84 
PACIFIC SCIENCE, VoL XIII, January 1959 
TABLE 2 
Comparison of Potential Evapotranspiration 
FROM Vegetables and Grass 
CROP 
TOMATOES 
CHINESE 
CABBAGE 
LETTUCE 
Planting date 
28 Oct. 
28 Oct. 
2 Jan. 
Days to reach 
1953 
1953 
1954 
maturity 
Total crop weight 
128 
(last fruit) 
61 
79 
(lbs.) 
85 (poor 
crop) 
341/2 
(1) Total P.E. (mm.) 
Average daily 
838 
282 
271 
P.E. (mm.) 
(2) Total P.E. of 
grass in same 
6.5 
4.6 
3.4 
period (mm.) . . 
Average daily 
P.E. of grass 
354 
182 
193 
in same period. 
(1) 
2.8 
3.0 
2.4 
Ratio 
(2) 
Mean temperature 
2.4 
1.5 
1.4 
during period (°F.) 
(1) 
Ratio for 1952 
(2) 
65 
68 
62 
winter 
2.3 
1.6 
1.7 
COMPARISON OF MEASURED POTENTIAL EVAPO- 
TRANSPIRATION WITH VALUES DERIVED FROM 
thornthwaite’s and penman’s formulae 
Thornthwaites Vormula 
The conclusions of the first report are amply 
confirmed by the longer period of record. 
Figure 2 shows monthly means of P.E. de- 
rived from the five years’ measurements. 
Again calculated values are too low in the dry 
winter and too high in the wet summer. Since 
investigators in Australia (Leeper, 1950), Ni- 
geria (Gamier, 1954), and Trinidad (Smith, 
1954) report similar shortcomings in the for- 
mula one must conclude that it should not be 
assumed to be valid for any monsoon or 
tropical maritime region. 
'Penman s Formula 
Penman (1948, 1950), using energy con- 
Fig. 1. Monthly potential evapotranspiration meas- 
ured at King’s Park (1), and at Kai Tak Airport (2). 
September 1954 through September 1955. "D” signi- 
fies that P.E. exceeded rainfall by at least 75 per cent 
and the letter "S” that rainfall was in excess by at 
least 35 per cent. 
ceptions, derives the following formula for 
estimating potential transpiration from vege- 
tated areas. (It checks well with observations 
made in America and Europe.) 
ET = EoXf=f (AH+0.27Ea) / 
(A+0.27) mm/day. 
Where E^ = potential transpiration 
Eo = the hypothetical evaporation that 
would take place from an extended 
sheet of open water exposed to the 
weather conditions found over the site, 
f, an empirically determined seasonal factor 
converting Eq to E^, has the following values: 
Nov. through Feb. 
0.6 
Mar., Apr., Sept., Oct. 
0.7 
May through Aug. 
0.8 
slope of vapor-pressure curve for water 
at mean air temperature 
Ta (mm mer- 
cury/°F) 
H = R^ (l-r)_(0.18+0.55n/N)-a-Ta(0.56 
-0.092 V’^)/(0T0-h0.90n/N) 
0.27 = the constant of the standard hygrom- 
eter equation (mm/°F) 
Ea = 0.35 (ea-Cd) ( 1 +U 2 X 10 ~ 2 ) mm/day 
Ra = theoretically calculable amount of radi- 
ation that would reach the earth in the 
