12t/ha on sufficiently moisted field, and declines as the soil becomes drier. Where the soil layer has a 30% moisture, it is as low as approx. 3t/ha or a quarter of the former. This is because the formation of plants substantially deteriorates due to the decreased photosynthesis caused by shortage of water supply to the roots of the plants. Meanwhile, the harvest index (= yields/dry-matter production) is also halved from 0.6 to 0.3. Therefore, the yield of maize given by the equation below is only about 1t/ha which is far below about 7t/ha under the condition of sufficient soil moisture. This result denotes how crucial is securing water resources for crop production.
Yields of Maize = [Harvest index] × [Dry-Matter Production]
Based on researches conducted to date, the water use efficiency (WUE) of dry-matter production of crop community of fields is expressed as follows.
(e.g., Loomis and Conner, 1993).
A number of researchers have reported on transpiration efficiencies of various crop species, of which average values are shown in Table 3. From the table, it is clear that the transpiration efficiency of the C3 crop species is relatively low and about half that of the C4 species.
Among the C3 crop species, the transpiration efficiency of leguminous crops, that allow parasitical leguminous bacteria in them, is about 15% lower than the rest.
