Can Stable Isotopes Indicate
Water Source of Major Indian Rivers?
Location:
Figure 1:
Himalayas Mountains,
Northern India (Figure 1)
Main Problem Illustrated:
How can we use isotope
tracers to quantify glacier melt
contribution to river flows in
the Himalayas?
How may this water supply be
in jeopardy in the future?
Summary:
Going into the future,
vanishing glaciers have the
potential to adversely affect
the water supply to a huge
population in southern Asia.
Figureof1.the
MapHimalayas
of the Himalayas
Region.
Map
Region
In this region, where many of
the rivers emanate from the ice
capped Himalayas mountains (the third biggest ice cap on the planet), glacier melt plays a large role in maintaining
river baseflows, so quantifying the affect of a reduced ice mass is of great concern for a great many people. As a first
step, it is important to accurately determine how much of the rivers’ flows depend on the existence of these glaciers.
Isotopically based hydrograph separations will prove to be vital to quantifying the importance of glacier meltwaters
to streamflow. Once we have an understanding of the current glacial contributions to the flows of these rivers, we can
begin to make predictions of future stream flows.
Tracers Used:
δ18O, δ2H
People Affected, Environmental, Ecological Impacts:
Rivers emanating from the Himalayas/Hindu Kush region provide water for hundreds of millions of people in
India. Loss of the Himalaya’s ice mass, could pose a significant problem for the people who depend on these rivers
for drinking water, especially in the summer. There has already been a notable decline in nearly all of the ice mass
in the Himalayas/Hindu-Kush region, and this trend is forecast to accelerate. For now, flows in many of the rivers
emanating from the Himalayas are normal or even above normal because of the melting ice, however, once the ice
mass declines enough, flows may decrease dramatically, and possibly abruptly. It is important, today, to attempt
to quantify the importance of this contribution to river flows in the region. Stable isotopes of water can be useful
indicators of water source, especially when combined with other measurements.
Hydrogeological Setting:
Many of the mountains of Southeast Asia, including the Himalayas, are a direct result of the collision of the IndoAustralian Plate and the Eurasian Plate. They are among the youngest and fastest growing mountains on earth
(averaging about 5 mm/yr). They are also some of the fastest eroding, accounting for nearly a quarter of the earth’s
sedimentary budget due to the erosive effects of water and ice in a very tectonically active region. Lithologies are
generally variable in the Himalayas, however a few general trends can be observed. Starting at the base of the
mountains, Miocene and Pleistocene sediments from the eroding mountain mass dominate. Moving further into the
range, there are older Cambrian age sediments, which become increasingly metamorphosed towards the core of the
Himalayas. Lastly, in the north, there is a suture zone, or joining of the plates, which is made up of a variety of facies
ranging from marine sediments to ophiolites to volcanics.
There are also dramatic climate variations within the Himalayas. The most notable trend is that higher precipitation
amounts are found in the south and the east, while it is drier in the north and west. Also, precipitation largely falls as
rain at lower elevations, and as snow at higher elevations, even during the summer (especially above 5,000 m where
there are permanent snow fields). The range also has a large influence on the climate of Southern Asia. Notably, it
prevents cold-arctic winds from penetrating into the subcontinent region, and it stops the advance of storms from
the west and moisture from the south, thus contributing to the formation of vast central Asian deserts. Its presence
(and that of the high Trans-Himalayan region to the north), has also been linked to the strength of the South Asian
Monsoon due to its affects on regional circulation.
Water Sampling and Analysis Summary:
So far, a number of studies have focused on trying to determine isotopic signatures and trends of both precipitation
and river water in the Himalayas region. To date, most of these studies have involved a broad generalization
of the isotopic effects of various factors such as altitude, location, and moisture source. Specifically, the studies
have examined rainout affects across the Indian subcontinent, major river isotopic compositions, and the effect
of evaporation on the stable isotopes of river water. There has also been sampling for other variables such as
conductivity and dissolved chemical species (anions+cations) to get a general idea of how different sources can be
labeled differently. However, despite these studies, robust hydrograph separations (which are necessary to determine
the contribution from glacial melt) are still lacking.
Results of Tracer Studies:
The isotopic trend of rainfall in India shows that there is a northeast/southwest gradient of precipitation, with the
lightest (most depleted in heavy isotopes) isotopic values in the northeast, and the heaviest (most enriched in heavy
isotopes) values in the southwest. In addition, there is considerable seasonality, and unexpected spatial variations. Many
of these variations can be attributed to the affects of evaporation and rainout. Notably, lower elevation sites are generally
subject to more evaporation, while higher elevation sites have less evaporation. Also, there is more rainout in certain
areas, and also during certain years. One particularly interesting finding that is related to rainout is that during wetter
monsoons, the rainfall is isotopically heavier than during monsoons that are drier. This is depicted in Figure 2, next
page, which shows that there is a negative correlation between precipitation intensity and how isotopically heavy the
precipitation is.
The stable isotopic composition of rivers generally reflects the contributions from the various inputs (snowmelt,
glacier melt, and rainfall), although, because the rivers integrate these inputs over time, the isotopic signals within
the rivers are significantly dampened. Again, there is a general northeast/southwest gradient across the region, with
the most depleted values in the trans-Himalayan region (see Figure 3, next page). Also, it has been shown that river
water from very cold, high altitude regions
have a very high deuterium excess and steeper
local meteoric water line (see Figure 3, next
page), indicating very little evaporation there.
There is also considerable isotopic variability
of Himalayan Rivers, with δ18O signatures
ranging from -12‰ to 6‰ (see Figure 4, next
page). It is also noted that in the Himalayas, the
isotopic composition of river water is sometimes
negatively correlated with its conductivity (an
easy to measure parameter in the field) - higher
conductivity values correlate with lighter, and
thus more glacial, isotopic values (see Figure 5,
next page).
a
b
Findings and Conclusions:
Initial estimates are that glaciers contribute
about 30 percent of the streamflow in typical
large Himalayan Rivers. However, these
numbers are not very precise because there
are many unknowns. In addition to the welldocumented spatial and temporal patterns of
isotopic variability in the region, there is also
small-scale variability that must be taken into
account, even within individual river systems.
Studies have also indicated that it is useful
to combine stable isotope measurements
with other river measurements. For example,
a)Figure
Measured
b) simulated
between precipitation
intensity and
2. a)and
Measured
and b) correlations
simulated correlations
between precipitation
conductivity appears to be a powerful
intensity and δ18O over the Indian Ocean. Both actual and modeled results
indicator for differentiating between snowmelt
reveal that there is a negative correlation between intensity and δ18O.
contributions and glacier melt contributions
(meltwater coming off of glaciers has a higher conductivity than snowmelt). Also, other chemical indicators (such as
sulfate and chloride) may become useful when the stable isotopes, themselves, do not reveal much information.
Take Home Message:
So far, there have been a number interesting studies that try to characterize spatial and temporal isotopic
variability in the streamflow of major Himalayas rivers, as well as attempts to interpret the isotopic signatures in
interesting ways (e.g. by looking at the deuterium excess) or by combining these with other measurements (such as
conductivity). These studies are a good start, but other studies, perhaps involving localized measurements that are
more telling of glacial contributions in specific watersheds will also be useful. In the end, there should be robust
isotopically based hydrograph studies that attempt to quantify how much of the rivers’ flows are dependent of glacial
melt water.
Further Reading:
Lambs L (2000), Correlation of
conductivity and stable O18 for the
assessment of water origin in river
system. Chemical Geology 164:
161–170.
Lambs L, Balakrishna F, Brunet, Probst
JL (2005). Oxygen and hydrogen
isotopic composition of majorIndian
rivers: a first global assessment.
Hydrol. Process. 19, 3345–3355
(2005)
Vuille M, Werner M, Bradley MS,
Keimig F (2005), Stable isotopes
in precioitation in the Asian
monsoon region, J. Geophys. Res,
110, D23108, doi:10.1029/
2005JD006022.
Pande K, Padia JT, Ramesh R, Sharma
KK. 2000. Stable isotope systematics
of surface water bodies in the
Himalayans and trans-Himalayan
(Kashmir) region. Proceedings of
the Indian Academy of Sciences 109:
109–115.
Ma
Figure
3. Plot showing
are related
at aatnumber
of locations
sampling sampli
Plot
showing
how how
δD δD
andandδ δ18OOare
related
a number
of locations
locations in the trans-Himalayan (Kashmir) region (Fig. 2, Kanchan Pande et. al 2001). Note
that the local meteoric water line is steeper than the world meteoric water line.
Figure 4. Visualization of the seasonal effects of the δ18O for the Himalayan rivers; Ganga:
rhombus; Brahmaputra: square; Indus: triangle (Fig. 4, Lambs et. Al 2005). Marked variability
exists both within and between rivers.
Correlation
conductivity
oxygen 18
foroxygen
the surface
in a high
Indian
Figure 5.between
Correlation
between and
conductivity
and
18 forwater
the surface
water
in aHimalayas
high
Indian Himalayas valley (Fig. 7, Lambs 1999). Higher conductivities are correlated with
lighter isotopic signatures, indicating the presence of more glacial meltwater.