span of the plantation were determined. The energy inputs
involve the direct energy required for driving a tractor and the indirect
energy required to produce the material used (for instance, the nitrogen in
the fertiliser).
3. RESULTS
3.1 Inputs required for various production systems
The main energy source of a crop is solar energy. Solar radiation provides
the energy for photosynthesis, which is the basis of crop production. The
amount available differs per climatic region. In Table 6.1, an indication is
given of the incoming solar radiation for the two regions considered. The
annual radiation in Portugal is nearly twice as high as in the Netherlands.
Furthermore, it is essential that water be available; the average annual
precipitation is also given. The precipitation levels in Portugal are much
higher than in the Netherlands, although still not enough to obtain the
potential growth level.
The fossil energy inputs required for the four production systems
studied are summarised in Table 6.2, and involve total inputs over the
complete life span of the plantation. The high-input systems require more
fossil energy per hectare than the low-input systems. The inputs for