ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 23, NO. 4, 2006, 513–520

Decreasing Reference Evapotranspiration in a Warming

Climate—A Case of Changjiang (Yangtze)

River Catchment During 1970–2000

Chong-yu XU∗1,2 (NÂ�), Lebing GONG3,4 (÷WX), JIANG Tong2 (ñ Õ), and Deliang CHEN4,5 (���)1Department of Geosciences, University of Oslo, P. O Box 1047 Blindern, NO-0316 Oslo, Norway

2Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008

3Department of Earth Sciences, Uppsala University, Villavagen 16, S-75236 Uppsala, Sweden

4Department of Earth Sciences, Gothenburg University, Sweden

5Laboratory for Climate Studies/National Climate Center, China Meteorological Administration, Beijing 100081

(Received 2 September 2005; revised 14 January 2006)

ABSTRACT

This study deals with temporal trends in the Penman-Monteith reference evapotranspiration estimatedfrom standard meteorological observations, observed pan evaporation, and four related meteorological vari-ables during 1970–2000 in the Yangtze River catchment. Relative contributions of the four meteorologicalvariables to changes in the reference evapotranspiration are quantified. The results show that both thereference evapotranspiration and the pan evaporation have significant decreasing trends in the upper, themiddle as well as in the whole Changjiang (Yangtze) River catchment at the 5% significance level, whilethe air temperature shows a significant increasing trend. The decreasing trend detected in the referenceevapotranspiration can be attributed to the significant decreasing trends in the net radiation and the windspeed.

Key words: reference evapotranspiration, Penman-Monteith method, temporal trend, Changjiang

(Yangtze) River catchment, China

doi: 10.1007/s00376-006-0513-4

1. Introduction

Reference evapotranspiration, Eref, i.e. the po-tential evapotranspiration of grass (e.g. Allen et al.,1998), is one of the most important things to con-sider when scheduling run times for an irrigation sys-tem, preparing input data for hydrological models ofa water balance study, or calculating actual evapo-transpiration for a region and/or catchment (Blaneyand Criddle, 1950; Dyck, 1983; Hobbins et al., 2001;Xu and Li, 2003; Xu and Singh, 2005). All else be-ing equal, one expects evapotranspiration to increasewith increases in air temperature, which has beenshown by Ramirez and Finnerty (1996) and Wanget al. (2003) and many others. However, severalstudies have reported decreasing trends in pan evap-oration (Epan) in several countries (e.g. Chattopad-hyay and Hulme, 1997; Kaiser, 2000, 2001; Thomas,2000; Hobbins et al., 2004; Liu et al., 2004; Chen

et al., 2005). This is at odds with the expectationthat global warming caused by increasing emissions ofgreenhouse gases would increase potential evapotran-spiration. The study of Chattopadhyay and Hulme(1997) showed that in spite of the general increasein temperature in recent decades over the Indian re-gion, both Epan and Eref have decreased and that in-creases in relative humidity and decreases in radia-tion were both important correlates with the decreas-ing trend in Eref. Kaiser (2000, 2001) analyzed thecloud amount, water vapour pressure and relative hu-midity over China and concluded that the marked de-creasing trend in the all-China mean cloud amount hascontinued; annual mean station pressure has increaseddramatically over most of China, and annual mean wa-ter vapor pressure has increased significantly in north-west and east-central China (no areas show any sig-nificant decreases); relative humidity has decreasedsignificantly in the northeast (where the largest de-

*E-mail: chongyu.xu@geo.uio.no

514 DECREASING REFERENCE EVAPOTRANSPIRATION IN CHANGJIANG CATCHMENT VOL. 23

creases in cloud amount are observed) but it has in-creased significantly in the northwest (consistent withwater vapor increases). The most likely explanationfor such a paradox is the air pollution over China.Thomas (2000) analyzed the time series (1954–1993)of Penman-Monteith evapotranspiration estimates for65 stations in China, for the country as a whole andfor individual stations. His analysis showed that forChina as a whole, the potential evapotranspiration hasdecreased in all seasons especially in northwest andsoutheast China. South of 35◦N, sunshine appears tobe most strongly associated with evapotranspirationchanges while wind, relative humidity and maximumtemperature are the primary factors in northwest, cen-tral and northeast China, respectively.

Previous studies have provided solid evidence thatseveral key meteorological variables have caused therich spatial temporal and spatial variations of Epan

and Eref and the role played by each variable variedwith region and season. Previous investigations havealso shown that additional analysis is needed to quan-tify the causes for the change in evaporation, to un-derstand the mechanisms behind the widely observeddecline in reference evapotranspiration and pan evapo-ration and its implications for understanding the influ-ence of climate change on the global hydrological cycle.As a part of the ongoing research with the main objec-tive of studying the impact of climate change on floodsin the Changjiang (Yangtze) River basin in China, de-tailed investigation of the variability of evaporation inthe Changjiang (Yangtze) River basin and quantifi-cation of the influence of key factors on evaporationin the region are needed. Recognizing the above con-cerns, the main aims of this study are formulated as (1)examining the temporal trend of the pan evaporationand the reference evapotranspiration calculated by thePenman-Monteith method (Allen et al., 1998) for dif-ferent geographic regions and for the whole catchmentof the Changjiang (Yangtze) River, and (2) analyz-ing the possible contributing factors for the trend inevapotranspiration and quantifying the contributionsof individual meteorological variables to that trend.

2. Study region and data

The Changjiang (Yangtze) River is about 6380 kmlong and has a drainage area of 1.8×106 km2 (Fig. 1).Originating from the Tibetan Plateau, the terrain ofthe basin is shaped like a staircase with 3 steps. Lo-cated on the Qingzang Plateau in the west, the high-est step has an average elevation of over 3000 metersabove sea-level; the second step—the middle part ofthe basin—has an average elevation of 1000 meters;then, water flows to the east China Plain, which isthe third step with an average elevation of about 100meters. In the following discussion and comparison,

the Changjiang catchment is divided into three geo-graphic regions corresponding to the three steps: theupper, the middle and the lower regions. The classi-fication of the upper, middle and lower regions of thecatchment in this study is different from what is de-termined by the Changjiang River Water ResourcesCommission (CWRC) in China, where flood control isthe main concern of the classification. According toCWRC, the section above the Yichang station (wherethe Three Gorges Dam is located) is called the Up-per Reach, 4500 km long, with a controlled catch-ment area of 1 million km2 accounting for 70.4% ofthe Yangtze’s total area. From Yichang to Hukou isthe Middle Reach, 955 km long with a catchment areaof 680 000 km2. The remaining part from Hukou tothe estuary is called the Lower Reach, 938 km longwith an interval catchment area of 120 000 km2. Theriver basin is located in the subtropical and temperateclimate zone. Monsoon winds dominate the climateof the region, giving rise to humid summers and drywinters. The advance and retreat of the summer mon-soon determines the timing of the rainy season and theamount of rainfall throughout the basin. The datasetof 115 National Meteorological Observatory (NMO)stations with daily observations of pan evaporation,maximum, minimum and mean air temperature, windspeed, relative humidity, sunshine hours, and absolutevapor pressure for the period of 1970 to 2000 was usedin this study. Data have been provided by the Na-tional Climate Center (NCC) of the China Meteoro-logical Administration (CMA). The locations of thesestations are also shown in Fig. 1.

3. Method and analysis procedure

The potential evapotranspiration is a function ofatmospheric forcing and surface type. In order to getrid of the influence of surface type and to study the

Fig. 1. Location of the Changjiang (Yangtze) River catch-ment and the meteorological stations (open circles).

NO. 4 XU ET AL. 515

evaporative demand of the atmosphere independent ofcrop type, crop development and management prac-tices, the concept of the reference evapotranspiration,Eref, was introduced by defining the reference crop asa hypothetical surface of green grass of uniform height,actively growing and adequately watered (Allen et al.,1998). The Food and Agriculture Organization (FAO)Penman-Monteith (P-M) method of reference evapo-transpiration (equation 1) is used in this study.

Eref =0.408∆(Rn −G) + γ

900Ta + 273

u2(es − ea)

∆ + γ(1 + 0.34u2),

(1)

where Eref=reference evapotranspiration mm d−1,Rn=net radiation at the crop surface (MJ m−2 d−1),G=soil heat flux density (MJ m−2 d−1), T=mean dailyair temperature at 2 m height (◦C), u2=wind speedat 2 m height (m s−1), es=saturation vapour pres-sure (kPa), ea=actual vapour pressure (kPa), es −ea=saturation vapour pressure deficit (kPa), ∆=slopevapour pressure curve (kPa ◦C−1), γ=psychrometricconstant (kPa ◦C−1). The computation of all datarequired for the calculation of the reference evapotran-spiration in this study follows the method and proce-dure given in Chapter 3 of Allen et al. (1998).

The procedure of analysis is as follows: (1) Dailyvalues of the reference evapotranspiration from 1970 to2000 for the 150 stations were calculated by the P–Mmethod. (2) The mean annual reference evapotran-spiration and pan evaporation for each station werecomputed by summing up the daily values; and theannual values for each region and the whole catchmentwere calculated as the arithmetic mean of the regions.

(3) The temporal trends of the reference evapotran-spiration and pan evaporation for each region and thewhole catchment were tested for significance by usingthe parametric t-test. (4) The cause of the decreas-ing trend in evapotranspiration was analyzed and thecontributions of individual meteorological variables tothe decreasing trend of evapotranspiration were quan-tified.

4. Results and discussion

4.1 Temporal trends in mean annual evapotran-spiration, pan evaporation and meteorolog-ical variables

The time series of annual Eref and Epan from 1970and 2000 averaged over the upper, the middle and thelower regions as well as the whole catchment are plot-ted in Fig. 2. The parametric t-test is then used totest the significance of the temporal trend for eachof these four time series. The normality of the data,which is required by the t-test, is pre-tested by usingthe Kolmogorov-Smirnov method and the result (notshown) reveals that all the variables shown in Fig. 2are normally distributed at the 95% confidence level.The p-values for the detected trends are given in Table1 (column 2). It is seen that (1) both the pan evapora-tion and reference evapotranspiration in all the regionshave decreased during the last 3 decades. (2) The de-creasing trend is the strongest in the upper region forboth Eref and Epan and becomes weaker in the middleand lower regions. (3) For the reference evapotranspi-ration, the decreasing trend is significant in the upper,the middle and the whole catchment at the 10% signif-icance level. For the pan evaporation, the decreasing

1970 1980 1990 2000800

850

900

950

1000

1050

Year

E ref (m

m y

r−1)

1970 1980 1990 20001000

1200

1400

1600

1800

Year

E pan (m

m y

r−1)

(a) (b)

L

W

U

M M

U

W

L

Fig. 2. Annual reference evapotranspiration, pan evaporation and their linear trends in the Changjiangcatchment and its sub-regions. Letters U, M, L, and W stand for upper, middle, and lower regions andthe whole catchment respectively.

516 DECREASING REFERENCE EVAPOTRANSPIRATION IN CHANGJIANG CATCHMENT VOL. 23

Table 1. Significance test p-values of the trends for reference evapotranspiration and relevant meteorological variablesin the three regions and the whole catchment of the Changjiang (Yangtze) River. (The italic numbers mean that thelinear trend is not significant at the 10% level).

Epan Eref TA WD RH RN

Upper region 0.03 <0.01 0.02 <0.01 <0.01 <0.01

Middle region 0.71 0.03 0.04 <0.01 0.20 <0.01

Lower region 0.17 0.48 <0.01 <0.01 0.15 <0.01

Whole catchment 0.14 0.05 <0.01 <0.01 0.80 <0.01

Note: Epan=pan evaporation, Eref=reference evapotranspiration, TA=air temperature, WD=wind speed, RH=relative

humidity, RN=net radiation.

1970 1980 1990 20008

10

12

14

16

18

Year

Air

tem

per

atu

re (

oC

)

1970 1980 1990 2000

0.8

1

1.2

1.4

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1.8

Year

Win

d s

pee

d (m

s−

1)

1970 1980 1990 2000

65

70

75

80

Year

Rel

ativ

e h

um

idit

y (%

)

1970 1980 1990 20007.5

8

8.5

9

Year

Net

rad

iati

on

(MJ

m−

2 d

−1

)

L

M

W

U

U

W

M

L

U

W

ML

U

W

M

L

(a) (b)

(c) (d)

Fig. 3. Annual temperature, wind speed, relative humidity and net radiation and their linear trends inthe Changjiang catchment and its sub-regions. Labels are as in Fig. 2.

trend is significant at the 10% significance level in theupper region and significant at the 20% significancelevel in the lower region and the whole catchment(when the p-values in the table are smaller than 0.20).(4) The significance of the decreasing trend in Eref andEpan differs in different regions. This is logical since itis well-known that the pan evaporation coefficient, i.e.the ratio of Etref to Etpan, is not a constant in eitherregion or season. This result is consistent with that of

other studies (e.g. Liu et al., 2004; Chen et al., 2005).In order to analyze the main causes of the decreas-

ing trend, the same trend analysis procedure is per-formed on the major meteorological variables in eachregion that determine the evaporation values. The me-teorological parameters that are examined include airtemperature, relative humidity, wind speed and netradiation. The results are shown in Fig. 3 and Table 1(columns 4–7), which reveal that the air temperature

NO. 4 XU ET AL. 517

1970 1975 1980 1985 1990 1995 2000

860

880

900

920

940

960

980

year

E ref (m

m y

r

)-1

(1)

(2)

(3)

(4)

(5)

Fig. 4. Comparison of the original reference evapotranspiration [Line (5)] with recalculated ref-erence evapotranspirations. Line (1): recalculated Eref with detrended data series for all variablesexcept air temperature; Line (2): recalculated Eref with detrended data series for all variables;Line (3): recalculated Eref with detrended data series for all variables except wind speed; Line (4):recalculated Eref with detrended data series for all variables except net radiation.

shows a significant increasing trend in all the regions,while wind speed and net radiation show a significantdecreasing trend in all regions at the 10% significancelevel. No significant temporal trend was detected forrelative humidity except in the upper region where anincreasing trend is detected. The increasing trend inair temperature over the past 3 decades is consistentwith the global warming as reported across most ofthe globe while the decreasing trend in wind speed re-quires more attention, although both can be due toa real climate change and/or a change in the envi-ronment of the measuring stations. Further study isneeded to determine the extent to which the decreasein wind speed is due to natural climate changes andchanges in the environment of the measuring stations.

As for the decreasing trend found for net radia-tion, the decrease in the global radiation (sunshineduration) is the most likely cause. Kaiser (2000, 2001)found that the most likely explanation for the decreaseof sunshine duration is the increasing air pollution inChina. The findings of Kaiser (2000, 2001) were citedand used by Liu et al. (2004) in studying the tem-poral trend and spatial variation of pan evaporationin China. By examining solar radiation measurementsin eastern China (Shanghai, Nanjing and Hangzhou),Zhang et al. (2004) also came to the conclusion thatthe global radiation has decreased in recent decadesand the acceleration of air pollution and decrease ofrelative sunshine are the possible causes for the de-crease of global and direct radiation.

Another study by Liu et al. (2004) also found thatunlike in other parts of the world, the decrease in solarirradiance in China was not always accompanied by anincrease in cloud cover and precipitation. Therefore,

they speculated that the increased air pollution in thearea may play an important role in the decrease ofsolar irradiance in China.

4.2 Contribution of meteorological variables tothe trend of reference evapotranspiration

In order to quantify the contributions of the trendsof meteorological variables to the decreasing trendof evapotranspiration, the following studies are per-formed. (1) Removing the increasing/decreasing trendin the meteorological variables to make them station-ary time series. (2) Recalculating the reference evapo-transpiration using the detrended meteorological vari-ables. (3) Recalculating the reference evapotranspira-tion by using, in each time, the original data (withtrend) for one variable and the detrended data forother variables. (4) Comparing the result with theoriginal evapotranspiration and considering the differ-ence as the influence of the trend by that variable.

The above analysis is done for each region and thewhole catchment. For illustrative purposes, the origi-nal reference evapotranspiration [Line (5)] and the re-calculated reference evapotranspiration time series forthe whole catchment are shown in Fig. 4. Line (2)shows the recalculated evapotranspiration when de-trended values of all four meteorological values areused to calculate the reference evapotranspiration. Itis plotted for comparison. Lines (1), (3) and (4) showthe recalculated Eref with the detrended data series ofall variables except for air temperature, wind speed,or net radiation, respectively. The difference betweenLine (2) and lines (1), (3) and (4) reveals the contri-bution of each variable to the recalculated evapotran-spiration. For example, the difference between Lines

518 DECREASING REFERENCE EVAPOTRANSPIRATION IN CHANGJIANG CATCHMENT VOL. 23

(1) and (2) shows that if the detrended data series areused for all variables except for air temperature, therecalculated evapotranspiration will have an increas-ing trend; in other words, the difference between thesetwo lines represents the contribution of temperatureto the trend in evapotranspiration. Same explanationapplies to the other lines. The difference between Line(2) and Line (5) reflects the combined effect of all vari-ables. The line with the influence of relative humidityis not shown because the trend of relative humidity isinsignificant in the whole catchment. Similar resultsare also obtained for the three sub-regions (not shown)except that in the upper region the effect of relativehumidity is notable.

To better understand the result presented in Fig.4, a sensitivity analysis of reference evapotranspira-tion to the meteorological variables is performed. Theprocedure of the study is as follows: (1) Determina-tion of scenarios for changes in meteorological vari-ables by adjusting the historic data series by addingdelta changes (∆X = 0,±10%,±20%,±30% of X(t),where X is the meteorological variable, and t is thedate), (2) recalculation of reference evapotranspirationfor each station by using one scenario at each time, (3)calculation of the relative changes between the recal-culated and the original reference evapotranspiration,and (4) plotting of the relative changes in referenceevapotranspiration against the relative changes in eachmeteorological variable.

Figure 5 shows the result of the sensitivity study. Itis seen that the sensitivity of reference evapotranspira-

tion to the meteorological variables varies in differentregions and the two most sensitive variables are rela-tive humidity and net radiation, followed by air tem-perature and wind speed. The combination of Fig. 5and Fig. 3 explains the result shown in Fig. 4. For ex-ample, on one hand, although relative humidity is oneof the most sensitive variables in almost all regions, ithas no significant contribution to the decreasing trendin reference evapotranspiration except in the upper re-gion, because the relative humidity in other regionsand over the whole catchment has no significant de-creasing or increasing trend. The temperature increasecompensates to some extent the decreasing trend inevapotranspiration, but the contributions from net ra-diation and wind speed are much larger, which resultsin the significant decreasing trend in evapotranspira-tion in the Changjiang catchment.

5. Summary and conclusions

The temporal trends in the time series of the ref-erence evapotranspiration, observed pan evaporation,and meteorological variables during 1970–2000 in theYangtze River catchment and its sub-areas were an-alyzed. The possible causes and quantitative contri-butions of the major meteorological variables to theevaporation trends were investigated. The followingconclusions may be drawn from the study. (1) For theupper and middle regions as well as the whole catch-ment, there exist significant decreasing trends in ref-erence evapotranspiration and pan evaporation from

0 5 10 15 20

0

5

10

15

20

R elative variation in meteorological variables (% )

Re

lativ

e v

ari

atio

n in

Ere

f (%

)

R N

T A

R H

WD

UpperMiddleLower

Fig. 5. The sensitivity of the reference evapotranspiration tothe four major meteorological variables in the Changjiang catch-ment and its sub-regions. TA=air temperature, WD=wind speed,RH=relative humidity, RN=net radiation.

NO. 4 XU ET AL. 519

1970 to 2000. (2) The sensitivity analysis shows thatthe most sensitive variables for the reference evapo-transpiration are relative humidity and net radiationfollowed by air temperature and wind speed. (3) Thedecreasing trend detected in reference evapotranspi-ration is a combined effect of all four meteorologicaldecreasing trend detected in reference evapotranspi-ration is a combined effect of all four meteorologicalvariables. The actual contribution of each variableto the decreasing trend in reference evapotranspira-tion depends on the sensitivity of evapotranspirationto the variable and the magnitude of the trend of thevariable. In the Changjiang (Yangtze) catchment, thedecreasing trends in reference evapotranspiration aremainly caused by the significant decreases in net radi-ation and wind speed. Temperature increases result inan increase in evapotranspiration in all 3 regions, butthis cannot compensate for all the contributions fromthe decrease in net radiation and wind speed.

The increasing trend in air temperature over thepast 3 decades is consistent with the global warmingas reported across most of the globe. Further studyis needed to determine the extent to which the de-crease in wind speed is due to natural climate changesand changes in the environment of the measuring sta-tions. Previous studies (Kaiser, 2000, 2001; Zhang etal. 2004) showed that the most likely explanation forthe decrease in the global radiation (sunshine dura-tion) over China is the increase in air pollution. Thispaper presents progress results from on-going researchthat studies the impact of climate change on floodsin the Changjiang (Yangtze) River basin in China.The ongoing research and planned studies include in-vestigation and quantification of natural and humaneffects on the changing trend of meteorological vari-ables, calculation and regional mapping of actual evap-otranspiration in the catchment by using complemen-tary relationship evaporation models and water bal-ance models, investigation of the effect of the changesin evapotranspiration on flooding and the hydrologi-cal cycle in the region, etc. Reference evapotranspi-ration and pan evaporation provide a measurement ofthe integrated effect of radiation, wind, temperatureand humidity on evaporation. In a humid climate,reference evapotranspiration provides an upper limitfor actual evapotranspiration and, in an arid climate,it provides a total available energy value for actualevapotranspiration. For a given precipitation event, adecrease in reference evapotranspiration will decreaseactual evapotranspiration in both humid and arid cli-mates, which in turn increases the magnitude and fre-quency of floods. Quantitative estimation of the effectof different meteorological variables on evaporation, aswas done in this study, is also an important step instudying the impact of climate change on evapotran-

spiration and water balance components.Acknowledgments. The authors thank the Na-

tional Climate Center of China for providing the meteo-

rological data. The overseas Chinese Scholar Fund of the

Chinese Academy of Sciences and the Key Project of the

Chinese Academy of Sciences (KZCX3-SW-331) are jointly

funded to do field work and data gathering in the Yangtze

River and to exchange visitors for this topic in China. The

Swedish Research Council (VR), the Swedish Foundation

for International Cooperation in Research and Higher Edu-

cation (STINT), the Chinese Ministry of Science and Tech-

nology, the Swedish International Development Coopera-

tion Agency (Sida) and the China Meteorological Adminis-

tration partly supported the research. The authors thank

the two anonymous reviewers for their detailed and con-

structive comments.

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