Ramage, C.S.

Pacific Science

ScholarSpace at University of Hawai'i at Manoa

Evapotranspiration in Hong Kong: A Second Report!c. S. RAMAGE2THE FIRST REPORT on evapotranspiration inHong Kong (Ramage, 1953)3 describes in-stallation of an evapotranspirometer battery,twenty months ' measurement of potentialevapotranspiration (P.E.) from grass surfaces,and comparisons of P.E. from grass and somecommon vegetables.Comparison of measured P.E. with thatcomputed from Thornthwaite's (1948) for-mula reveals that the formula when applied inHong Kong gives values consistently low in. winter and high in summer.Th is leads to two possible conclusions :1) The site of the evapot ranspi rometers isnot representative of the area.2) Thornthwaite's formula is not valid forHong Kong.The present paper will summarise the firstfive years' P.E. measurements at Hong Kong(October 1951 thro ugh September 1956),present arguments on the two possibilitiesmentioned above, discuss Penman's (1948)transpiration formula, and, finally, describe1 Contribution No. 7 of Hawaii Institute ofGeophysics. Scient ific Report N o.1 of Cont ract N o.AF 19 (604)-1942. The research in chis docume nt ha-been spo nsored in part by the Air Force CambridgeResearch Center, Air Research and Developm ent Com-mand, un der Cont ract No. AF 19 (604)-1942. Presentedat the Ninth Pacific Science Congres s, Bangk ok,Novemb er, 1957. Manuscript received September 19,1957.2 Me teorology Division, Hawaii In stitute of Geo-physics, University of Hawaii.3 Th is report also appea red on pages 112- 124 ofJ ohns Hopkins University Laboratory of Climatol ogy,Publications in Climatology Vol. VII , No .1, edited byJ . R. M ather. (Contracts AF19 (604)-193, AF 19 (604)-289.)81the derivation of a simple P.E. formula forHong Kong.FIVE YEARS' POTENTIALEVAPOTRANSPIRATION MEASUREMENTSTable I shows monthly totals of P.E. meas-ured for three grass-covered soil tanks at theKing's Park Radioson de Station (22°19'N .,114°lO'E., 213 feet above mean sea level) to-gether with relevant meteorological data .Each tank has an internal area of four squaremeters. To save material, the central tank hasa wall in common with each of the others .The site was unchanged throughout the pe-riod and there were no variations in methodsof watering or of measuring rainfall and over-flow. Except when one or two tanks were usedbriefly for growing vegetables, the results aremeans derived from all three tanks in thebattery . Readin gs varied insignificantly andrandomly between tanks, indicating a satis-factory local exposure and unlikel ihood ofleaks. In wet weather the overflow tanksoccasionally flooded . Result ing errors havebeen eliminated from the final values.POTENTIAL EVAPOTRANSPIRATIONOF VEGETABLESValidity of P.E. relative to grass for to-matoes, Chinese cabbage, and lettuce de-termined during the 1952 winter (table 1 offirst report) was tested in the winter of 1953-54. The results appear in Table 2. It can beseen that ratios of vegetable P.E. to grassP.E. varied little from one winter to the next .The method of comparison thus appears togive results of real value to irrigation planners.82 PACIFIC SCIENCE, Vol. XIII , January 1959TABLE 1MONTHLY POTENTIAL EVAPOTRANSPIRATION AND M ETEOROLOGICAL OBSERVATIONS MADE AT KING'S PARK(22°19' N., 114°1O'E., 213 FT. ABOVE M.S.L.) AND AT ROYAL OBSERVATORY (22°18'N .; 114°1O 'E.,109 FT. ABOVE M.S.L.) FOR THE FIVE·YEAR PERIOD, OCTOBER 1951-SEPTEMB ER 1956,AND ALSO FOR OCTOBER 1956- DECEMBER 1956IYEAR JAN. FEB. MAR. APR. MAY JUN. JUL. I OCT. NOV. DEC.AUG. SEP.- - ------- - - - --- - - - - - - - --1951 144 100 911952 74 79 80 88 120 95 113 104 81 139 100 76Potencial 1953 72 49 73 72 99 130 133 100 119 122 93 81evapotranspiration(Ki ng' s Park) mm. 1954 72 77 88 80 140 102 148 147 161 133 136 831955 79 81 106 107 110 116 118 101 125 147 121 87- - - - --1956 64 84 95 131 142 157 157 124 140 141 116 78--- - - - - - - - --- - - - --- - - - - -5·YEARMEAN 72 74 88 96 122 120 134 115 125 137 110 84----------------------------------------1951 104 75 151952 26 32 40 183 184 560 189 376 838 17 9 24Rainfall 1953 27 64 135 95 354 407 138 374 564 48 41 76(King's Park) mm .1954 53 30 58 160 89 207 147 294 171 20 133 01955 2 0 61 162 446 322 546 580 115 11 49 18- - - - - -1956 67 80 7 79 299 367 170 455 165 37 34 7- - --------- - - - - - - - --- -5- YearMean 35 41 60 136 274 373 238 416 371 40 61 27---------------------------------------1951 77 70 641952 62 61 65 72 80 82 83 81 80 77 73 61Mean temperature 1953 61 61 65 66 76 82 83 83 80 79 71 64(KinK's Park) of .1954 63 61 61 71 80 82 83 83 81 76 70 611955 57 64 69 72 79 81 82 82 83 77 68 66- - - - - -1956 58 61 66 75 79 83 84 82 83 77 68 62REPRESENTATIVENESS OF THEKING'S PARK SITEAn oil-drum evapotranspirometer (see Gar-nier , 1954) was installed at Kai Tak Airport(22°20'N ., 114°12'E., 12 feet above mean sealevel)and readin gs began in September, 1954.The tank was turfed with and surrounded bygrass clipped to 1-2 inches and was wateredfrom abo ve. The site being in an extensiveflat area near sea level differs considerablyfrom Ki ng's Park where the evapo transp irom-eter is atop a small isolated hill.Figure 1 shows results of the first year' ssimultaneous readin gs. For this perio d it issafe to assume that the oil drum did not de-velop leaks. At K ai Tak, in moisture deficientEvapotranspiration in Hong Kong - RAMAGE 83TABLE 1, continuedM ONTHLY POTENTIAL EVAPOTRANSPIRATION AND M ETEOROLOGICAL OBSERVATIONS MADE AT KING'S PARK(22°19' N. , 114°10'E. , 213 FT. ABOVE M.S.L.) AND AT R OYAL OBSERVATORY (22°18 'N. , 114°lO'E .,109 FT. ABOVE M.S.L.) FOR THE FIVE·Y EAR PERIOD, O CTOBER 1951- SEPTEMBER 1956,AND ALSO FOR O CTOBER 1956- DECEMBER 1956YEAR JAN. FEB. MAR. APR. MAY JUN. JUL. AUG. SEPT. OCT. NOV. DEC.- - - - --- - - [-- --- --- - - - - - - - - - - ---- -- - - --5- Ye arMean 60 62 65 71 79 82 83 82 81 77 70 631951 68 61 501952 52 55 61 68 75 77 77 77 75 68 62 47Mean dew 1953point temperatu re(R oya l O bser vatory) OF. 1954515756 60 60 7154 53 67 7477777777787774 7373 6561 5559 481955 41 53 62 64 72 75 77 76 76 64 53 531956 46 55 59 67 73 76 77 76 72 65 52 511951195 2 168197 135 18978 84 114 196 152 192 177 165 240 264 138Sunshine du ration 1953 130(Royal Observatory) hours59 75 61 125 194 275 241 114 211 141 1531954 160 116 109 69 221 159 254 206 218 256 213 1501955 210 147 121 174 169 137 160 208 253 289 181 2291956 165 66 102 175 137 214 263 229 229 242 172 164--- ---- - - [--- - - - - - - --- ----------- - - - - -5 · Ye arM ean 167 93 98 119 170 171 229 212 196 239 187 172---------[---------------------------1951 9.5 10.9 9.91952 9.6 11.7 13.2 10.8 8.9 8.1 8. 1 7.0 9.0 9.8 7.5 8.4Mean wind spee d 1953 10.2 11.5 10.9 11.3 9.5 8.8 6.4 5.7 10.3 9.5 8.3 9-4(Royal Observatory) kn ot s1954 9.3 9.4 9.1 11.0 6.7 7.7 5.9 7.3 10.5 8.8 10.6 9.31955 9-4 11.6 ILl 9.3 8.0 9.1 6.8 4.7 6.5 11.2 11.6 10.8--- - - -1956 9.3 12.1 12.5 10.4 9.5 8.2 6.6 9.6 9. 1 9.9 9.4 7.8-:1-:--- - - -----------:1-:--15-YearM ean 11.4 10.6 8.5 8.4 6.8 6.9 9.1 9.6months, grass surrounding the tanks couldnot be watered , while some floodin g possiblyoccurred in moisture-surplus months. SinceKing's Park is not subject to these mensuraldrawbacks they may account for some of thedifferences between the two curves. It seemsreasonable then to conclude that the King 'sPark site is probably a representative one.84TABLE 2COMPARISON OF POTENTIAL EVAPOTRANSPIRATIONFROM VEGETABLES AND GRASSCHINESECROP TOMATOES CABBAGE LETTUCEPlanting date . .. . . . . 28 Oct. 28 Oct. 2 ]an.1953 1953 1954Days to reachmaturity ... ... . . . 128 61 79(last fru it)Total crop weight(lbs .) . . . . . . . .. . . 85 (poor 34Y2crop)(1) Total P.E. (rnm. ) 838 282 271Average dailyP.E . (mm.) .. .. 6.5 4.6 3.4(2) Total P.E. ofgrass in sameperiod (mm.) . . 354 182 193Average dailyP.E. of grassin same period . 2.8 3.0 2.4(1)Ratio-. .... .. . . . 2.4 1.5 1.4(2)Me an temperatureduring period (OF.) 65 68 62(1)Ratio - for 1952(2)winter . .. . . . .. . . . 2.3 1.6 1.7COMPARISON OF MEASURED POTENTIAL EVAPO-TRANSPIRATION WITH VALUES DERIVED FROMTHORNTHWAITE 'S AND PENMAN 'S FORMULAETborntbtoaite's FormulaThe conclusions of the first report are amplyconfirmed 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 drywinter and too high in the wet summer. Sinceinvestigators in Australia (Leeper, 1950), Ni -geria (Garnier, 1954), and Trinidad (Smith,1954) report similar shortcomings in the for -mula one must conclude that it should not beassumed to be valid for any monsoon ortropical maritime region .Penman's FormulaPenman (1948, 1950), using energy con-PACIFIC SCIENCE, Vol. XIII, January 1959IX X XI Xli I II 111 IV it VI VII VIII IX1954 1955FIG. 1. M onthly potential evapotranspir ation meas -ured at King's Park (1), and at K ai Tak Airport (2).September 1954 through September 1955. "D" signi -fies th at P.E. exceeded rainfall by at least 75 per centand the letter " S" that rainfall was in excess by atleast 35 per cent.ceptions, derives the following formula forestimating potential transpiration from vege -tated areas. (It checks well with observationsmade in America and Europe.)ET=EoXf= f (.6H + 0.27Ea) /(.6 +0.27) mrn/ day.Where ET= potential transpirationEo=the hypothetical evaporation thatwould take place from an extendedsheet of open water exposed to theweather conditions found over the site.f, an empirically determined seasonal factorconverting Eo to ET , has the following values:No v. through Feb. 0.6Mar., Apr., Sept., Oct. 0.7May through Aug . 0.8.6 = slope of vapor -pressure curve for waterat mean air temperature Ta (mm mer-cury;CF)H =RA (I-r) (0.18+ 0.55n/ N) -(1T~(0.56- 0.092 "ect)/(0.1O+ 0.90n/ N)0.27 = the constant of the standard hygrom-eter equation (mm;CF)Ea=0.35 (ea- ed) (1+u2XlO-2) mm/dayRA = theoretically calculable amount of radi-ation that would reach the earth in the.Evapotranspiration in Hong Kong - RAMAGE 85absence of an atmosphere, convertedto an evaporation equivalent by put-ting 59 cal/em- = 1 mm . evaporation .r = reflection coefficient, to be taken as0.05. For standard evaporation tankswith light walls and clean water it maybe very much greater.n/N = actual/possible hours of sunshine. Inthe first term in H it is a factor limitingincoming short-wave radiation; in thesecond it is a transform of a cloudinessfactor limiting outward long-waveradiation.aTa 4 = theoretical black-body radiation atmean air temperature Ta ; this too is inevaporation units like RA •ed= saturation vapor pressure at dewpoint(mm Hg).ea= saturation vapor pressure at mean airtemperature ; hence (ea-ed) is themean saturation deficit.U2 = average wind speed in miles/day at. 2 m. above the ground.Since the formula incorporates both hu-midity and wind parameters it appearedlikely to give more reasonable results thanThornthwaite's formula when applied inHong Kong. The data listed in Table 1 havebeen used to calculate mean monthly P.E.according to Penman's formula and the val-ues are plotted in Figure 2. They approachthe observed values more closely than thosedetermined by Thornthwaite's formula buterr in the same sense, overestimating summerand underestimating winter P.E.POTENTIAL EVAPOTRANSPIRATIONFORMULA FOR HONG KONGDeriving the FormulaIn Penman's formula, the expressionEa=O .35 (ea- ed) (1+u2X10-2) mm/dayincorporates humidity and wind parameters.Figure 3 shows a plot of both E, and meandaily observed P.E. at Hong Kong derivedfrom the monthly means for five years. FromOctober through January and from February40100/\i' -.. \.' ",I \I \i \ (2),jI!I........... .... I<. , . •, !', !876453(mm)9i1I~ /I",,,' ,..". I80" ~., ,'x ~ . . ," I ,><."'- ; I60 <. i ".... ..J I..",,,,---'140120p.E.(rnrn)160II III IV V VI VII VIII IX X XI ~II 2FIG. 2. Monthly means of potential evapotranspira-tion measured at King's Park (1), and calculated ac-cording to Thornrhwaire's formula (2), Penm an'sformula (3), and the new formula (4) . Based on fiveyears' data.II III IV V VI VII VIII IX X XI XIIFIG. 3. Mean daily potential evapotranspirationmeasured at King's Park (1), and Ea (2). Based on fiveyears' data. .86 PACIFIC SCIENCE, Vol. XIII, January 1959TABLE 3M EASURED AND C ALCULAT ED MONTHLY P OT ENT IAL EVAPOTRANS PIR ATIO N AT HONG KONGFOR 1953 AND O CT OBE R THROUGH D ECEMBER 1956 (MM.)1953 1956--- - - - --- - ----------- - - ------ -Jan. Feb. Mar. Apr. May J un. JuI. Aug. Sep. Oct. N ov . Dec. Oct. N o v. Dec .- - --- - - - - - - --- - - - - ------------ -Measured . . ... . 72 49 73 72 99 130 133 100 119 122 93 81 141 116 78Thorn thwai teformula .. . . . 35 33 53 59 122 165 175 168 138 135 73 45 115 58 38Di fference .. .... + 37 + 16 + 20 +13 - 23 - 35 - 42 - 68 - 19 - 13 + 20 + 36 +26 +58 + 40Penman formula 64 46 65 76 106 140 160 144 114 104 69 59 125 85 57Difference . . . . . + 8 + 3 + 8 - 4 -7 - 10 - 27 - 44 +5 +18 +24 + 22 +16 + 31 + 21New formula... 81 57 78 81 106 126 127 121 129 124 105 84 142 111 81Difference . . . . . -9 - 8 - 5 - 9 -7 + 4 + 6 - 21 - 10 - 2 - 12 - 3 -1 +5 - 3"." ........,160140120P.E.(rnmline whose equation isP.E.= VEa+ vea- 3.5 mm/ dayValues calculated from this equation andplotted in Figure 2 are in reasonable agree-ment with mean monthly measured P.E.Testing the Formula1953 was the most abnormal year of thefive-year period . Table 3 and Figure 5 depictoIXo8VIIXJ/i~<> 7 / SO+'0'" IV/""> /XII / 60 '> (310 / / ,~'III \ (21I 0 406 o gthrough August the curves are similar al-though between these periods they undergosharp relative displacemenrs. Nevertheless,E, seems to bear some inrraseasonal relation-ship to measured P.E. This may explain whyPenman's formula approximates Ho ng Ko ngobservations more closely than does Thorn-thwaite's formula which does not explicitlyembody humidity and wind parameters.Thornthwaite attaches great importance totemperature in the evapotranspiration process.This was allowed for by combining ea with E,in the form VEa+ vea' When this expressionis plotted against mean daily measured P.E.(Fig. 4), a good fit is provided by a:straight3 4Doily P.E.(mm)FIG, 4. Mean daily potential evapotranspiration meas-ured at King 's Park plotted against .,;E;. +ve;;.Based on five years' data.III rv v VI VII VIII IX X XI XIIFIG. 5. Mo nthly potential evapotranspiration for1953 measured at Ki ng's Park (1), and calculated ac-cording to Thornthw aite's formula (2), Penman' sformula (3), and the new formula (4).Evapotranspiration in Hong Kong - RAMAGEmeasured P.E. and P.E.calcula ted by theThornthwaite, Penman , and new formulae.In addition, Table 3 compares the formulaefor the three months following the five-yearperiod. Again the new formula best approx i-mates H on g K on g measurements.CONCLUSIONSEvapotranspiration formulae based on mid-latitude data should not be uncritically usedin estimating P.E. for mon soonal or tropicalmaritime regions. .In such regions, humidity and wind may beas important parameters as temperature, andsimple formu lae similar to that derived forHong Kong might fairly be used to calculatepotential evapotranspiration.REFERENCESGARNIER, B. J. 1954. Measuring potent ialevapotranspiration in N igeria . J ohns Hop-87kins Univ., Pub. Climatol. 7(1) : 140- 176.LEEPER, G . W: 1950. Thornthwaite's climaticformula. Austral. Inst. Agr. Sci. J our. 16(1) :2-6.PENMAN, H. 1. 1948. N atural evaporationfrom open water, bare soil and grass. Roy.Soc. London, Proc. Ser. A. 193: 120- 145.--- 1950. Evaporation over the BritishIsles. Roy. Met. Soc., Quart. Jour. 76: 372-383.R AMAGE, C. S. 1953. Evapotranspira tionmeasurements made in H ong K ong . Firstreport - O cto ber 1951 to M ay 1953. Roy.Observ. Hong Kong, Tech . Note 7: 1- 4.SMITH, G . W. 1954. Evapotrans piration inTr inidad, B.W.I. J ohns Hopkins Univ. Pub.Climato/. 7(1) : 125-1 39.THORNTHWAITE, C. W. 1948. An approachtoward a ration al classification of climate.Geog. Rev. 38: 55-94.

Evapotranspiration in Hong Kong: A Second Report

Ramage, C S

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Evapotranspiration in Hong Kong: A Second Report

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