Climatology and Sugar Cane — CHANG 
393 
the fact that there were considerable differences 
in crop age, solar radiation, cultural practice, 
variety, etc. 
The average slope of the regression lines for 
the five unirrigated fields is 4.6 tons of cane 
per acre for every 10 inches of water. This is an 
important figure in determining the economy 
of irrigation. It is thought that by extending the 
regression lines to the point of zero deficit the 
approximate yield potential of the area could be 
estimated. The average yield for the five unirri- 
gated fields was 65.2 tons of cane per acre. The 
estimated potential would be 91.4 tons cane per 
acre, or an increase of 40% over the present 
yield. This estimate is probably reasonable when 
compared with the yield of 109 tons of cane per 
acre at Waialua, Oahu, an irrigated plantation 
with slightly better climatic conditions. 
Admittedly the use of linear extrapolation as 
a means of estimating yield potential is a crude 
procedure. Yet this seems to be the most rea- 
sonable method we can adopt at present. The- 
oretically the relationship between water and 
yield should be curvilinear, as shown in Figure 
17. The exact shape of the curve cannot be 
determined until results of the Waipio experi- 
ments become available. The difference in yield 
potential between the two curves in Figure 17 
is caused primarily by radiation, if soil and 
other factors are constant. Experiments are also 
underway to assess the effect of solar radiation 
on yield. It is hoped that in future this con- 
certed research program will enable us to draw 
with certainty the climate-and-yield relationship 
now presented hypothetically in Figure 17. 
By analyzing the daily water balance at Ko- 
hala, we also derived an empirical relationship 
between the actual rainfall and the effective 
rainfall for plant use. The computation was car- 
ried out by assuming varying soil-moisture 
storage capacities of 2, 3, and 4 inches. Figure 
18 shows the curvilinear relationships between 
the annual rainfall and effective rainfall for all 
the five stations combined. Multiple regression 
analyses were carried out to the fourth power 
and a few selected values are given in Table 2. 
It is noted that the effective rainfall increases 
with the storage capacity only slightly, espe- 
90 
80 
UJ 
cr 
70 
(T 
LU 
CL 
U 60 
< 
O 
CO 
-I — J 1 1 I 1 I _L l 
10 20 30 40 50 60 70 80 90 
WATER DEFICIT (INCHES) IN 24 MONTHS 
FIG. 16. Relationships between cane yield and water deficit for six fields at Kohala. 
