Climatology and Sugar Cane — CHANG 
389 
Fig. 11. Five-year moving mean of rainfall at 
Makiki, recorded at middle year, for the period 
1920-58. 
time climate the Penman estimates are far better 
than the Thornthwaite formula. 
The various radiation components were meas- 
ured at Makiki to evaluate the energy budget 
approach to evapotranspiration. Over a mature 
cane field the partition of incoming radiation is 
approximately as follows: 16% reflected radia- 
tion, 17% back radiation, and 67% net radia- 
tion (Fig. 12). The relationship between the 
net and incoming radiation is almost a constant 
throughout the year and over different vegeta- 
tive surfaces. It was observed at Wahiawa that 
over short grass and pineapple fields, which 
have an albedo of about 5%, the net radiation 
remains two-thirds of incoming radiation. 
Theoretically it is difficult to reconcile the fact 
that the net radiation is not affected by the de- 
crease of albedo, but from a practical standpoint 
this is convenient. 
In the tropics, especially under the cover of 
tall vegetation, heat flux to and from the soil 
is negligible; therefore, the net radiation is 
consumed either in heating the air or in evapo- 
transpiration. At Makiki 82 % of the annual net 
radiation, or 55% of the incoming radiation, is 
used in evapotranspiration. There is a small sea- 
sonal variation of this percentage value, which 
is some 10% higher in summer than in winter. 
There is also a regional variation of the 
energy partition. For instance, the percentage of 
insolation used in evapotranspiration increases 
with total radiation. In Hawaii the regional 
variations of energy budget are rather small, and 
the equation at Makiki could be used to esti- 
mate with reasonable accuracy the monthly 
potential evapotranspiration for other cane- 
growing areas in Hawaii. For estimating short 
term, say weekly, potential evapotranspiration 
it is advisable to use an evaporimeter which 
integrates not only radiation but other meteoro- 
logical elements as well. 
The simple energy budget presented here is 
applicable only in the humid tropics. In middle 
and high latitudes the energy budget is subject 
to enormous seasonal variations. In an arid cli- 
mate the advective heat, which is extremely dif- 
ficult to evaluate, may introduce a significant 
error. 
PAN EVAPORATION AS A MEASURE OF 
EVAPOTRANSPIRATION 
Until a cheap and readily installable piece of 
equipment capable of recording vertical vapor 
transfer is available, agricultural meteorologists 
will continue to use evaporation pans. In an arid 
climate pan evaporation is accentuated by the 
oasis effect to such an extent that its usefulness 
as a climatic parameter is greatly impaired. In 
humid climates, pan evaporation has been found 
to be more accurate than the Penman and 
Thornthwaite estimates (Suzuki and Fukuda, 
1958), and a satisfactory guide to irrigation 
control (Krogman and Lutwick, 1961). In this 
INCOMING RADIATION (100%) 
FIG. 12. Energy budget over a sugar-cane field at 
Makiki. 
