392 
PACIFIC SCIENCE, Vol. XVII, October 1963 
I960 JULY AUG SEPT OCT NOV DEC 
FIG. 15. Running daily soil-moisture balance at Waipio. Dashed lines indicate irrigation. 
DAILY WATER BALANCE 
If the soil storage capacity is known, the daily 
water balance can be computed by comparing 
the daily rainfall and potential evapotranspira- 
tion. Figure 15 shows the running daily soil 
moisture balance at Waipio, where the storage 
capacity is 2.5 inches. The rainfall in excess of 
the amount that a soil can hold is regarded as 
surplus. By subtracting the surplus from the 
rainfall, the effective rainfall can be determined. 
When the daily water balance reaches zero, 
drought occurs and irrigation is called for. The 
water deficits during the drought days can be 
added to determine the total water deficit dur- 
ing the growing season of a crop. 
This daily water-balance scheme assumes that 
the depletion rate of soil moisture is equal to 
potential as long as the soil moisture is above 
the wilting point. This assumption is adopted 
primarily for simplicity of computation. It 
should not be taken to mean that we endorse 
the well-known Veihmeyer and Hendrickson 
argument (1955). Although the relationship 
between the depletion rate and the moisture 
tension is still debatable, there are indica- 
tions that the relationship probably varies with 
weather conditions (Denmead, 1961). The con- 
stant depletion rate reported by Veihmeyer and 
Hendrickson is probably less in error in a humid, 
cloudy climate, especially under the cover of 
tall vegetation, than in a dry continental climate. 
The computed daily water balance at Waipio 
has been compared with gypsum-block readings. 
They agree in general. When the water balance 
reaches zero, the gypsum-block resistance is 
usually between 3,000 and 5,000 ohms. The 
latter value calls for irrigation. In general, the 
irrigation date determined by the pan evapora- 
tion is one or two days earlier than that indi- 
cated by the block reading. 
KOHALA RESULTS 
Kohala is a plantation on northern Hawaii 
with an annual water deficit varying from 20 
to more than 50 inches. In I960 only about 
27% of the plantation was irrigated. The ques- 
tion then arises as to whether an expanded irri- 
gation program will be profitable in the long 
run. Apparently this question cannot be an- 
swered without an investigation of the climate- 
and-yield relationship. 
The daily water balances during more than 
10 years for one irrigated and five unirrigated 
stations in Kohala were computed by using a 
storage capacity of 3 inches. The total water 
deficits for different crops were summarized 
and correlated with the yields (Fig. 16). The 
correlations are highly significant for Hawi, 
Niulii 9, Halawa 3, and Upolu 6, but are poor 
for Puakea 6 and Union 8. In general they are 
very good for field crops, especially in view of 
