UTILITY OF EMPIRICAL FORMULAE FOR COMPUTING POTENTIAL EV APOTRANSPIRA TION

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Annals of Arid Zone 27 (1) : 2935, 1988 UTILITY OF MPIRIAL FORMULA FOR OMPUTING POTNTIAL V APOTRANSPIRA TION P. R. RDDY, A. HANDRASKHAR,J. VNKATARAMUDUand T.

1 Annals of Arid Zone 27 (1) : 2935, 1988 UTILITY OF MPIRIAL FORMULA FOR OMPUTING POTNTIAL V APOTRANSPIRA TION P. R. RDDY, A. HANDRASKHAR,J. VNKATARAMUDUand T. DAYANANJI) Agricltral Research StationAnantapr ABSTt<AT ompting potential evapotranspiration (Pn, which is generally inflenced by varios weather parameters, is an important step in estimation of water reqirements of crops. An attempt was made to tilise for prediction methods, recommended by FAO, lor compting PT for Anantapr region. Modified Penman method was more sitable since a nmber of weather parameters are taken in to consideration. PT vales from the other three methods can be converted into modified Penman vales with the ratios worked Ollt. INTRODUTION ompting water reqirement of crops is an essential step in the proper tilization of irrigation water. Water reqirement can be estimated by differnt methods, the most accrate being the field method. Bt this method is laborios and time consming. stimation of potential evapotranspiration (PT) by sing empirical formlae is one of the steps in compting water reqirement. The FAa grop of scientists have selected for sch formlae as more sitable for compting PT (Doorenbos and Pritt 1977). An attempt was made to estimate the PT vales for Bapatla in Andhra Pradesh and padegaon in Maharashtra tilising the formlae recommended by the FAa (Ramachandra Reddy et ai., 1984 and hhabda et al ). Anantapr in Andhra Pradesh receives an annal rainfall of less than 55 mm and its irrigation potential is less than 2% of the cltivated area. As the irrigation water availability is meagre, there is every need to tilise the irrigation water most economically for profitable crop prod'.:tion. Hence an attempt was made to estimate PT vales for the Anantapr region tilizing the for prediction methods. MATRIAL AND MTHODS Anantapr is located on the Deccan Platea at I4 41'N latitde 77 4' longitde and at an altitde of 35 m above mean sea level. Most of the rainfall is being received dring sothwest monsoon period (Jly to October). Delayed onset and early withdrawal of monsoon with prolonged dry spells is a common featre of the monsoon in this region. The weather data recordeq at Agricltral Research Station, Anantapr (TableI) were tilised for compting PT vales. For methods recommended by FAa (Doorenbos and Pritt, 1977) were employed:

2 3 : RDDY et al. A. Blaneyriddle method PT = IP (.46 T + 8) I mm/day (1} where T = Mean daily temperatre (O) over the month, P = Mean daily percentage of total annal day time hors which depends on latitde and the month. = Adjstment factor, which depends on minimm relative hmidity, snshine hors and day time wind B. Radiation method PT = (w. Rs) mm/day (2) wind where = adjstment factor which depends on mean hmidity and day time w = weighting factor which depends on temperatre and altitde eqation (3) Rs= solar radiation in eqivalent evaporation in mm/day, calclated by Rs= ( n/n) Ra (3) where Ra= extra terrestrial radiation which is a fnction of latitde and tho month (Table2) n = actal snshine hors per day N possible snshine hors per day The actal (n) and compted (N) snshine hors are given in Tables 1 and 2, respectively.. Modified penman method For the areas where measred data on temperatre, hmidity, wind and snshine hors are available, modified penman method is more sefl. The procedre to calclate PT by this method is rather complicated as it takes into consideration many weather parameters. PT = W.Rn + (lw. fe). (ea ed) mm/day (4) where W Weighting factor which depends on temperatre and altitde Rn = net radiation mm/day III eqivalent evaporation f () = wind related fnction (ea ed) = difference between satration vapor pressre at mean air temperatre and mean actal vapor pressre of air, both in milli bar.

$FORMULA FOR POTNTIAL VAPOTRANSPIRATION: 31,....c '\ eo M \ \ '\ M "'l r: Vl '\ S r.: N '\ \ \ Vl vi...: '..:.:... <> ;> '. '(;.... Vl Vl \ " Vl.' " r.: "'!... '\ d Vl d s;. D vi.i.i ll.. '\ = ;.$

3 FORMULA FOR POTNTIAL VAPOTRANSPIRATION: 31,....c '\ eo M \ \ '\ M "'l r: Vl '\ S r.: N '\ \ \ Vl vi...: '..:.:... <> ;> '. '(;.... Vl Vl \ " Vl.' " r.: "'!... '\ d Vl d s;. D vi.i.i ll.. '\ = ;....l:: ' U ' to.c:2 =.,., M "l '\ '<t; (.).;:: c =' \ \.. Vl vi.. G) r:..: eo.,.d =' M N M ni,... \ G :?O, c :c =' '\ :::....,. " ='. U c:: >< d M..,. M c < ; '\ \." Vl '\ \..,.... Vl..,. M M M M..,. Vl M M '\ M.....,. \ OQ ' '. '\ M N..,. Vl M '<t;... N... M M... N (.)..,. M 9 U Vl Vl c "'!..,. M "1 IS..., M.. \ N N \ lei.,;. ai M <"l :: lu (.). '.... >. to >....D..cl IS.,. e (.) ';:: >. ='. >. ;> <J IS. d o ti d : c:: =' =' III ::.::,< :: "3 G) =' =' z i.li , < rn

$32 ; RDDY et at... \ N S' \ \ e N.,.,.,.,.., : ""l ""l ""l \ \ \ ' c:i c:i c:i e e ci c:i c:: c::.2 III c:: '<I"....,..., '<I" e > > > > \ \. > e \ U,..., Z.,., \ N \..". 5 ' N In.."..,., \ r c:i c:i c:i c:i c c:i c:i c:i c:i \.$

4 32 ; RDDY et at... \ N S' \ \ e N.,.,.,.,.., : ""l ""l ""l \ \ \ ' c:i c:i c:i e e ci c:i c:: c::.2 III c:: '<I"....,..., '<I" e > > > > \ \. > e \ U,..., Z.,., \ N \..". 5 ' N In.."..,., \ r c:i c:i c:i c:i c c:i c:i c:i c:i \...,... \ ,..."..., ,.,....,..,. N N N N N N N c:i c:i c:i c:i c:i c:i c:i c:i \ e \..,.., \..,.., \ \ '<l:.., \ """ \..". N N N \ ""l N.,.,....,.:,.:.. \ \.,., V) ' :' \ e... '<I" tn : \ N.,.,..,...,..,., <:..,. \ r;.,., : ":. \..,. \,.: \ vi.. N...."...,..,....,.., N g Il.. <'l <'l N N <'l N N "! N N N f'l \ \ \ \ \ \.,., ci c:i c:i c:i c:i c:i <:5.,., N N.....,..,. N N f'l N N N N N N N N N ci c:i c:i <:5 c:i ci c:i c:i c:i c:i N li) e \ \.., 1' ::l c:i c:i. c:i c:i c:i c:i c:i c:i ci c:i : " :>. os c: "<t 1 \ "': I"; r; <: : N M i li) li).,., li) li) i N N c::: :>. U" ": 1 e li) \ \ N ' "! "': ::: N N N.., N N N... co z: :c co. li) li) li) :>. \ \ e co c:i ci ci c:i c:i c:i c:i ci c:i = a: " li).,., "5 o U ::.. " ::l.a. :>...8 > " ::l U.. lj., "3 <:.. ::l <: Z Q :>....n N co 5 ::l ::l : c:...;: >, ([) c:: co. eo co c: I... t/) ""

FORMULA FOR POTNTIAL VAPOTRANSPIRATION: 33 = adjstment factor to compensate for the effect of day and niiht weather eonditions alclations of Rn Rn = Rns Rnl Rns (Net shoet wave radiation) a:: (l.

5 FORMULA FOR POTNTIAL VAPOTRANSPIRATION: 33 = adjstment factor to compensate for the effect of day and niiht weather eonditions alclations of Rn Rn = Rns Rnl Rns (Net shoet wave radiation) a:: (l.25) Rs Rnl (Net long wave radiation) """ f (t). f (ed). f (n/n) which are correction factors for temperatre, vapor pressre and the ratio of actal to maximm snshine hors, respectively. alclation of (eaed) ea = satration vapr pressre at mean air temperatre in milli bar RH mean ed = ea x 1 D. Pan vaporation method vaporatin pans provide a measrement of integrated effeet of radiation, wind, temperatre and hmidity on evaporation from open water srface. PT = Kp.. pan pan = pan evaporation in mm/day from US class A pan Kp = pan coefficient which depends on hmidity, wind and pan environment RSULTS AND DISUSSION The estimated PT vales for all for methods for each month in the year are presented in Tabcl3. High PT vales were recorded dring the smmer (March to May) ;nd early kharif (Jne to Agst) in all the methods de to high temperatre, low relative hmidity and strong wind conditions prevailing at that time. Low PT vales were recorded dring rabi and winter season (October to December) de to mild climatic conditions. It is an established fact that temperatre, relative hmidity and wind velocity are the major factors which inflence evapotranspiration. Highest PT vales were recorded in modified Penman method becase it took into consideration varios weather parameters inflencing evapotranspiration. Ratios of estimated PT from the three methods with modified Penman method were also calclated and are presented in Table3. The ratios are helpfl in converting the PT vales from the other three methods into vales by the modified Penman method. Pan evaporation data can be tilised for compting PT as as it is the easiest of all the for methods. By mltiplying the PT data from Pan evaporation method with the concerned ratio, we can arrive at the PT vale by modi fled Penman method.

$34 RDDY et a1. e: t<l ll r oo N \ r M N N r N V "'l.,., a c:l N \ e: "" ii:" "'l "'l <") <") M <") i:l. <c: : a os v r v v N \ \ r M M N \ ii; r \..". v \ t<l : : : M "! "': ::...: t<l.;; s:l ai U :II \ v M r r M r \ \ \ ii;.$

6 34 RDDY et a1. e: t<l ll r oo N \ r M N N r N V "'l.,., a c:l N \ e: "" ii:" "'l "'l <") <") M <") i:l. <c: : a os v r v v N \ \ r M M N \ ii; r \..". v \ t<l : : : M "! "': ::...: t<l.;; s:l ai U :II \ v M r r M r \ \ \ ii;..". r \ \ : "! "! M "" "'l..c:...:...:., g ' ' :1 'B :: r v v v \..s 6 N \ "! N M M r: \ r r»....". r i :; c. (ij :1 Ul 1 > e:. c e: e: «i:l. \ N \ N M r M \..". M.,., \ \ r oo.."...".,..;..; on on..;..;..;,..; M ; e: ;::. III ; g M N.. N "tl N \ N :..". r: r: r: on on on on on..;..; g :: s:l e ;» ". e: : N N N "tl al.;: on.."...;,..;,..;,..; c<i r:..; " G e: U >.. "tl..c:.. U 15 ll.. e > M os..c: = e.. :1 on.. c::l :1.. :1.. :c B.. c:...;: >.» OIl 'S. ;.. os. c: :; :1 I<. II. <... :1... <.. (/] Z Q

FORMULA FOR POTNTIAL V APOTRANSPIRA TION : 35 The correlation coefficients between modified Penman and rest of the methods were calclated. Their vales were.946,.8559 and.

7 FORMULA FOR POTNTIAL V APOTRANSPIRA TION : 35 The correlation coefficients between modified Penman and rest of the methods were calclated. Their vales were.946,.8559 and.8534 between modified Penman and Pan evaporation method, modified Penman and radiation method and modified Penman and Blaneyriddle method, respectively which were highly significant. The highly significant correlation between PT vales from modified Penman method to those of other methods indicates the sitability of all the methods for compting PT vales. Depending on the availability of climatic data, any of the for prediction methods can be tilised for compting potential evapotranspiration for a given place. RFRNS hhabda, P.R., More, S.D., Palaskar, M.S., and Varade, S.B. }86. A comparison between Penman and Hargreaves methods in comptation of reference crop evapo transpiration. Jornal of Indian Society of Soil Science. 34 : Doorenbos, J., and Pritt, W.O., Gidelines for predicting crop water reqirements. FAa, Rome.:} 4. Ramachandra Reddy, P., Rami Reddy, S., Sbramanyam, M.V.R., Ramakrishna Reddy, M. G. and Gopala Rao, P stimation of potential evapotranspiration sing prediction methods for Bapatla region. The Andhra Agricltral Jornal. 31(1) : 2328.

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