Hydrological Research Letters 3, 1 5 (2009)
Published online in J-STAGE (www.jstage.jst.go.jp/browse/HRL). DOI: 10.3178/HRL.3.1

The Implication of Heat and Water Balance Changes
in a Lake Basin on the Tibetan Plateau
Jianqing Xu1, Shumei Yu2, Jingshi Liu2, Shigenori Haginoya3, Yasushi Ishigooka4,
Tsuneo Kuwagata4, Masayuki Hara1, and Tetsuzo Yasunari1, 5
1
Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology,
Kanagawa, 236-0001, Japan
2
Institute of Tibetan Plateau Research, Chinese Academy of Sciences, 100085 Beijing, China
3
Meteorological Research Institute, Tsukuba, 305-0052, Japan
4
National Institute for Agro-Environmental Sciences, Tsukuba, 305-8604, Japan
5
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, 464-8601, Japan

Abstract:
The numerous lakes found throughout the Tibetan
Plateau have a large effect on evaporation from theplateau. We analyzed variation in the water cycle in
Yamdrok Yumtso Lake basin under the temperature
increase of 1.13°C during the 45 years from 1961 2005.
Both vapor pressure and relative humidity increased byapproximately 17%, resulting in a 13.7% increase inlongwave radiation flux and a smaller daily range of
surface air temperature. A 5.7% decrease in sunshine
duration caused solar radiation flux to decrease by 3%.
Heat and water balances were simulated over land andthe lake. The lake covers 10% of the total basin area.
Evaporation from the lake was larger than that fromland, with lake evaporation making up 26% of the basin
total. Evaporation from the lake decreased 7% (May
September) over the study period, and observed small
pan evaporation also decreased. This trend was notfound in the evaporation from land and from precipitation. With water vapor in the air increasing and evaporation from the ground surface (lake and land) decreasing, the water vapor may have come from areas outside
the basin and possibly even beyond the plateau.
KEYWORDS
Climate change; lake evaporation;
Tibetan Plateau; water vapor increase.

INTRODUCTION
Lakes play a key role in continental hydrologicalcycles. The Tibetan Plateau contains numerous lakes
covering a total area of 44,993.3 km2 , including morethan 1091 with individual areas greater than 1.0 km2 .
Most of these lakes are located in sparsely populatedareas at elevations over 4000 m. However, the plateau is
extremely sensitive to global warming. Satellite images
have revealed that the areas of lakes across the plateau
have changed in recent years (Lu et al., 2006). The
mechanism of the change is still unknown, due to thelack of observational data in this high-altitude region.
Many weather observatories were established on the
Tibetan Plateau in the 1950s, and intensive meteorological observations have been conducted in the area
since the 1970s. The First Global Atmospheric Research
Program (GARP) Global Experiment (FGGE) II-b began
upper-air observations in summer 1979. Yanai et al.
(1992) analyzed FGGE upper-air data and found that

Figure 1. Map of the Yamdrok Yumtso Lake basin (top),
Tibetan Plateau (bottom). The photograph in the upper
right shows the lake. The large and small pans areshown on the left and right sides, respectively.

sensible heating dominated in early summer, but latentheating dominated during the mid-summer monsoon.
The Global Energy and Water Cycle Experiment(GEWEX) Asian Monsoon Experiment (GAME-Tibet)
conducted an intensive observation period over Tibet in
1998. Using GAME data, Xu and Haginoya (2001) and
Xu et al. (2005a) estimated the heat and water balancesof daily and seasonal variations over the Tibetan
Plateau.
Yamdrok Yumtso Lake (90°41´E, 28°56´N, Figure 1)
is one of the three largest lakes of the Tibetan Plateau.
Situated 120 km south of Lhasa at 4441 m above sea
level, the lake has a catchment area of approximately
6110 km2 and surface area of 638 km2 . Its depth isCorrespondence to: Jianqing Xu, Frontier Research Center for
Global Change, Japan Agency for Marine-Earth Science andTechnology, Kanagawa, 236-0001, Japan. E-mail: jxu@jamstec.
go.jp ©2008, Japan Society of Hydrology and Water Resources.

―1―

Received 1 Septemer 2008
Accepted 21 December 2008

J. XU ET AL.
unknown. Many snowcapped mountains surround thelake, and numerous small streams feed it. It is almost aclosed system; only a small amount of natural runoff
flows through a small tributary of the Yarlung-Tsangpo
River, with the inflow balanced mainly by evaporation.
Kongmucuo Lake and Bajiucuo Lake are independent of
Yamdrok Yumtso Lake.
Baidi hydrological station (Figure 1) is located on
the lake shore at 90°26´20´´E, 29°07´25´´N. Evaporation
was observed at this station using a large pan. The laketemperature was observed during 1977 1995. Nagarze
weather station (ID 55681, 90°24´E, 28°58´N, 4432.4 m)
is located 18 km south of Baidi. The air temperature,humidity, and wind speed observed in Nagarze werevery similar to those observed at Baidi, and the observation period was longer there.
To reveal the long-term change over the plateau, weused the meteorological data from Nagarze weather
station to simulate the evaporation and water temperature of the lake and land. We then compared theseestimates with the lake temperature and large pan evaporation that were observed at Baidi station.

DATA SETS AND CALCULATION METHODS
Data sets
Routine meteorological data including daily temperature, humidity, sunshine duration, and wind speed
covered 45 years (1961 2005) at the Nagarze Meteorological Station (Table I).

The monthly water temperature and large pan data
were acquired from Baidi hydrological station. A largepan (lower left of the upper image in Figure 1) was set
up at Baidi. This type of pan is known as a “20 m2 basin”;
it has a surface area of 20 m2, with a diameter of 5 m anddepth of 2 m. The evaporation from this type of large
pan is nearly the same as the real evaporation of a lake
(Brutsaert, 1982), but these pans are costly to install.
The large pan was observed from 1984 to 1995 in thewarm season (May September), when 95% of precipitation occurred because of the monsoon climate. Water
temperature was measured on one side of the lake from1977 to 1995.

Calculation methods
The evaporation from three kinds of surfaces wascalculated using the same routine meteorological data
as input. The surface types were water (lake), soil (land,
including snow and frozen processes in winter), andimaginary humid surfaces. Evaporation from land
surfaces (represented by Es in this paper) has been described in detail by Kondo and Xu (1997) and applied to
the Tibetan Plateau by Xu and Haginoya (2001) and Xu
et al. (2005a). Xu et al. (2005b) described potential
evaporation Ep from an imaginary humid surface. The
ratio WI = Pr/Ep was defined as a wetness index (WI) to
highlight climatic wetness or dryness, where Pr represents the precipitation.
Estimation of evaporation from a shallow lake
We assumed that the lake was shallow (less than 10m). A one-layer water model has been produced toTable I. Statistical amounts during 1961 2005. Observations and calculations are included (column “O/C”).
The values in the “Annual” column are the annual amounts, while those in “May September” are the warm
season amounts. The significance level of every trend was tested at = 5%, with “T” indicating True and “F”
indicating False. Changes during the 45 years are also shown. TAM, TAMAX, and TAMIN represent the surface airmean, maximum, and minimum temperature, respectively. N is the sunshine duration, RH is the relative
humidity, and Vpr is the vapor pressure. Other notations are given in the text. Here, “water” means from thelake, and “soil” means from the land surface.
Annual
Item

TAM
TAMAX
TAMIN
U
N
RH
Vpr
S
L
Pr
Lpan
SPan
Ep
WI
Rn (water)
H (water)E (water)Ew (water)
Tw (water)
Tw (obs.)Rn (soil)
H (soil)
E (soil)Es (soil)
Ts (soil)

Unit

°C
°C
°C
m s1
hour
%
hPa
W m2
W m2
mm
mm
mm
mm
W m2
W m2
W m2
mm
°C
°C
W m2
W m2
W m2
mm
°C

O/C

Obs.
Obs.
Obs.
Obs.
Obs.
Obs.
Obs.
Cal.
Cal.
Obs.
Obs.
Obs.
Cal.
Cal.
Cal.
Cal.
Cal.
Cal.
Cal.
Obs.
Cal.
Cal.
Cal.
Cal.
Cal.

May Sept.

Mean

Trend
yr 1

Change
45yr


5%

2.8
9.7
3.7
2.6
2891.7
42.7
3.8
246.0
241.9
358.8

0.025
0.009
0.050
0.015
3.659
0.159
0.015
0.172
0.738
0.548

1.13
0.41
2.25
0.68
164.66
7.16
0.68
7.74
33.21
24.66

T
T
T
T
T
T
T
T
T
F

1392.5
0.26

2.749
0.001

123.71
0.05

T
F

7.2

0.0395

1.78

T

62.8
41.2
20.8
263.2
9.1

0.068
0.079
0.004
0.055
0.026

3.06
3.56
0.18
2.48
1.16

T
T
F
F
T

*: Averaged from the May September total amount during 1984 1995.
**: Averaged from the May September mean amount during 1977 1995.

―2―

Mean

Trend
yr 1

Change
45yr


5%

8.4
14.7
2.8
2.0
1071.3
61.5
6.8
268.7
280.0
340.7
604.2*
902.9
671.1
0.51
148.6
26.0
116.5
621.0
14.1
14.2**
91.3
48.9
39.4
208.3
16.4

0.013
0.002
0.034
0.010
1.992
0.072
0.015
0.243
0.157
0.302

0.59
0.09
1.53
0.45
89.64
3.24
0.68
10.94
7.07
13.59

T
F
T
T
T
T
T
T
T
F

1.446
1.064
0.001
0.198
0.017
0.184
0.975
0.023

65.07
47.88
0.05
8.91
0.77
8.28
43.88
1.04

F
T
F
T
F
T
T
T

0.083
0.092
0.004
0.014
0.008

3.74
4.14
0.18
0.63
0.36

T
T
F
F
F

THE IMPLICATION OF HEAT AND WATER BALANCE CHANGES
estimate the evaporation from lakes (Kondo andKuwagata, 1992).
The downward radiation flux input to the lake canbe written as
R = (1ref) S + L,

(1)

where Re is the Reynolds number, is the kinematic
viscosity coefficient of air, and D is the molecular diffusion coefficient of vapor. We assumed CE U  CH U andconsidered the case of natural convection, where CHU isexpressed as
CHU = max(a + b × 0.7U, c × (Twwhere R is the input radiation flux, ref = 0.06 is thealbedo of the water surface, = 0.98 is the surface
emissivity, S is the solar radiation, and L is the longwave radiation flux from the atmosphere.
The heat balance equation is written as
R G = Tw + H + 
E,
4

(2)

Where

Tw
,
t
H = cp CHU(Tw TAM),

G = cw w ZE=*CHU(hqSAT(Tw)

(3)
(4)
qAM).

(5)

Here, G is the heat flux of conduction to the water,
 is the Stefan-Boltzmann constant, and Tw is the calculated water temperature, which satisfies the heat balance equations (Equations (2) (5)). Sensible and latent
E were computed simultaneously. In
heat fluxes H and 
addition, cw is the specific heat of water, w is the densityof the water, Z is the depth of the lake, 
Tw/t is thevariation rate of water temperature with time, and cp
and are the specific heat and density of air, respectively. The air temperature is given by TAM , and is thelatent heat of vaporization, with evaporation efficiency
* = 1. Finally, qSAT (Tw) is the specific humidity at saturation, qAM is the specific humidity of air, and U is thewind speed. The evaporation rate E under these conditions only takes into account evaporation from the lake.
We used a simple one-layer water model in thepresent study for several reasons. First, the heat flux
conducting into the lake G depended on the temperaturechange of the lake. The daily range of water temperature was less than the monthly range because of thelarge thermal physical coefficient of water. Second, thetemperature changed at various water depths in different seasons. We assumed depths of 2, 4, 6, 8, and 10 m
and performed calculations using these values. Amongthese five cases, the largest difference in the total evaporation during the warm season (May September) was
10%, and the largest difference between months was
17%.
The air temperature was lower than 0°C from
November to March, and the lake surface froze in
winter. However, no observational data were available
for winter from the Baidi hydrological station. Therefore, changes in the thermal physical coefficients withfreezing and melting processes could not be tested forthe freezing season. Hence, we did not consider freezing
effects in our model. This omission will not affect theresults, because we did not consider evaporation during
the cold season in this study.
In Equations (4) and (5), CHU is the exchange speed.
The exchange speed in the bulk formula depends on theevaporation area of a surface. The lake can be treated asa finite water surface area, with averaged width of approximately 3 km (Figure 1). According to Kondo (1994,
p. 172), when the wind speed is constant in height overa finite surface, the bulk transfer coefficient for latent
heat flux CE is
CE = 0.2275( /D) 2/3(log10 Re) 2.58, Re : 6.5 × 106 ~ 109,
(6)
Re =

XU
.


(7)

1

TAM)1/3),

a = 0.0023 ms , b = 0.0025, c = 0.003 ms 1 K 3.

(8)
(9)

Here, a, b, and c were derived from Equations (6) and(7).
Determining the depth of the lake
The depth of the lake was unknown. We assumed
depths of 2, 4, 6, 8, and 10 m and calculated heat andwater balances with those depths. The minimum[Tw(obs.) Tw(cal.)2 ] was found for a depth of 4 m.
Therefore, we assumed a lake depth of 4 m.
Figure 2 shows the calculated results (solid lines).
The circles are the observed water temperature (upper
panel) and large pan evaporation from 1984 1995. The
dotted line is the observed small pan evaporation at
Nagarze weather station. The simulated values agreed
closely with the monthly mean temperature and total
evaporation observations in the warm season. Thechange pattern of the small pan evaporation was similar
to that of the large pan and the lake evaporation, but
the annual amplitude of the small pan evaporation was
the largest.
Figure 2 verifies our one-layer water model of evaporation on the Tibetan Plateau. Evaporation, sensible
heat flux, and other quantities relevant to heat andwater balances over the lake surface were calculated for
45 years.

RESULTS
Seasonal changes
Figure 3 shows the seasonal variations of the heat
and water balances over the lake and land surfaces inthe basin. The net radiation Rn into the lake (149
Wm 2, Table I) was much larger than that into the land
(91 Wm 2 ) in the warm season (May September). Thiscan be explained by the differences in albedo and surface temperature for these two types of surfaces. The

Figure 2. Seasonal changes of the observed water temperature (top) and pan evaporation (bottom). The solidlines and black circles show the calculated results andthe observations, respectively. The dotted line shows
the observed small pan data.

―3―

J. XU ET AL.
annual mean surface temperatures of the land and lakewere 9.1°C and 7.2°C, respectively, meaning that the
upward longwave radiation of the land surface waslarger than that of the lake surface. The peak of Rn
occurred in May for the lake (Figure 3a) and in August
for the land (Figure 3b).
The latent heat flux E (warm season mean 116
Wm 2) dominated the heat balance over the year on thelake surface. On land, however, sensible heat flux domE domininated the heat budget in early summer, with ating during the mid-summer monsoon, as in othersoutheastern parts of the plateau (see Figure 7 of Xu
et al. 2005a). The land surface temperature Ts (Figure3c) was highest in early summer, becoming similar tothe lake temperature Tw in the monsoon season due torainfall Pr (Figure 3e). As noted above, lake area
accounts for 10% of the whole basin, and the evaporation from the lake makes up 26% of the basin total;
evaporation from the lake (Ew) was 621 mm (Table I)
during the warm season, which far exceeded the precipitation of 341 mm, while evaporation from the land
surface was 208 mm (Figure 3d). The seasonal change ofsmall pan evaporation (SPan) was similar to that of thelake and to large pan (LPan) evaporation. However, thedifference between SPan and LPan was larger in earlysummer. August was the most humid month, with WIover 1.0 (Figure 3e). The soil water content generally
followed the precipitation changes (Figure 3f), in agreement with previous findings for the southeastern area(see Figure 7f of Xu et al., 2005a)

Long-term variation
Over the study period, the basin had an averagepressure of 591 hPa and vapor pressure of 3.85 hPa, just
one third that of Tokyo. Figure 4 shows long-termchanges in heat and water balances from 1961 to 2005.
The increase in temperature of 1.13°C (Figure 4a, TableI), combined with the vapor pressure Vpr (Figure 4c)
increase of 0.68 hPa (17.5%) and the 7.16% (16.8% in 45
years) increase in relative humidity RH (Table I),
resulted in downward longwave radiation flux L increasing by 33.21 Wm 2 (13.7%, Figure 4f) during the 45year period. The decreased sunshine duration (Figure4b) reduced the downward solar radiation flux S
(Figure 4e).
The same trends were found on an annual basis andfor the warm period (May September). In the warm
season (May September), evaporation from the lake Ew(Figure 4h) decreased 43.88 mm (7%), a result that corresponded to the decrease in SPan (Figure 4g). Thisdecrease can be explained by the increase of qAM (Equation (5)), which is directly proportional to Vpr. The
decrease in the daily air temperature range (TAMIN TAMAX, Table 1) would have resulted from the decreasein radiative cooling (L). Precipitation Pr (Figure 4j) increased slightly, and evaporation from the land surface
Es (Figure 4i) barely changed; however, the reliabilitiesof these trends were not significant (Table I). The basin
is located in a semi-arid climate region with WI = 0.26;
the land surface is not always sufficiently wet, therefore, Es mainly depended on Pr, and most of the precipitation (approximately 60%) evaporated annually.
Although WI (Figure 4k) increased, the increase was not
significant.

SUMMARY AND CONCLUDING REMARKS

Figure 3. Monthly variations of the heat balance andwater balance of the basin. Heat fluxes over (a) the lake’s
surface and (b) land surface. (c) Land surface temperature Ts (dotted line), water temperature Tw (the dashedline and black circles show the calculated values and theobservations, respectively), and observed air temperature (solid line). (d) Observed SPan evaporation (dashedline with open circles), LPan evaporation (black circles),and calculated evaporation from the lake Ew and land
surface Es. (e) Precipitation Pr and wetness index WI. (f)Volumetric soil water content for four layers.

We successfully used a one-layer water model, multilayer soil model, and potential evaporation method tosimulate long-term changes in heat and water balances
for the land and lake areas of the Yamdrok Yumtso
Lake basin. Warming has been confirmed in this region.
For the water cycle under this warming condition, we
found that both vapor pressure and relative humidity
increased in the basin. These increases resulted in an
increase in the longwave radiation flux and reduction in
the daily temperature range. Evaporation from the lake
made up approximately 26% of the total amount of
evaporation in the basin; over the study period, lake
evaporation decreased 7% (May September), a trendthat was not found in evaporation from land and fromprecipitation, which showed no significant change.
Given the increase in water vapor in the air (Vpr andRH) and the decrease of evaporation from the ground―4―

THE IMPLICATION OF HEAT AND WATER BALANCE CHANGES
surface (sum of lake Ew and land Es), it is possible thatthe water vapor originated outside the basin and maybe
even outside the plateau. Future research will analyze
long-term change in large-scale weather conditions
around the plateau.

ACKNOWLEDGEMENTS
We thank Dr. Yuping Yan of the China Meteorological Administration. We also thank the Climate
Center of the China Meteorological Administration andthe Hydrological Bureau of Tibet for providing the data.
Thanks are also extended to Prof. Fujio Kimura and Dr.
Rikie Suzuki of FRCGC, JAMSTEC. This project was
supported by JSPS and China NSFC (No. 40571037 andNo. 40711140130) under the Japan-China Scientific
Cooperation Program. DIAS was used to prepare thedata.

SUPPLEMENTS
Supplement 1 Enlarged figure of Figure 1 is included.
Supplement 2 Enlarged figure of Figure 2 is included.
Supplement 3 Enlarged figure of Figure 3 is included.
Supplement 4 Enlarged figure of Figure 4 is included.

REFERENCES

Figure 4. Interannual variations (solid lines) for thebasin from 1961 to 2005. Dotted lines represent thetrends obtained by linear regression. (a) Air temperature, (b) sunshine duration, (c) vapor pressure, (d) windspeed, (e) solar radiation flux, (f) longwave radiation
flux, (g) small pan evaporation, (h) evaporation from thelake and large pan (black circle with dashed line), (i)evaporation from the land surface, (j) precipitation, (k)WI.

Brutsaert W. 1982. Evaporation into the Atmosphere. D. Reidel
Publishing Company, Holland; 299 pp.
Kondo J. 1994. Meteorology of the Water Environment - Waterand Heat Balance of the Earth’s Surface. Asakura Shoten
Press, Japan; 348 pp. (in Japanese).
Kondo J, Kuwagata T. 1992. Hydrological climate in Japan (1):Radiation and evaporation from shallow lakes. Journal ofJapan Society of Hydrology and Water Resources 5: 13 27(in Japanese with English summary).
Kondo J, Xu J. 1997. Seasonal variations in the heat and waterbalances for nonvegetated surfaces. Journal of Applied
Meteorology and Climatology 36: 1676 1695.
Lu A, Wang L, Yao T. 2006. The study of Yamzho Lake andChencuo Lake variation using remote sensing in Tibet
Plateau from 1970 to 2000. Remote Sensing Technology
and Application 21: 173 177 (in Chinese with English
summary).
Xu J, Haginoya S. 2001. An estimation of heat and water balances in the Tibetan Plateau. Journal of the MeteorologicalSociety of Japan 79(1B): 485 504.
Xu J, Haginoya S, Masuda K, Suzuki R. 2005a. Heat and waterbalance estimates over the Tibetan Plateau in 1997 1998.
Journal of the Meteorological Society of Japan 83: 577 593.
Xu J, Haginoya S, Saito K, Motoya K. 2005b. Surface heatbalance and pan evaporation trends in eastern Asia in the
period 1971 2000, Hydrological Processes 19: 2161 2186,
DOI: 10.1002/hyp.5668.
Yanai M, Li C, Song Z. 1992. Seasonal heating of the Tibetan
Plateau and its effects on the evolution of the Asiansummer monsoon. Journal of the Meteorological Society ofJapan 70: 319 351.

―5―