99:). Actual evapotranspiration is said to equal potential evapotranspiration when there is ample water present. Evapotranspiration can never be greater than potential evapotranspiration, but can be lower if there is not enough water to be evaporated or plants are unable to transpire maturely and readily. Some US states utilize a full cover alfalfa reference crop that is 0.5 m (1.6 ft) in height, rather than the general short green grass reference, due to the higher value of ET from the
17:
139:. Often a value for the potential evapotranspiration is calculated at a nearby climate station on a reference surface, conventionally on short grass. This value is called the reference evapotranspiration, and can be converted to a potential evapotranspiration by multiplying by a surface coefficient. In agriculture, this is called a crop coefficient. The difference between potential evapotranspiration and actual precipitation is used in
116:
21:
20:
24:
23:
18:
25:
929:
was developed as a substitute to the PenmanâMonteith equation to remove dependence on observations. For
PriestleyâTaylor, only radiation (irradiance) observations are required. This is done by removing the aerodynamic terms from the PenmanâMonteith equation and adding an empirically derived constant
106:
Potential evapotranspiration is higher in the summer, on clearer and less cloudy days, and closer to the equator, because of the higher levels of solar radiation that provides the energy (heat) for evaporation. Potential evapotranspiration is also higher on windy days because the evaporated moisture
953:
The underlying concept behind the
PriestleyâTaylor 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 potential evapotranspiration. However, observations
683:
describes evaporation (E) from an open water surface, and was developed by Howard Penman in 1948. Penman's equation requires daily mean temperature, wind speed, air pressure, and solar radiation to predict E. Simpler
Hydrometeorological equations continue to be used where obtaining such data is
22:
166:
is a zone of climate with hot and humid summers, and cold to mild winters. Subarctic regions, between 50°N and 70°N latitude, have short, mild summers and freezing winters depending on local climates. Precipitation and evapotranspiration is low (compared to warmer variants), and vegetation is
1001:, is not a closed box, but constantly brings in dry air from higher up in the atmosphere towards the surface. As water evaporates more easily into a dry atmosphere, evapotranspiration is enhanced. This explains the larger than unity value of the Priestley-Taylor parameter
94:
Often a value for the potential evapotranspiration is calculated at a nearby climatic station on a reference surface, conventionally on land dominated by short grass (though this may differ from station to station). This value is called the reference evapotranspiration
842:
282:
538:
622:
1106:
19:
954:
revealed that actual evaporation was 1.26 times greater than potential evaporation, and therefore the equation for actual evaporation was found by taking potential evapotranspiration and multiplying it by
974:. 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
712:
182:
79:
is considered the net result of atmospheric demand for moisture from a surface and the ability of the surface to supply moisture, then PET is a measure of the demand side (also called
72:
from the ground up into the lower atmosphere and away from the initial location. Potential evapotranspiration is expressed in terms of a depth of water or soil moisture percentage.
410:
1021:. The proper equilibrium of the system has been derived and involves the characteristics of the interface of the atmospheric boundary layer and the overlying free atmosphere.
127:
Potential evapotranspiration is usually measured indirectly, from other climatic factors, but also depends on the surface type, such as free water (for lakes and oceans), the
997:
The assumption that an air mass moving over a vegetated surface with abundant water saturates has been questioned later. The lowest and turbulent part of the atmosphere, the
29:
This animation shows the projected increase in potential evaporation in North
America through the year 2100, relative to 1980, based on the combined results of multiple
660:
1019:
992:
972:
948:
339:
917:
N.B.: The coefficient 0.408 and 900 are not unitless but account for the conversion from energy values to equivalent water depths: radiation = 0.408 radiation .
310:
1374:
403:
381:
359:
68:
if there was sufficient water available. It is a reflection of the energy available to evaporate or transpire water, and of the wind available to transport the
1297:
van
Heerwaarden, C. C.; et al. (2009). "Interactions between dry-air entrainment, surface evaporation and convective boundary layer development".
1070:
544:
1030:
702:(ET) estimates of vegetated land areas. This equation was then derived by FAO for retrieving the potential evapotranspiration
1246:
107:
can be quickly moved from the ground or plant surface before it precipitates, allowing more evaporation to fill its place.
1238:
837:{\displaystyle ET_{o}={\frac {0.408\Delta (R_{n}-G)+{\frac {900}{T}}\gamma u_{2}\delta e}{\Delta +\gamma (1+0.34u_{2})}}}
277:{\displaystyle PET=16\left({\frac {L}{12}}\right)\left({\frac {N}{30}}\right)\left({\frac {10T_{d}}{I}}\right)^{\alpha }}
146:
Average annual potential evapotranspiration is often compared to average annual precipitation, the symbol for which is
1442:
1405:
1386:
1211:
1050:
1206:. FAO Irrigation and drainage paper 56. Rome, Italy: Food and Agriculture Organization of the United Nations.
1081:
694:
533:{\displaystyle \alpha =(6.75\times 10^{-7})I^{3}-(7.71\times 10^{-5})I^{2}+(1.792\times 10^{-2})I+0.49239}
998:
163:
87:, and wind all affect this. A dryland is a place where annual potential evaporation exceeds annual
119:
Monthly estimated potential evapotranspiration and measured pan evaporation for two locations in
906:
684:
impractical, to give comparable results within specific contexts, e.g. humid vs arid climates.
631:
1004:
977:
957:
933:
140:
1437:
1344:
1306:
1271:
1174:
317:
8:
857:Δ = Rate of change of saturation specific humidity with air temperature. (Pa K)
665:
Somewhat modified forms of this equation appear in later publications (1955 and 1957) by
289:
1348:
1310:
1275:
1262:
Culf, A. (1994). "Equilibrium evaporation beneath a growing convective boundary layer".
1178:
1147:
1130:
Thornthwaite, C. W. (1948). "An approach toward a rational classification of climate".
699:
666:
388:
366:
344:
76:
53:
115:
1401:
1382:
1362:
1242:
1207:
1186:
706:. It is widely regarded as one of the most accurate models, in terms of estimates.
1352:
1314:
1279:
1182:
1139:
1237:. ASCE Manuals and Reports on Engineering Practices. Vol. 70. New York, NY:
679:
132:
1201:
1165:
Black, Peter E. (2007). "Revisiting the
Thornthwaite and Mather water balance".
896:
1431:
617:{\displaystyle I=\sum _{i=1}^{12}\left({\frac {T_{m_{i}}}{5}}\right)^{1.514}}
159:
88:
30:
1232:
341:
is the average daily temperature (degrees
Celsius; if this is negative, use
1366:
1357:
1332:
854:= Potential evapotranspiration, Water volume evapotranspired (mm day)
1421:
1045:
1040:
1035:
69:
1203:
1283:
867:
625:
136:
84:
1151:
65:
1318:
1199:
1143:
1379:
Evaporation into the
Atmosphere: theory, history, and applications
876:= Ground heat flux (MJ m day), usually equivalent to zero on a day
100:
405:
is the average day length (hours) of the month being calculated
120:
1107:"Humid subtropical climate (Cfa) | SKYbrary Aviation Safety"
1333:"Natural evaporation from open water, bare soil, and grass"
128:
61:
57:
1233:
M. E. Jensen, R. D. Burman & R. G. Allen, ed. (1990).
687:
312:
is the estimated potential evapotranspiration (mm/month)
1200:
Allen, R.G.; Pereira, L.S.; Raes, D.; Smith, M. (1998).
1299:
Quarterly
Journal of the Royal Meteorological Society
1007:
980:
960:
936:
715:
634:
547:
413:
391:
369:
347:
320:
292:
185:
170:
383:
is the number of days in the month being calculated
1235:
Evapotranspiration and Irrigation Water Requirement
1167:
Journal of the American Water Resources Association
1063:
1013:
986:
966:
942:
836:
654:
628:which depends on the 12 monthly mean temperatures
616:
532:
397:
375:
353:
333:
304:
276:
1429:
1296:
175:
167:characteristic of the coniferous/taiga forest.
870:(MJ m day), the external source of energy flux
1123:
131:type for bare soil, and also the density and
1129:
1031:Effects of climate change on the water cycle
920:
1373:
1356:
672:
1071:"Kimberly Research and Extension Center"
114:
15:
52:) is the amount of water that would be
1430:
1330:
1158:
688:FAO 56 PenmanâMonteith equation (1998)
1395:
1164:
1424:Global map of potential evaporation.
1261:
1239:American Society of Civil Engineers
13:
797:
738:
171:Estimates of potential evaporation
14:
1454:
1415:
1381:. Dordrecht, Holland: D. Reidel.
83:). Surface and air temperatures,
1187:10.1111/j.1752-1688.2007.00132.x
361:) of the month being calculated
1343:(1032). London, U.K.: 120â145.
110:
1290:
1255:
1226:
1193:
1099:
828:
806:
760:
741:
518:
496:
480:
458:
442:
420:
1:
1056:
1051:Köppen climate classification
888:= Wind speed at 2m height (m)
176:Thornthwaite equation (1948)
38:Potential evapotranspiration
7:
1024:
882:= Air temperature at 2m (K)
10:
1459:
1264:Boundary-Layer Meteorology
999:atmospheric boundary layer
927:PriestleyâTaylor equation
921:PriestleyâTaylor equation
655:{\displaystyle T_{m_{i}}}
164:humid subtropical climate
77:actual evapotranspiration
54:evaporated and transpired
1443:Meteorological phenomena
1400:. Cambridge, U.K.: CUP.
695:PenmanâMonteith equation
150:. The ratio of the two,
1014:{\displaystyle \alpha }
987:{\displaystyle \alpha }
967:{\displaystyle \alpha }
943:{\displaystyle \alpha }
1398:Ecological Climatology
1396:Bonan, Gordon (2002).
1358:10.1098/rspa.1948.0037
1015:
988:
968:
944:
907:Psychrometric constant
838:
698:refines weather based
673:Penman equation (1948)
656:
618:
574:
534:
399:
377:
355:
335:
306:
278:
124:
34:
1331:Penman, H.L. (1948).
1016:
989:
969:
945:
839:
657:
619:
554:
535:
400:
378:
356:
336:
334:{\displaystyle T_{d}}
307:
279:
141:irrigation scheduling
118:
46:potential evaporation
28:
1078:extension.uidaho.edu
1005:
978:
958:
934:
713:
632:
545:
411:
389:
367:
345:
318:
290:
183:
1349:1948RSPSA.193..120P
1311:2009QJRMS.135.1277V
1276:1994BoLMe..70...37C
1179:2007JAWRA..43.1604B
1132:Geographical Review
305:{\displaystyle PET}
1305:(642): 1277â1291.
1284:10.1007/BF00712522
1011:
984:
964:
940:
834:
700:evapotranspiration
667:C. W. Thornthwaite
652:
614:
530:
395:
373:
351:
331:
302:
274:
125:
81:evaporative demand
35:
1248:978-0-87262-763-5
832:
774:
602:
398:{\displaystyle L}
376:{\displaystyle N}
354:{\displaystyle 0}
262:
231:
213:
123:, Hilo and Pahala
26:
1450:
1411:
1392:
1370:
1360:
1323:
1322:
1294:
1288:
1287:
1259:
1253:
1252:
1230:
1224:
1223:
1221:
1220:
1197:
1191:
1190:
1173:(6): 1604â1605.
1162:
1156:
1155:
1127:
1121:
1120:
1118:
1117:
1103:
1097:
1096:
1094:
1092:
1086:
1080:. Archived from
1075:
1067:
1020:
1018:
1017:
1012:
993:
991:
990:
985:
973:
971:
970:
965:
949:
947:
946:
941:
843:
841:
840:
835:
833:
831:
827:
826:
795:
788:
787:
775:
767:
753:
752:
733:
728:
727:
661:
659:
658:
653:
651:
650:
649:
648:
623:
621:
620:
615:
613:
612:
607:
603:
598:
597:
596:
595:
581:
573:
568:
539:
537:
536:
531:
517:
516:
492:
491:
479:
478:
454:
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404:
402:
401:
396:
382:
380:
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360:
358:
357:
352:
340:
338:
337:
332:
330:
329:
311:
309:
308:
303:
283:
281:
280:
275:
273:
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267:
263:
258:
257:
256:
243:
236:
232:
224:
218:
214:
206:
27:
1458:
1457:
1453:
1452:
1451:
1449:
1448:
1447:
1428:
1427:
1418:
1408:
1389:
1375:Brutsaert, W.H.
1327:
1326:
1295:
1291:
1260:
1256:
1249:
1231:
1227:
1218:
1216:
1214:
1198:
1194:
1163:
1159:
1128:
1124:
1115:
1113:
1105:
1104:
1100:
1090:
1088:
1087:on 4 March 2016
1084:
1073:
1069:
1068:
1064:
1059:
1027:
1006:
1003:
1002:
979:
976:
975:
959:
956:
955:
935:
932:
931:
923:
913:â 66 Pa K)
865:
853:
822:
818:
796:
783:
779:
766:
748:
744:
734:
732:
723:
719:
714:
711:
710:
705:
690:
680:Penman equation
675:
644:
640:
639:
635:
633:
630:
629:
608:
591:
587:
586:
582:
580:
576:
575:
569:
558:
546:
543:
542:
509:
505:
487:
483:
471:
467:
449:
445:
433:
429:
412:
409:
408:
390:
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386:
368:
365:
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346:
343:
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325:
321:
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291:
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268:
252:
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244:
242:
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223:
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205:
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184:
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173:
113:
98:
16:
12:
11:
5:
1456:
1446:
1445:
1440:
1426:
1425:
1422:ag.arizona.edu
1417:
1416:External links
1414:
1413:
1412:
1406:
1393:
1387:
1371:
1325:
1324:
1319:10.1002/qj.431
1289:
1270:(1â2): 34â49.
1254:
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1192:
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1144:10.2307/210739
1122:
1098:
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1026:
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897:vapor pressure
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56:by a specific
31:climate models
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1008:
1000:
995:
981:
961:
951:
937:
928:
918:
912:
908:
904:
901:
899:deficit (kPa)
898:
894:
890:
887:
884:
881:
878:
875:
872:
869:
862:
859:
856:
850:
847:
846:
823:
819:
815:
812:
809:
803:
800:
792:
789:
784:
780:
776:
771:
768:
763:
757:
754:
749:
745:
735:
729:
724:
720:
716:
709:
708:
707:
701:
697:
696:
685:
682:
681:
670:
668:
663:
645:
641:
636:
627:
609:
604:
599:
592:
588:
583:
577:
570:
565:
562:
559:
555:
551:
548:
540:
527:
524:
521:
513:
510:
506:
502:
499:
493:
488:
484:
475:
472:
468:
464:
461:
455:
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437:
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423:
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406:
392:
384:
370:
362:
348:
326:
322:
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285:
269:
264:
259:
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228:
225:
220:
215:
210:
207:
202:
198:
195:
192:
189:
186:
168:
165:
161:
160:aridity index
157:
153:
149:
144:
142:
138:
134:
130:
122:
117:
108:
104:
102:
92:
90:
89:precipitation
86:
82:
78:
73:
71:
67:
63:
59:
55:
51:
47:
43:
39:
32:
1397:
1378:
1340:
1337:Proc. R. Soc
1336:
1302:
1298:
1292:
1267:
1263:
1257:
1234:
1228:
1217:. Retrieved
1202:
1195:
1170:
1166:
1160:
1138:(1): 55â94.
1135:
1131:
1125:
1114:. Retrieved
1110:
1101:
1089:. Retrieved
1082:the original
1077:
1065:
996:
952:
926:
924:
916:
910:
902:
892:
885:
879:
873:
860:
848:
693:
691:
678:
676:
669:and Mather.
664:
541:
407:
385:
363:
314:
286:
179:
155:
151:
147:
145:
126:
111:Measurements
105:
103:reference.
93:
80:
74:
49:
45:
41:
37:
36:
1438:Climatology
1046:Water cycle
1041:Water vapor
1036:Evaporation
70:water vapor
1432:Categories
1219:2007-10-08
1116:2023-10-19
1057:References
868:irradiance
626:heat index
137:vegetation
85:insolation
1009:α
982:α
962:α
938:α
804:γ
798:Δ
790:δ
777:γ
755:−
739:Δ
556:∑
511:−
503:×
473:−
465:×
456:−
435:−
427:×
415:α
270:α
158:, is the
133:diversity
66:ecosystem
1377:(1982).
1367:18865817
1025:See also
994:values.
930:factor,
1345:Bibcode
1307:Bibcode
1272:Bibcode
1175:Bibcode
528:0.49239
101:alfalfa
75:If the
1404:
1385:
1365:
1245:
1210:
1152:210739
1150:
911:γ
866:= Net
284:Where
121:Hawaii
1148:JSTOR
1091:4 May
1085:(PDF)
1074:(PDF)
736:0.408
624:is a
610:1.514
500:1.792
44:) or
1402:ISBN
1383:ISBN
1363:PMID
1341:A193
1243:ISBN
1208:ISBN
1093:2018
925:The
816:0.34
692:The
677:The
462:7.71
424:6.75
162:. A
129:soil
62:soil
58:crop
1353:doi
1315:doi
1303:135
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