390 
PACIFIC SCIENCE, Vol. XVII, October 1963 
respect Hawaii is ideally suited to the use of 
evaporation pans, thanks to the existence of a 
moist mixing layer below the trade inversion. 
Lysimeter and pan-evaporation records in Ha- 
waii indicate that the potential evapotranspira- 
tion of mature sugar cane is approximately the 
same as evaporation from a ground-level U.S. 
Weather Bureau Class A pan. The ground-level 
pan, however, evaporates some 10% less than an 
elevated pan at cane top level, and some 15% 
more than a buried pan in an irrigated plot. 
The one-to-one ratio obtained by using the 
ground-level pan is very much in line with 
ratios found for many other crops. From the 
standpoint of water use, crops may be divided 
into two groups: conventional and nonconven- 
tional. The potential evapotranspiration of a 
nonconventional crop is influenced to a large 
extent by its physiology. Rice, with a ratio of 
1.2 (Suzuki and Fukuda, 1958), and pineapple, 
with a ratio of 0.35, are good examples. The 
water need of a conventional crop is deter- 
mined primarily by the ^weather conditions, al- 
though it increases with the roughness of the 
crop, especially in areas of large advective heat 
(Tanner and Pelton, I960). Thus the ratio in- 
creases from 0.75 for short grass (Penman, 
1948) to 0.87 for corn (Fritschen, 1960^), and 
to 1.0 for sugar cane. 
The fact that potential evapotranspiration of 
sugar cane approximates pan evaporation does 
not necessarily mean that irrigation based on a 
ratio of one-to-one is most economical. A new 
experiment has been set up at Waipio to de- 
termine sugar-cane growth and yield by using 
different pan ratios, i.e., 1.30, 1.15, 1.00, 0.85, 
0.70, and 0.55. The yield data over a number of 
years should permit determination of the most 
economical level of irrigation. 
Fig. 13- Monthly water balance at Opaeula. 
MONTHLY AND ANNUAL WATER BALANCE 
The monthly water balance of a place can 
be portrayed by comparing the pan evaporation 
and the median rainfall. At Opaeula, for ex- 
ample, rainfall exceeds pan evaporation during 
the winter months from November to February 
(Fig. 13). From March to October, however, 
supplemental irrigation is needed to fill the 
water deficits, which total 29.6 inches in a year. 
By analyzing the water balance of some 30 
stations we have been able to construct an an- 
nual water-deficit map for the cane-growing 
areas in Hawaii (Fig. 14). The map is tentative, 
as the stations are not well distributed. Recently 
we have added 20 pan stations in order to 
strengthen the network. In constructing the map 
the radiation records were also used to fill the 
gap. 
By planimetering the deficit areas in the map, 
TABLE 1 
Estimated Annual Water Deficit for Growth of Sugar Cane 
ISLAND 
AREA 
( acre ) 
AVERAGE DEFICIT 
( inches ) 
TOTAL DEFICIT 
( acre-inches ) 
Hawaii 
98,601 
16.5 
1,628,390 
Maui 
42,424 
72.8 
3,087,612 
Oahu 
33,223 
41.0 
1,361,141 
Kauai 
47,088 
33.4 
1,573,502 
Total 
221,336 
34.6 
7,650,645 
