Precipitation as rain, or in the form of snow, sleet, or hail, depending on atmospheric temperature and other conditions, is governed by movement of air and is generally abundant wherever air currents are predominately upward. The greatest precipitation should therefore occur near the equator. The average annual precipitation in the continental United States is shown in Fig. 4.2 (Visher, 1954); Table 4.3 (U. S. Dept, of Commerce, 1995) is a summary of the average monthly and annual precipitation at different locations in the United States. The average annual rainfall is about 79 cm.

The moisture needs of aquatic biomass are presumably met in full because growth occurs in liquid water, but the growth of terrestrial biomass is often water-limited. The annual requirements for good growth have been found for many biomass species to be in the range 50 to 76 cm (Roller et al, 1975). Some crops, such as wheat, exhibit good growth with much less water, but they are in the minority. Without irrigation, water is supplied during the growing season by the water in the soil at the beginning of the season and by

rainfall. Figure 4.3 illustrates the normal precipitation recorded in the continen­tal United States during the growing season, April to September (Visher, 1954). This type of information and the established requirements for the growth of terrestrial biomass can be used to divide the United States into precipitation regions as shown in Fig. 4.4 (Visher, 1954). The regions that are more produc­tive for biomass generally correlate with the precipitation regions, as shown in Figs. 4.5 and 4.6 (Visher, 1954). It should be realized, though, that rainfall alone is not quantitatively related to productivity of terrestrial biomass because of the differences in soil characteristics, water evaporation rates, and infiltra­tion. Also, as suggested in Fig. 4.5, certain areas that have low precipitation can be made productive through irrigation. Some areas of the country that vary widely in precipitation as a function of time, such as many western states, will produce moderate biomass yields and often sufficient yields of cash crops without irrigation to justify commercial production.

The transpiration of water to the atmosphere through biomass stomata is proportional to the vapor pressure difference between the atmosphere and the saturated vapor pressure inside the leaves. Transpiration is obviously affected by atmospheric temperature and humidity. The internal water is essential for biomass growth. The efficiency of utilizing this water (water-use efficiency, WUE) has been defined as the ratio of biomass accumulation to the water consumed, expressed as transpiration or total water input to the system. Analy­sis of the transpiration phenomenon and the possibilities for manipulation

“U. S. Dept, of Commerce (1995).

of WUE have led some researchers to conclude that biomass production is inextricably linked to biomass transpiration. Agronomic methods that mini­mize surface runoff and soil evaporation, and biochemical alterations that reduce transpiration in C3 plants, have the potential to increase WUE. But for water-limited regions, the fact remains that without additional water, the research results indicate that these areas cannot be expected to become regions of high biomass yields (Sinclair, Tanner, and Bennett, 1984). Irrigation and full exploitation of humid climates are of highest priority in attempting to increase biomass yields in these areas.

A. Temperature

Most biomass species grow well in the United States at temperatures between 15.6 and 32.3°C (60 and 95°F). Typical examples are corn, kenaf, and napier grass. Tropical grasses and certain warm-season biomass have optimum growth

temperatures in the range 35 to 40°C (95 to 104°F), but the minimum growth temperature is still near 15°C (Ludlow and Wilson, 1970). Cool-weather bio­mass such as wheat may show favorable growth below 15°C, and certain marine biomass such as the giant brown kelp only survive in water at temperatures below 20 to 22°C (North, 1971). The average number of days per year in the continental United States where the temperature is less than 6.1°C (43°F) and essentially no biomass growth occurs are shown in Fig. 4.7 (Visher, 1954). Table 4.4 is a summary of average monthly and annual temperature fluctuations with time and location in the United States (U. S. Dept, of Commerce, 1995). The growing season is clearly longer in the southern portion of the country. In some areas such as Hawaii, the Gulf states, southern California, and the southeastern Atlantic states, the temperature is usually conducive to biomass growth most of the year.

The effect of temperature fluctuations on net C02 uptake is illustrated by the curves in Fig. 4.8 (El-Sharkawy and Hesketh, 1964). As the temperature increases, net photosynthesis increases for cotton and sorghum to a maximum value and then rapidly declines. Ideally, the biomass species grown in an area should have a maximum rate of net photosynthesis as close as possible to the average temperature during the growing season in that area.