Evapotranspiration in Hong Kong: A Second Report ^ 
C. S. Ramage^ 
The first report on evapotranspiration in 
Hong Kong (Ramage, 1953)^ describes in- 
stallation of an evapotranspirometer battery, 
twenty months’ measurement of potential 
evapotranspiration (P.E=) from grass surfaces, 
and comparisons of P.E» from grass and some 
common vegetables. 
Comparison of measured P.E. with that 
computed from Thornthwaite’s (1948) for- 
mula reveals that the formula when applied in 
Hong Kong gives values consistently low in 
winter and high in summer. 
This leads to two possible conclusions: 
1) The site of the evapotranspirometers is 
not representative of the area. 
2) Thornthwaite’s formula is not valid for 
Hong Kong. 
The present paper will summarise the first 
five years’ P.E. measurements at Hong Kong 
(October 1951 through September 1956), 
present arguments on the two possibilities 
mentioned above, discuss Penman’s (1948) 
transpiration formula, and, finally, describe 
^ Contribution No. 7 of Hawaii Institute of 
Geophysics. Scientific Report No. 1 of Contract No. 
AF 19 (604)-1942. The research in this document ha- 
been sponsored in part by the Air Force Cambridge 
Research Center, Air Research and Development Com- 
mand, under Contract No. AF 19 (604)-1942. Presented 
at the Ninth Pacific Science Congress, Bangkok, 
November, 1957. Manuscript received September 19, 
1957. 
^ Meteorology Division, Hawaii Institute of Geo- 
physics, University of Hawaii. 
2 This report also appeared on pages 112-124 of 
Johns Hopkins University Laboratory of Climatology, 
Publications in Climatology VoL VII, No. 1, edited by 
J. R. Mather. (Contracts AF19 (604)-193, AF 19 (604)- 
289.) 
the derivation of a simple P.E. formula for 
Hong Kong. 
FIVE years’ potential 
EVAPOTRANSPIRATION MEASUREMENTS 
Table I shows monthly totals of P.E. meas- 
ured for three grass -covered soil tanks at the 
King’s Park Radiosonde Station ( 22 ® 19 ^N .5 
114°10^E., 213 feet above mean sea level) to- 
gether with relevant meteorological data. 
Each tank has an internal area of four square 
meters. To save material, the central tank has 
a wall in common with each of the others. 
The site was unchanged throughout the pe- 
riod and there were no variations in methods 
of watering or of measuring rainfall and over- 
flow. Except when one or two tanks were used 
briefly for growing vegetables, the results are 
means derived from all three tanks in the 
battery. Readings varied insignificantly and 
randomly between tanks, indicating a satis- 
factory local exposure and unlikelihood of 
leaks. In wet weather the overflow tanks 
occasionally flooded. Resulting errors have 
been eliminated from the final values. 
POTENTIAL EVAPOTRANSPIRATION 
OF VEGETABLES 
Validity of P.E. relative to grass for to- 
matoes, Chinese cabbage, and lettuce de- 
termined during the 1952 winter (table 1 of 
first report) was tested in the winter of 1953- 
54. The results appear in Table 2. It can be 
seen that ratios of vegetable P.E. to grass 
P.E. varied little from one winter to the next. 
The method of comparison thus appears to 
give results of real value to irrigation planners. 
81 
