Full text of DOI:10.1007/BF02907089

Vol. 44 No. 9
SCIENCE IN CHINA (Series D)
September 2001
Variation of precipitation δ 18O in Langtang Valley,
Himalayas
ZHANG Xinping (章新平)1,2, Masayoshi Nakawo (中尾正义)3,
Koji Fujita (藤田耕史)3, YAO Tandong (姚檀栋)4 & HAN Jiankang (韩健康)2
1. Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China;
2. Institute of Territory and Environment Research, Hunan Normal University, Changsha 410081, China;
3. Institute for Hydrospheric-Atmospheric Sciences, Nagoya University, Nagoya 464-8601, Japan;
4. Cold and Arid Regions Environmental and Engineering Research Institute, Lanzhou 730000, China
Correspondence should be addressed to Zhang Xinping (email: zhangxp@niglas.ac.cn)
Received October 20, 2000
Abstract The variation of the δ 18O in precipitation and the relationship with precipitation amount
at Kyangjin Base House and Yala Glacier Camp in Langtang Valley, Nepal Himalayas were ana-
lyzed. The variations of the δ 18O with precipitation had great scatter, and the correlations between
the δ 18O and precipitation changed with time on the synoptic scale. On the seasonal scale, there
was marked amount effect at Kyangjin Base House. However, the δ 18O-precipitation gradient was
smaller than that on the synoptic scale. Because of the maintenance of the basic equilibrium be-
tween stable isotopic compositions in atmospheric vapor and precipitation, the evaporation en-
richment was light during the rainy season. Therefore, the variation of stable isotopic compositions
in precipitation was independent on the sampling intervals. Simulations show that the rainfall in
Langtang Valley was not the outcome of the initial condensation of ocean vapor that originated
from low latitudes. The stable isotopic compositions in precipitation were greatly depleted due to
the strong rainout of the vapor from oceans as the vapor was raised over the Himalayas.
Keywords: Himalayas, Lantang Valley, stable isotopic ratio, precipitation.
Precipitation is an important link in the water cycle. The variation of stable isotopic composi-
tions in precipitation is closely related to the weather process of generating rainfall and the initial
—
conditions in vapor origins, and changes with space and time[1 6]. The investigation of the stable
isotopic changes in the water cycle started in about 1950[7]. The large-scale and organized collec-
tion of precipitation samples was initiated in 1961[2]. The International Atomic Energy Agency
(IAEA) in co-operation with the World Meteorological Organization (WMO) set up a global net-
work with more than 550 meteorological stations that continually collected monthly precipitation
samples for the environmental isotopic compositions in precipitation and meteorological elements.
The isotopic compositions in precipitation and routine meteorological data of all IAEA/WMO
stations were regularly published by IAEA. The main objective of the global survey program was
to determine temporal and spatial variations of environmental isotopes in precipitation and, con-
sequently, to provide basic isotope data for the use of environmental isotopes in hydrological in-
vestigations for water resources inventory, planning and development[2].
Temperature effect, namely the marked positive correlation between stable isotopic ratio and

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temperature, is recognized as the most important content on the stable isotopic changes in precipi-
—
tation[2,5,6, 8 11]. It mainly appears in middle-high latitudes. The climatic information recorded in
snow and ice of middle-high latitudes can be reasonably interpreted using a simple linear rela-
—
tionship between the stable isotopic ratio and the temperature[12 15]. Temperature effect arises
from the fact that the fractionation of stable isotopes in the atmosphere is related to the
phase-change temperature in the process of the phase change[8,16,17]. However, vapor is affected by
many different phase-change processes from its origin to fall site: evaporation in source region,
initial condensation of evaporated vapor, growth of condensate in cloud under different tempera-
ture and humidity conditions, evaporation of raindrops in falling process, etc[2,17,18]. Therefore, the
stable isotopic compositions in precipitation are not a single-valued function of sampling tem-
perature at the sampling site.
Amount effect, namely the notable negative correlation between the stable isotopic ratio in
precipitation and precipitation amount, mainly appears in low-middle latitudes, especially in
tropical coasts and islands. It is related to strong convective phenomenon[2,19,20]. Temperature
situation in convective cloud is very different from that in surface because of the complex tem-
perature and humidity conditions in the cloud, which overshadows the temperature effect.
The Qinghai-Tibet Plateau is famous for its gigantic extent, high altitude and complex to-
pography. It affects the climatic latitudinal and meridianal structure over wide areas in the North-
ern Hemisphere[21]. Vapor producing rainfall comes mainly from the west sources, low-latitude
oceans and inner plateau, and is influenced by special geographic position and topography. The
interactions among different vapor sources cause the stable isotopic variations in precipitation on
the Qinghai-Tibet Plateau to be different from that in south Asia where precipitation is mainly in-
fluenced by low-latitude oceans, and in central Asia where vapor comes from the westerlies. The
analyses show that there are notable positive correlations between δ 18O in precipitation and tem-
perature on the northern plateau[6,22]. However, on the southern plateau, the δ 18O in precipitation
is related to vapor origins: δ 18O in precipitation is lower under the influence of the vapor from
low-latitude oceans, but higher from middle-high-latitude continent[23,24]
.
Langtang Valley is located in the south slope of the Himalayas and is a region of transition
from the monsoon climate in south Asia to the plateau monsoon climate. In order to understand
the mechanism of the water cycle and basic features of stable isotopic compositions in precipita-
tion in the valley, a sampling program for stable isotopic compositions in precipitation was carried
out by Institute for Hydrospheric-Atmospheric Sciences, Nagoya University, Japan and the Minis-
try of Water Resources, Nepal from 1993 to 1996. The sampling sites of the co-operation program
were set up at Kyangjin Base House (3880 m a.s.l.) and Yala Glacier Camp (5100 m a.s.l.) in
Langtang Valley (fig. 1). The precipitation sampling and the measurement of precipitation amount
were made basically once a day in the forenoon from May 10, 1993 to October 5, 1996 at Kyang-
jin Base House (the dataset noted as Kyangjin). In order to assess the possible error caused by the

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evaporation of rainfall in the rain gauge, another sampling and the measurement of precipitation
amount were made everyday at 06h NST (Nepal standard time) and 18h NST from May 28 to Oc-
tober 26, 1996 at the same site (the dataset noted as Kyangjin*). The sampling method at Yala
Glacier Camp was the same as that at Kyangjin* from May 20 to October 4, 1996 (noted as Yala).
Fig. 1. Map of Langtang Valley showing sampling sites. Gray parts denote glaciers.
The stable oxygen isotope of all collected precipitation samples was measured by the
MAT-250 mass spectrometer, at the Institute for Hydrospheric-Atmospheric Sciences, Nagoya
University, Japan. The measured ratio of the stable oxygen isotope (18O/16O) in precipitation sam-
ples is expressed as parts per thousand of their deviation relative to the Standard Mean Ocean Wa-
ter (SMOW), and is defined by
18
16
⎛
⎜
⎞
( O/ O)sample
δ 18O =
−1 ×1000 ,
(1)
⎟
18
16
⎜
⎟
( O/ O)SMOW
⎝
⎠
where (18O/16O)sample and (18O/16O)SMOW stand for the isotopic ratio 18O/16O in the water sample
and standard mean ocean water, respectively.
1
Variation of δ 18O on the synoptic scale
1.1 Correlation between δ 18O in precipitation and precipitation amount
The global survey result on stable isotopic compositions in precipitation shows that there was
amount effect in low-middle latitudes, especially on tropical islands and coasts[2,5]. Fig. 2 presents
the δ 18O in precipitation versus the precipitation amount at Kyangjin for the period from May
1993 to October 1996.

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Fig. 2. Scatter of δ 18O in precipitation versus precipitation at Kyangjin during May 1993 to October 1996.
It can be seen that the δ 18O in precipitation had great scatter. The correlated coefficient be-
tween δ 18O and precipitation was only −0.19 for the collected 339 precipitation days, reaching
0.05 confidence limits, which shows that there was amount effect to a certain degree in the period
of sampling at Kyangjin.
If the 339 precipitation days are grouped by calendar year and their annual correlation be-
tween δ 18O and precipitation are analyzed, it can be found that there were not only marked dif-
ferences among correlated coefficients between the δ 18O and precipitation but also the different
δ 18O-precipitation gradients for different calendar years (table 1). Interactions among different
influencing factors, such as vapor origins, different condensation height, different condensation
temperature and different transportation patterns, lead to the great scatter in δ 18O variation[8,12,17]
.
Table 1 Correlations between the δ 18O in precipitation and precipitation amount at Kyangjin for different calendar years
1993
−0.06
−0.0688
79
1994
−0.27
−0.4504
1995
−0.21
−0.1578
1996
−0.28
−0.2998
Correlated coefficient
dδ 18O/dP
a)
81 (α ＜0.02 )
106 (α ＜0.05 )
73 (α ＜0.02 )
Sample number
a) α is confidence limit.
1.2 Comparison between δ 18O variations with different sampling intervals
Fig. 3 presents the δ 18O in precipitation versus precipitation at Kyangjin* and Yala. Both sta-
tions have a marked amount effect, with correlated coefficients of −0.376 and −0.344, reaching
0.001 and 0.05 confidence limits, respectively. The dδ 18O/dP is slightly greater at Yala compared
with that at Kyangjin*, which is attributed to the influence of altitude. Generally, an air mass will
undergo a large descending-temperature range at a higher altitude for generating the same quantity
of precipitation as that in low altitude due to the descent of air humidity, and thus the δ 18O values
in precipitation have a great variation range.
The dδ 18O/dP of Kyangjin* was smaller compared with that at Kyangjin in 1996, although
there was amount effect to a certain degree at Kyangjin*. Such a difference is related to the dif-
ferent sampling intervals.

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Fig. 3. Scatters of δ 18O in precipitation versus precipitation at Kyangjin* (a) and Yala (b) in 1996.
2
Variation of δ 18O on seasonal scale
2.1 Seasonal variation of δ 18O at Kyangjin
Here, the δ 18O in the precipitation day can be transformed into the monthly weighted mean
values according to
δ 18Ow
=
Pδ 18Oi
P ,
(2)
i
i
∑
∑
where δ 18Oi and Pi are the δ 18O and corresponding precipitation on a precipitation day. The sea-
sonal variation of the δ 18Ow at Kyangjin is shown in fig. 4.
Fig. 4. Monthly variations of the weighted mean δ 18O and precipitation at Kyangjin from May, 1993 to October, 1996.
1, Jan.; 3, Mar., 5, May; 7, Jul., 9, Sep.; 11, Nov.

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Although there were pronounced variations in daily δ 18O versus daily precipitation on the
synoptic scale, the mean δ 18O in every calendar year displayed the similar patterns on the sea-
sonal scale: the maximum δ 18O appeared in May before the onset of the rainy season; from June,
the mean δ 18O was progressively depleted and reached a minimum in September or October. The
variation of the mean δ 18O was inconsistent with the monthly precipitation. However, the mean
δ 18O during the rainy season was lighter than that during the dry season.
2.2 Comparison of mean δ 18O variations with different sampling intervals
A comparison (fig. 5) between the weighted mean δ 18O values for Kyangjin in 1996 and
Kyangjin* shows that, although there was a difference between two sampling precipitation
amounts, their variations of the monthly weighted mean δ 18O were similar from June to October
except for in May. Such a result displays, on one hand, that a certain sampling interval played a
smaller role at a high altitude like Kyangjin Base House during the rainy season; on the other hand,
it shows that the stable isotopic compositions in atmospheric vapor and raindrops basically kept an
equilibrium during the rainy season at Kyangjin Base House.
Fig. 5. Comparison of the monthly weighted mean δ 18O and monthly precipitation between two different sampling
intervals in 1996 at Kyangjin Base House. 1, Precipitation (Kyangjin, 1996); 2, precipitation (Kyangjin*); 3, δ 18O
(Kyangjin, 1996); 4, δ 18O (Kyangjin*).
2.3 Correlation between δ 18O and precipitation on seasonal scale
After removing random influences on the synoptic scale, the correlation between the monthly
weighted mean δ 18O and monthly precipitation became notable at Kyangjin Base House (fig. 6).
There was a lower dδ 18O/dP on the seasonal scale compared with that on the synoptic scale.
This is because the weighted mean and the summing have the variational ranges of the mean δ 18
O
and precipitation on seasonal scale less and greater than those on synoptic scale respectively.
Therefore, it can be concluded that the mean annual dδ 18O/dP is also different from the gradients
on synoptic and seasonal scales.
2.4 Mathematical simulation of amount effect
The generation of amount effect is linked to the strong convective phenomenon[8,19]. Al-

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Fig. 6. Scatter of the monthly weighted mean δ 18O versus the monthly precipitation, and simulations of the amount
effect at Kyangjin.
though there is no temperature effect in low latitudes, stable isotopic fractionation in every micro-
scopic phase-changed process is connected to phase-changed temperature of generating condensa-
tion or sublimate in a strong convective cloud inducing amount effect. It is assumed that the cool-
ing of a rainfall air parcel and the generation of condensation are completely caused by the verti-
cal motion, namely by the wet adiabatic cooling. Thus, the stable isotopic ratio in accumulative
condensation water should be the weighted mean of the stable isotopic ratio in the condensation
water during every stage of cooling[19]
:
tp
δ 18Ot dM v
∫
t0
δ 18O =
,
(3)
tp
dM v
∫
t0
where t0 and tp are the temperature at cloud base and top, and δ 18Ot is the stable isotopic ratio in
hydrometeors at every layer:
dδ 18Ot
1+ δ 18O
M vdα + α(α −1)dM v
α(M v + αMl )
=
,
(4)
where α is fractionation factor of stable isotope, Mv the mixing ratio of air parcel and Ml the liq-
uid-water content in cloud:
t
⎧
− c1 dM v ,
0
t,
≤
⎪
∫
t0
⎪
−t
⎪
M = − c
dM v ,
t
t
0,
(5)
＜
⎨
l
2
∫
t0
⎪
0,
− t ,
≤
⎪
0
⎪
⎩
where c1 and c2 are the liquid-water content coefficients in cloud, and all are greater than zero.
They stand for the condensation amount also remaining in cloud.
Moreover it is assumed that: (1) the air temperature in vapor origins is 29℃, water tempera-

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ture is 26℃, air pressure is 1010hPa and relative humidity is 80%; (2) the super saturation ratio at
the ice surface Si = Ev/Ei = 1.0 − 0.003t, where Ev and Ei are the actual saturated vapor pressure
and that at ice surface; (3) the liquid-water content coefficients in cloud are c1 = 1/3 and c2 = 1/4.
Setting the δ 18O in surface water of vapor origins to zero, the simulation I is obtained (see
fig. 6). The curve stands for the relationship between the mean δ 18O in initial hydrometeors from
ocean vapor and precipitation (condensation amount). It can be seen that there is a great difference
between the actual regression line and simulation I.
By setting the δ 18O in surface water of vapor origins to −7.95‰, there is a very good agree-
ment between the regression line and the obtained simulation II. In fact, it is impossible that the
δ 18O in surface water of the oceans is as light as −7.95‰. Therefore, it can be concluded that the
rainfall in Langtang Valley was not the outcome of the initial condensation of ocean vapor origi-
nated from low latitudes. The stable isotopic compositions in precipitation were greatly depleted
due to the strong rainout of the vapor from oceans as the vapor was raised over the Himalayas.
The mean depleted degree was about −7.95%.
3
Discussion
Temperature effect and amount effect are two most important ones in isotopic variations in
the water cycle. The analyses display that[1,5,6] the variations of the δ 18O are consistent with those
of temperature without any exception for the stations with temperature effect, which shows that
temperature is the main factor controlling the stable isotopic compositions in precipitation in these
stations. Although there is the marked amount effect in Lantang Valley, it can be seen, from fig. 4,
that the variation of δ 18O is inconsistent with that of the precipitation intensity. Therefore, it can
be concluded that precipitation intensity is not the main factor controlling the stable isotopic
compositions in precipitation. Unlike temperature effect, the amount effect is only an accompani-
ment linked to the strong convective phenomenon at low latitude. Because the magnitude of the
stable isotopic compositions in strong rainfall is linked to the fractionations of the stable isotopes
during every condensation process in a convective cloud[19] and because the mean surface tem-
perature is different from that in cloud, the magnitude of surface temperature cannot determine the
magnitude of the stable isotopic compositions in precipitation uniquely and the clean interpreta-
tion is hampered.
An important fact is that, in Kyangjin, the maximum δ 18O in precipitation usually appears in
May before the rainy season, whereas the minimum values in September or October as the rainy
season ends. A cause generating such a seasonal distribution is probably related to the variations
of stable isotopic compositions in vapor sources. The observations show that the δ 18O values from
the shallow corals in the tropical Pacific have markedly seasonal variability. There are light δ 18O
in these corals with high sea surface temperature during summer and heavy ones with low sea
surface temperature during winter[25,26]. Although the magnitude of the δ 18O in coral reef is

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closely related to the complex biochemical process of coral growth to a great degree, δ 18O of
ocean water is another important controlling factor. Usually, after entering the warm season, the
stable isotopic compositions in surface water of oceans are depleted gradually because more and
more fresh water from lands is poured into oceans continuously and because the intense rainfall
having relatively light δ 18O dilutes the marine δ 18O pool during the rainy season. This conse-
quently induces a corresponding depletion in vapor and precipitation. The minimum δ 18O in
ocean water, vapor and rainfall probably appears in late summer or early fall. During the cold
season, the vapor evaporated from oceans is mostly retained on lands in the form of snow or ice
and the rainfall decreases rapidly. The net loss in oceans produced owing to ocean evaporation has
stable isotopic compositions in ocean surface water gradually enriched. This trend will continue
until late winter or early spring.
In addition, the evaporation enrichment of raindrops in the falling process and in the rain
gauge is stronger during the spring than during other seasons because of high temperature and low
air humidity, which will lead to high stable isotopic compositions in precipitation[19]
Conclusions
(1) There are variational negative correlations of a certain degree between the δ 18O in pre-
.
4
cipitation in Langtang Valley, Nepal Himalayas and precipitation. The variations of the δ 18O ver-
sus precipitation has great scatter on the synoptic scale.
(2) There is a notable amount effect on the seasonal scale, and the δ 18O-precipitation gradi-
ent is less than that on synoptic scale.
(3) The influence of the different sampling intervals on the mean δ 18O was unimportant dur-
ing the rainy season at Kyangjin Base House, which shows that the stable isotopic compositions in
atmospheric vapor and precipitation basically kept an equilibrium during the rainy season.
(4) Simulations show that the rainfall in Langtang Valley was not the outcome of the initial
condensation of ocean vapor originated from low latitudes. The stable isotopic compositions in
precipitation were greatly depleted due to the strong rainout of the vapor from oceans as the vapor
was raised over the Himalayas. The mean depleted degree was about −7.95‰.
Acknowledgements This work was supported by the National Key Research Project (G1998040802), the CAS Knowl-
edge Innovation Program (XZ-301), LICCRE Program (BX2001-3) and the CAREERI foundation (TZ2000-02).
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