388 
PACIFIC SCIENCE, Vol XVII, October 1963 
Fig. 9- Solar radiation at Makiki, 1932-60. 
general the irrigated areas have less than 60 
inches of rainfall in a year. 
Intelligent management of irrigation requires 
a knowledge of the water need by a particular 
crop. Das (1936) advocated the use of day- 
degree as a measure of water need and as a 
guide to irrigation interval control. Numerous 
field experiments were conducted in Hawaii to 
determine the relationship between sugar yields 
and the number of day-degrees between irriga- 
tion. It was found that temperature is a poor 
indicator of the solar energy, which determines 
to a large extent the water needs of a crop. In 
fact, the monthly temperature and radiation at 
Makiki for the period 1932-60 have a correla- 
tion coefficient of only 0.57. The correlation is 
even poorer when the maximum temperature 
is used instead of the mean. In other parts of 
the world also, the use of day-degree as a 
phenological index has been refuted (Schneider, 
1952; Wang, I960). 
As interest in the day-degree approach waned, 
Baver (1954) properly called attention to the 
rapid development of the evapotranspiration 
and other micrometeorological concepts and 
their application in irrigation. He contended 
that the meteorological approach has the ad- 
vantage of simplicity of operation when com- 
pared with methods based upon measurement 
of soil moisture change. More important, how- 
ever, are the many applications of evapotrans- 
piration in determining the regional water 
balance and in disentangling the climate-yield 
relationship. With these ideas in mind the Ex- 
periment Station started an intensive study of 
micrometeorology in 1957. 
EVAPOTRANSPIRATION AND ENERGY BUDGET 
Sugar cane is a tall, ungainly plant with an 
aerodynamically rough canopy. Wind profile 
measurements indicate that the roughness pa- 
rameter of a mature sugar cane of 4 cm height 
is 9 cm, as against 2.3 cm for thick grass of 10 
cm height and 0.1 cm for short lawn grass 
(Sutton, 1953). The high roughness of a sugar- 
cane crop renders the aerodynamic approach to 
evapotranspiration a difficult task (Deacon et 
al., 1958). This, together with the lack of a 
suitable instrument for measuring vapor flux, 
accounts for the fact that the aerodynamic 
method was not assayed in our experimental 
work. 
The potential evapotranspiration of sugar 
cane was measured by drainage lysimeters and 
the data were analyzed by the IBM computer 
according to the Penman (1948) and Thorn- 
thwaite (1948) formulae. Detailed discussion 
of the instrumentation and the results has been 
reported elsewhere (Chang, 1961). It needs 
only to be emphasized that in a tropical mari- 
FlG. 10. Five-year moving mean of temperature 
at Makiki, recorded at middle year, for the period 
1918 - 61 . 
