518:
168:
22:
848:. It is defined as the evapotranpiration for " hypothetical reference crop with an assumed crop height of 0.12 m, a fixed surface resistance of 70 s m-1 and an albedo of 0.23." This reference surface is defined to represent "an extensive surface of green grass of uniform height, actively growing, completely shading the ground and with adequate water". The corresponding equation is:
513:{\displaystyle {\overset {\text{Energy flux rate}}{\lambda _{v}E={\frac {\Delta (R_{n}-G)+\rho _{a}c_{p}\left(\delta e\right)g_{a}}{\Delta +\gamma \left(1+g_{a}/g_{s}\right)}}}}~\iff ~{\overset {\text{Volume flux rate}}{ET={\frac {\Delta (R_{n}-G)+\rho _{a}c_{p}\left(\delta e\right)g_{a}}{\left(\Delta +\gamma \left(1+g_{a}/g_{s}\right)\right)L_{v}}}}}}
1113:
equation was developed as a substitute for the Penman-Monteith equation to remove dependence on observations. For
Priestley–Taylor, only radiation (irradiance) observations are required. This is done by removing the aerodynamic terms from the Penman-Monteith equation and adding an empirically derived
1137:
The underlying concept behind the
Priestley–Taylor model is that an air mass moving above a vegetated area with abundant water would become saturated with water. In these conditions, the actual evapotranspiration would match the Penman rate of reference evapotranspiration. However, observations
99:, yet are often not emphasized in results because the precision of this component is often weak relative to more directly measured phenomena, e.g., rain and stream flow. In addition to weather uncertainties, the Penman-Monteith equation is sensitive to vegetation-specific parameters, e.g.,
1185:, is not a closed box but constantly brings in dry air from higher up in the atmosphere towards the surface. As water evaporates more readily into a dry atmosphere, evapotranspiration is enhanced. This explains the larger-than-unity value of the Priestley-Taylor parameter
984:
827:
While the Penman-Monteith method is widely considered accurate for practical purposes and is recommended by the Food and
Agriculture Organization of the United Nations, errors when compared to direct measurement or other techniques can range from -9 to 40%.
788:
818:
concentration in the air, that is to say plant reaction to external factors. Different models exist to link the stomatal conductance to these vegetation characteristics, like the ones from P.G. Jarvis (1976) or Jacobs et al. (1996).
1138:
revealed that actual evaporation was 1.26 times greater than reference evaporation. Therefore, the equation for actual evaporation was found by taking reference evapotranspiration and multiplying it by
841:
To avoid the inherent complexity of determining stomatal and atmospheric conductance, the Food and
Agriculture Organization proposed in 1998 a simplified equation for the reference evapotranspiration
1158:. The assumption here is for vegetation with an abundant water supply (i.e. the plants have low moisture stress). Areas like arid regions with high moisture stress are estimated to have higher
854:
648:
1205:. The proper equilibrium of the system has been derived. It involves the characteristics of the interface of the atmospheric boundary layer and the overlying free atmosphere.
1181:
The assumption that an air mass moving over a vegetated surface with abundant water saturates has been questioned later. The atmosphere's lowest and most turbulent part, the
1203:
1176:
1156:
1132:
675:
1062:
N.B.: The coefficients 0.408 and 900 are not unitless but account for the conversion from energy values to equivalent water depths: radiation = 0.408 radiation .
79:(ET) from meteorological data as a replacement for direct measurement of evapotranspiration. The equation is widely used, and was derived by the United Nations
1573:
van
Heerwaarden, C. C.; et al. (2009). "Interactions between dry-air entrainment, surface evaporation and convective boundary layer development".
40:
1618:
1342:
Jarvis, P. (1976). "The interpretation of the variations in leaf water potential and stomatal conductance found in canopies in the field".
807:
accounts for aerodynamic effects like the zero plane displacement height and the roughness length of the surface. The stomatal conductance
1435:
1514:
1255:
1506:
1090:
979:{\displaystyle ET_{o}={\frac {0.408\Delta (R_{n}-G)+{\frac {900}{T}}\gamma u_{2}\delta e}{\Delta +\gamma (1+0.34u_{2})}}}
1369:
Jacobs, C.M.J (1996). "Stomatal behaviour and photosynthetic rate of unstressed grapevines in semi-arid conditions".
80:
58:
1648:
1478:
1461:
1250:. FAO Irrigation and drainage paper 56. Rome, Italy: Food and Agriculture Organization of the United Nations.
1069:
can then be used to evaluate the evapotranspiration rate ET from unstressed plants through crop coefficients K
1312:
150:
effects, and salinity). Models of native vegetation cannot assume crop management to avoid recurring stress.
36:
1271:
Keith Beven (1979). "A sensitivity analysis of the Penman-Monteith actual evapotranspiration estimates".
531:
797:
refers to the resistance to flux from a vegetation canopy to the extent of some defined boundary layer.
1182:
1397:
543:= Volumetric latent heat of vaporization. The energy required per unit volume of water vaporized. (
1051:
783:{\displaystyle g_{a}={\tfrac {1}{r_{a}}}~~\And ~~g_{s}={\tfrac {1}{r_{s}}}={\tfrac {1}{r_{c}}}}
658:
635:
1439:
1188:
1161:
1141:
1117:
1582:
1539:
1282:
1273:
8:
999:Δ = Rate of change of saturation specific humidity with air temperature. (Pa K)
569:Δ = Rate of change of saturation specific humidity with air temperature. (Pa K)
1586:
1543:
1530:
Culf, A. (1994). "Equilibrium evaporation beneath a growing convective boundary layer".
1286:
1638:
1633:
1598:
1555:
76:
1643:
1602:
1559:
1510:
1483:
1462:"On the Assessment of Surface Heat Flux and Evaporation Using Large-Scale Parameters"
1417:
1382:
1324:
1294:
1251:
1243:
1590:
1547:
1473:
1409:
1378:
1351:
1290:
1093:
modify the standard Penman-Monteith equation for use with an hourly time step. The
107:
1505:. ASCE Manuals and Reports on Engineering Practices. Vol. 70. New York, NY:
1344:
Philosophical
Transactions of the Royal Society of London. B, Biological Sciences
127:
1413:
1245:
1041:
814:
accounts for the effect of leaf density (Leaf Area Index), water stress, and CO
622:
1627:
1487:
1421:
1218:
1214:
1101:-integrated hydrologic models estimating ET using Penman-Monteith equations.
598:
147:
139:
96:
1500:
1355:
1328:
1247:
1221:. Penman published his equation in 1948, and Monteith revised it in 1965.
996:= Reference evapotranspiration, Water volume evapotranspired (mm day)
1551:
1094:
1009:
579:
114:) account for differences between specific vegetation modeled and a
1594:
1244:
Richard G. Allen; Luis S. Pereira; Dirk Raes; Martin Smith (1998).
1018:= Ground heat flux (MJ m day), usually equivalent to zero on a day
534:. The energy required per unit mass of water vaporized. (J g)
95:
Evapotranspiration contributions are significant in a watershed's
1398:"A discussion on and alternative to the Penman–Monteith equation"
611:
143:
126:) account for reductions in ET due to environmental stress (e.g.
669:
Note: Often, resistances are used rather than conductivities.
100:
1479:
10.1175/1520-0493(1972)100<0081:OTAOSH>2.3.CO;2
588:= Ground heat flux (W m), usually difficult to measure
131:
1501:
M. E. Jensen, R. D. Burman & R. G. Allen, ed. (1990).
1098:
762:
740:
693:
558:= Mass water evapotranspiration rate (g s m)
1575:
Quarterly
Journal of the Royal Meteorological Society
1191:
1164:
1144:
1120:
857:
678:
171:
1503:
Evapotranspiration and
Irrigation Water Requirement
31:
may be too technical for most readers to understand
1197:
1170:
1150:
1126:
978:
836:
782:
512:
1460:Priestley, C. H. B.; Taylor, R. J. (1972-02-01).
1625:
1317:Symposia of the Society for Experimental Biology
1572:
1459:
1012:(MJ m day), the external source of energy flux
831:
582:(W m), the external source of energy flux
83:for modeling reference evapotranspiration ET
1270:
638:of air, atmospheric conductance (m s)
564:= Water volume evapotranspired (mm s)
336:
332:
1477:
59:Learn how and when to remove this message
43:, without removing the technical details.
1395:
1310:
1626:
1368:
1341:
41:make it understandable to non-experts
1529:
1306:
1304:
1239:
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1235:
1233:
1065:This reference evapotranspiration ET
15:
1507:American Society of Civil Engineers
1371:Agricultural and Forest Meteorology
1104:
1091:American Society of Civil Engineers
13:
939:
880:
717:
601:capacity of air (J kg K)
436:
355:
270:
194:
122:) standard. Stress coefficients (K
14:
1660:
1612:
1301:
1230:
81:Food and Agriculture Organization
20:
1566:
1523:
1033:= Wind speed at 2m height (m/s)
837:FAO 56 Penman-Monteith equation
649:surface or stomatal conductance
90:
1494:
1453:
1428:
1396:Widmoser, Peter (2009-04-01).
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1:
1402:Agricultural Water Management
1313:"Evaporation and environment"
1224:
1084:
1438:. 2007-07-03. Archived from
1383:10.1016/0168-1923(95)02295-3
1295:10.1016/0022-1694(79)90130-6
1213:The equation is named after
1089:The standard methods of the
800:The atmospheric conductance
116:reference evapotranspiration
7:
1436:"Hydrology Models in GRASS"
1414:10.1016/j.agwat.2008.10.003
1024:= Air temperature at 2m (K)
832:Variations and alternatives
822:
532:Latent heat of vaporization
160:Evaporation and Environment
153:
103:resistance or conductance.
10:
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1619:Derivation of the equation
1532:Boundary-Layer Meteorology
1208:
1183:atmospheric boundary layer
647:= Conductivity of stoma,
1311:Monteith, J. L. (1965).
73:Penman-Monteith equation
1649:Meteorological concepts
1198:{\displaystyle \alpha }
1171:{\displaystyle \alpha }
1151:{\displaystyle \alpha }
1127:{\displaystyle \alpha }
1466:Monthly Weather Review
1356:10.1098/rstb.1976.0035
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1052:Psychrometric constant
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659:Psychrometric constant
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1274:Journal of Hydrology
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169:
1587:2009QJRMS.135.1277V
1544:1994BoLMe..70...37C
1287:1979JHyd...44..169B
162:, the equation is:
1581:(642): 1277–1291.
1552:10.1007/BF00712522
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77:evapotranspiration
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1257:978-92-5-104219-9
1114:constant factor,
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708:
550:= 2453 MJ m)
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106:Various forms of
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1377:(2–4): 111–134.
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1068:
1058:≈ 66 Pa K)
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1007:
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97:water balance
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1472:(2): 81–92.
1469:
1465:
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1444:. Retrieved
1440:the original
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555:
544:
537:
524:
159:
157:
115:
105:
94:
91:Significance
72:
70:
55:
46:
30:
1323:: 205–234.
614:(kg m)
49:August 2024
1628:Categories
1446:2022-02-21
1225:References
1095:SWAT model
1085:Variations
1010:irradiance
651:(m s)
610:= dry air
580:irradiance
118:(RET or ET
1639:Hydrology
1634:Equations
1603:123228410
1560:123108265
1488:1520-0493
1422:0378-3774
1193:α
1166:α
1146:α
1122:α
946:γ
940:Δ
932:δ
919:γ
897:−
881:Δ
718:&
443:γ
437:Δ
409:δ
385:ρ
372:−
356:Δ
334:⟺
277:γ
271:Δ
248:δ
224:ρ
211:−
195:Δ
177:λ
1644:Agronomy
1178:values.
1073:: ET = K
823:Accuracy
154:Equation
142:induces
130:reduces
101:stomatal
1583:Bibcode
1540:Bibcode
1329:5321565
1283:Bibcode
1209:History
793:where r
612:density
134:-zone O
35:Please
1601:
1558:
1513:
1486:
1420:
1327:
1254:
1056:γ
1008:= Net
724:
721:
715:
712:
663:γ
578:= Net
338:
330:
138:, low
1599:S2CID
1556:S2CID
878:0.408
1511:ISBN
1484:ISSN
1418:ISSN
1325:PMID
1252:ISBN
1217:and
1109:The
1077:* ET
958:0.34
144:wilt
132:root
71:The
1591:doi
1579:135
1548:doi
1474:doi
1470:100
1410:doi
1379:doi
1352:doi
1348:273
1291:doi
1099:GIS
911:900
39:to
1630::
1597:.
1589:.
1577:.
1554:.
1546:.
1536:70
1534:.
1509:.
1482:.
1468:.
1464:.
1416:.
1406:96
1404:.
1400:.
1375:80
1373:.
1346:.
1321:19
1319:.
1315:.
1303:^
1289:.
1279:44
1277:.
1232:^
1134:.
1081:.
1050:=
1040:=
991:ET
843:ET
657:=
634:=
621:=
597:=
562:ET
530:=
146:,
110:(K
87:.
1605:.
1593::
1585::
1562:.
1550::
1542::
1519:.
1490:.
1476::
1449:.
1424:.
1412::
1385:.
1381::
1358:.
1354::
1331:.
1297:.
1293::
1285::
1260:.
1079:0
1075:c
1071:c
1067:0
1054:(
1048:γ
1038:e
1036:δ
1030:2
1028:u
1022:T
1016:G
1006:n
1003:R
994:0
971:)
966:2
962:u
955:+
952:1
949:(
943:+
935:e
927:2
923:u
914:T
906:+
903:)
900:G
892:n
888:R
884:(
872:=
867:o
863:T
859:E
846:0
816:2
812:s
809:g
805:a
802:g
795:c
773:c
769:r
765:1
759:=
751:s
747:r
743:1
737:=
732:s
728:g
704:a
700:r
696:1
690:=
685:a
681:g
661:(
655:γ
645:s
642:g
632:a
629:g
619:e
617:δ
608:a
605:ρ
595:p
592:c
586:G
576:n
573:R
556:E
548:v
545:L
541:v
538:L
528:v
525:λ
497:v
493:L
488:)
483:)
477:s
473:g
468:/
462:a
458:g
454:+
451:1
447:(
440:+
433:(
425:a
421:g
416:)
412:e
405:(
399:p
395:c
389:a
381:+
378:)
375:G
367:n
363:R
359:(
350:=
347:T
344:E
317:)
311:s
307:g
302:/
296:a
292:g
288:+
285:1
281:(
274:+
264:a
260:g
255:)
251:e
244:(
238:p
234:c
228:a
220:+
217:)
214:G
206:n
202:R
198:(
189:=
186:E
181:v
136:2
124:s
120:0
112:c
85:0
62:)
56:(
51:)
47:(
33:.
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