COMPUTING POTENTIAL EVAPOTRANSPIRATION BY DIFFERENT METHODS*

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.Annals f Arid ne 27 (3 & 4) : 253-261, 1988 COMPUTING POTNTIAL VA

1 .Annals f Arid ne 27 (3 & 4) : , 1988 COMPUTING POTNTIAL VAPOTRANSPIRATION BY DIFFRNT MTHODS* V. UMA.MAHSWARA RAO ANO K.P.R.V. MURTyl Haryana Agriltral University, Hisar-1254 India ABSTRACT Ptential evaptranspiratin PT) vales at twenty tw statins in-semi-arid 'l"egin f soulh India were evalated by sing {he three methds (Diffsin. Thrnthwaite, Penman). Cmparisn f mnthly and.annal vales f PT by the three methds revealed PT vales by diffsin methd t be.lse 1 thse by PeRman methd. INTRODUCTION An estimate f the water ptential in different perids f tbe year is pref«jisite fr planned harnessing f water fr irrigatin prpses. One f the imprtant parameters in water bdgeting is the estimat;n f the PT t wrk t the atal (sses f water vapr frm sil t atmsphere. In d present stdy,.an attempt was made t estimate the PT vales fr 22 statins vering the semi-arid regin 'Of atn India (Fig 1) by tbing diffsin m::thd (Mrty, 1979). The allated vales were mpared with the ther methds viz. Thrnthwaite (1948) and Penman (1"948) mdified frmla (Ra et al ). MATRIAL AND MTHODS The prindple f the diffsin methd is based n the estimatin f vert;a1 water flx treating the prblem as a bndary vale prblem. The details f slving the simltanes eqatins were desribed by Mrty (1979), Ra (1981) and Ra et al. (1983). This methd prvides fr a mre arate estimatin f the atal water vapr flx as a fntin f the dependent prperties at tw bndaries, that ;s, the sil srfae and the inming mass f air. Fr ease f allatin, a nmgram was prepared (Fig. 2). Utilizing this nmgram, the rate f evaptranspiratin in mm/ <lay etd be estimated frm the bserved vales f temperatre, wind and atmspheri hmidity. In PT vale is read frm the given vale f atal vapr pressre ;n the hrizntal axis by mving vertially p t the pint f intersetin with the bserved tmperatre istherm and then by mving hrizntally p t the given vale f wind. Part f the Ph D. wrk ftirst athr.!prfessr, Deptt. f Meterlgy and Oeangraphy, Andhra University, Waltair 533, India.

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3 COMPUTING POTNTIAL V APOTRANSPIRA TION: 255 Fig. 2 Nmgram fr the determinatin f evaptranspiratin. The mnthly PT vales were estimated sing the three methds (Diffsin, Thrnthwaite, Penman's mdified frmla) fr 22 statins in the semi-arid regin f sth India frm the limatlgial nrmals fr the perid pblished by India Meterlgial Department. RSULTS AND DISCUSSION The annal variatins f the mnthly PT vales btained in the three methds fr different statins are peresented in Table 1. The annal marh is generally similar at all the statins shwing higher vale in the mnth f May and a sendary peak vale in the mnth f Otber. The

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COMPUTING POTNTIAL VAPOTRANSPIRATION: 259 inrease in PT frm winter t smmer is fairly rapid while the PT latter dereases slwly in almst all the ases.

7 COMPUTING POTNTIAL VAPOTRANSPIRATION: 259 inrease in PT frm winter t smmer is fairly rapid while the PT latter dereases slwly in almst all the ases. Hwever, there are ertain nspis differenes amng the statins with regard t the sendary maximm and minimm. ven thgh the three methds fr eah statin bradly shw similar trends, sharp differenes in sme ases are ntieable. At the statins Dhad, Akla, Arangabad, Glbarga and Tirhirapalli the annal variatin f PT is fnd t be very large and this is partilarly refleted in the diffsin methd whih is shwing relatively higher range f variatin, frm minimm t maximm at all these statins exept Glbarga. This is mainly de t the high wind speeds assiated with the water vapr transprt whih are taken int ant nly by the diffsin methd. It is interesting t nte that the PT vales f Visakhapatnam, Mahilipatnam, Nellre, Khammam and Vellre (all statins nearer t the east ast) are strikingly similar. PT by diffsin methd is nsistently estimated lwer than by the ther tw methds. This is prbably de t lwer magnitdes f water transprts nder sea breeze effets. Hwever, Tirhirapalli, thgh nearer t the ast, presents a qite different type f variatin prbably de t the very high winds. At ther statins the three methds are giving vales whih are mparable. At these statins the wind effets and the assiated water vapr transprt rrespnd t average nditins and this may be ne f the reasns fr the lw vales mpted by the diffsin methd agreeing fairly well with thse by the ther methds. Fr all the statins, vales frm the Penman methd are in agreement with the diffsin methd better than thse frm Thrnthwaite methd. The diffsin methd, nsidering mre nmber f inpt parameters, ffers a realisti explanatin f the basi physial presses invlved and gives a detailed pitre f the PT variatins. The annal ttals f PT as btained frm the three methds at the 22 statins were sed t prepare the annal PT harts fr the semi-arid regin f sth India (Fig. 3 a t ). In all the three methds, the eastern parts f Karnataka are shwing relatively higher PT. Hwever there are sme differenes in the three harts with regard t the gradients and latitdinal and lngitdinal rientatins f the islines. In the diffsin methd (Fig. 3 a), the PT islines are generally lngitdinally riented and there is a narrw belt f high PT frm Anantapr twards nrth -nrth-west. The PT vales generally derease twards sth-east twards the ast. There are large variatins f PT and the islines are very lse t ne anther indiating high latitdinal gradients f PT. There is anther pket f high PT tward the sth near Tirhirapalli. In the Penman methd the high PT vales are mainly nentrated in the eastern Karnataka regin (Fig. 3 b). PT vales derease twards nrth, sth and east. The lngitdinal natre f the islines as bserved in the diffsin methd is present nly twards nrth. xept in this regin, the latitdinal gradients are relatively small when mpared t diffsin methd.

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8 26 : RAO & MURTY (a) 1 FFU51H ' ARA81AN 8 A Y r 15 SA 8 N GAl 1 1 N l)thornthwait 2',- " J5' ARA 81AN 8 A Y r 15 SA. 8NGAl II 1 N 7 75' 8' 85' ' Fig. 3 Distribtin f mean annal PT mm

9 COMPUTING POTNTIAL V APOTRANSPIRA TION : 261 The Thrnthwaite methd shws smewhat different harateristis f PT variatin (Fig. 3). The PT inreases frm nrth-east t sth-east and a tnge-like strtre f high PT vales extends int eastern Karnataka regin frm the sth. Frther the islines are mre latitdinally riented and the gradients are direted mre lngitdinally. Hwever a regin f lw PT vales is nspis in the regin arnd Pna in all the three methds. In all the three harts an inreasing tendeny is bserved frm Salem t TirhirapaIli. RFRNCS Mrty, K.P.R.V The physial priniples in determining the rate f irrigatin. Indian Jrnal f lgy. 6(1): Penman, H.L Natral evapratin frm pen water, bare sil and grass. Preedings f the Ryal Siety Lndn. Series A. 193: Ra, K.N., Gerge,.J. and Ramasastri, K.S The limati water balane f India. Memirs f the India Meterlgial Department. III: 42. Ra, V. Umamaheswara, Sme Agrmeterlgial aspets f the semi-arid regin f sth India. Ph.D. Thesis, Andhra University, Waltair (npblished). 196 pp. Ra, V. Umamaheswara, Kmar, K. Kaman and Mrty, K.P.R.V A new apprah t water management. The Natinal Gegraphial Jrnal f India. 26: Thrnthwaite, C.W An apprah twards a ratinal lassifiatin f limate. Gegraphial Review. 38:

UTILITY OF EMPIRICAL FORMULAE FOR COMPUTING POTENTIAL EV APOTRANSPIRA TION

UTILITY OF EMPIRICAL FORMULAE FOR COMPUTING POTENTIAL EV APOTRANSPIRA TION 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