This study quantified changes in evaporation, evapotranspiration, latent heat flux, and surface temperature in the Central Asian region around the Aral Sea from pre-1950 to 1983-2002. Increased irrigation for agriculture over this period yielded regional cooling of -0.6°C due to higher evapotranspiration from irrigated land. However, the shrinking of the Aral Sea due to diversion of over 80% of its inflow for irrigation resulted in decreased evaporation and warming of 0.5°C. The study found the regional temperature increase over this period was the net result of offsetting cooling and warming effects of irrigation and Aral Sea shrinkage, respectively.

1. Inland hydro‐climatic interaction: Effects of human water use
on regional climate
Georgia Destouni,1
Shilpa M. Asokan,1
and Jerker Jarsjö1
Received 30 May 2010; revised 29 July 2010; accepted 9 August 2010; published 22 September 2010.
[1] This study has quantified the regional evaporation and
evapotranspiration changes, and the associated latent heat
flux and surface temperature changes in the Central Asian
region of the Aral Sea drainage basin and the Aral Sea itself
from the pre‐1950 period of the 20th century to 1983–2002.
The human water use for irrigation yielded an average
regional cooling effect of −0.6 °C due to increased evapo-
transpiration and latent heat flux from the irrigated land. The
runoff water diverted for irrigation was more than 80% of the
pre‐1950 runoff into the terminal Aral Sea, and was largely
lost from the regional water system by the evapotranspiration
increase. The Aral Sea shrank due to this water loss, resulting
in decreased evaporation and latent heat flux from the pre‐
1950 Aral Sea area extent, with an average regional warming
effect of 0.5 °C. In general, the endorheic (land‐internal)
runoff and relative consumptive use of irrigation water from
that runoff determine the relative inland water area shrinkage,
its warming effect, and to what extent the warming counter-
acts the cooling effect of irrigation. Citation: Destouni, G.,
S. M. Asokan, and J. Jarsjö (2010), Inland hydro‐climatic interac-
tion: Effects of human water use on regional climate, Geophys.
Res. Lett., 37, L18402, doi:10.1029/2010GL044153.
1. Introduction
[2] Land use alteration of soil moisture by irrigation can
considerably affect regional climate [Boucher et al., 2004;
Bonfils and Lobell, 2007; Kueppers et al., 2007; Lobell et al.,
2009]. The geographic variability of this effect has been
suggested to depend on two main factors that vary between
regions [Lobell et al., 2009]: 1) the extent of irrigated land
area; and 2) the linkage between the climate regime and the
land surface processes [e.g., Koster et al., 2004]. Both of
these factors also involve to some degree the regional water
cycling and its use and alteration by humans.
[3] Regarding factor 1, Lobell et al. [2009] found a lack
of agreement between climate simulation results and tem-
perature observations in the Aral Sea region in Central Asia.
Climate simulations that neglected irrigation yielded a tem-
perature bias that shifted to an opposite bias in climate
simulations that accounted for irrigation. Lobell et al. [2009]
suggested that the resulting bias in simulations that did
account for irrigation might depend on inaccuracy with
regard to how much water was actually used for irrigation.
This explanation indicates a possible direct effect of human
water use on the regional climate, in addition to the effects
of changes in global climate and regional land‐use.
[4] Regarding factor 2, Schär et al. [2004] and Seneviratne
et al. [2006] identified an important soil moisture‐
precipitation feedback to regional climate as responsible for
significant correlation between temperature and precipita-
tion anomalies, even in regions where low‐resolution global
climate modeling would indicate weak soil moisture‐
atmosphere coupling [Koster et al., 2004]. However, soil
moisture does not only change as a feedback to climate
change, but is also directly altered by human land and water
use. In irrigated land, soil moisture depends directly on the
applied amount of irrigation water. Dams and engineered
water diversions also change soil moisture conditions by
turning former land areas into surface water areas and vice
versa. Also with regard to factor 2, human water use change
may thus have a direct effect on regional climate change.
[5] The Aral Sea region in Central Asia is a prime example
of major land and water use changes, in combination with
climatic changes during the 20th century. The former have
led to increased evapotranspiration due to irrigation in the
Aral Sea drainage basin (ASDB) [Shibuo et al., 2007], and to
dramatic shrinkage of the Aral Sea itself with effects on the
hydrology and local climate at and around it [Small et al.,
2001a, 2001b; Jarsjö and Destouni, 2004; Shibuo et al.,
2006; Alekseeva et al., 2009]. Previous studies have investi-
gated these changes separately in different sub‐areas of the
ASDB and the Aral Sea. However, the combined effects of all
hydro‐climatic interactions and changes over the whole
coupled regional system of the ASDB and the Aral Sea has
yet to be quantified. In this study, we investigate the evapo-
ration and evapotranspiration changes, and the associated
latent heat flux and surface temperature changes in this
coupled system from the pre‐1950 period of the 20th century
to 1983–2002. These results are then used for quantifying
and distinguishing the role of human water use for the
observed regional hydro‐climatic changes.
2. Materials and Methods
[6] Figure 1a shows the location and boundaries of the
whole ASDB and Aral Sea system, which extends over a total
area of 1 888 810 km2
. Figure 1b illustrates the shrinkage of
the Aral Sea area and corresponding increase of the ASDB
area within this regional system, along with the average
Aral Sea area and ASDB area in the pre‐1950 period, before
the Aral Sea shrinking started, and in the 1983–2002 period,
after the Aral Sea had undergone much of its still ongoing
shrinkage. Shibuo et al. [2007] investigated the distribution
of hydrological fluxes within the ASDB and their changes
between these two periods. We have chosen the same two
1
Department of Physical Geography and Quaternary Geology,
Bert Bolin Centre for Climate Research, Stockholm University,
Stockholm, Sweden.
Copyright 2010 by the American Geophysical Union.
0094‐8276/10/2010GL044153
GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L18402, doi:10.1029/2010GL044153, 2010
L18402 1 of 6

2. periods for the present quantification of changes in latent heat
flux and surface temperature over the whole regional system
of the ASDB and the Aral Sea.
[7] Shibuo et al. [2007] developed their spatially distrib-
uted hydrological model based on observed temperature (T)
and precipitation (P) data within the ASDB [see Shibuo et al.,
2007, Figure 2], as reported in the CRU TS 2.1 database by
Mitchell and Jones [2005] for the pre‐1950 period and the
1983–2002 period. Furthermore, their hydrological modeling
accounted for the distribution of irrigated area within the
ASDB and engineered water diversions from the rivers for
irrigation [see Shibuo et al., 2007, Figure 1 and Table 1]. In
the present study, we used the spatially distributed results of
Shibuo et al. [2007, Figure 3 and Table 1] for evapotranspi-
ration (ET), river runoff (R), and their changes from the pre‐
1950 period to 1983–2002, by averaging the results from two
different ET process representations considered in their
hydrological modeling. This study further extends the pre-
vious analysis by quantifying also the changes in evaporation
(E) from the Aral Sea itself, and the shift from E to ET in the
Figure 1. (a) The location and boundaries of the Aral Sea Drainage Basin (ASDB, with water divide shown by the red line).
The shrinking Aral Sea (AS) is shown at the North‐Western part of the ASDB (green‐blue fields show the AS extent in 2005;
surrounding white‐grey fields indicate the earlier, pre‐shrinkage AS extent). (b) The AS and ASDB area development shown
along with the average AS and ASDB areas in the pre‐1950 period and the 1983‐2002 period. The AS area development data
are from the Central Asia Water Information database CAWATERinfo (http://www.cawater‐info.net/aral/index_e.htm), and
the ASDB area development is determined from the drainage basin delineation of Shibuo et al. [2007] in combination with the
dried seabed area, which adds to the ASDB area so that it and the AS together cover a constant total area of 1 888 810 km2
.
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3. dried seabed area (see Text S1, section AI‐1, of the auxiliary
material).1
[8] From the changes in ET within the ASDB area, E in
the Aral Sea area, and E and ET in the dried seabed area,
the corresponding latent heat flux changes (DF; see Text S1,
section AI‐2) were quantified for each sub‐area and nor-
malized by averaging over the same, fixed total regional area.
The normalization made the separate DF results directly
comparable with each other and representative for the entire
regional system of the ASDB and the Aral Sea. For this whole
regional system, the different components of average surface
temperature change (DT; see Text S1, section AI‐3) were
then calculated and associated with the different causes of
latent heat flux change.
[9] For the DT component calculations, the total observed
annual and regional average T change DTa = 1.1°C (from
7.5°C in the pre‐1950 period to 8.6°C in the 1983–2002
period [Mitchell and Jones, 2005; Shibuo et al., 2007]) is
expressed as DTa = DTcl − DTshr + DTirr = 1.1°C, where
DTcl, DTshr and DTirr are the T change components of the
regional manifestation of global climate change, the Aral Sea
shrinkage, and the irrigation in the ASDB, respectively. The
DTirr component must then be zero in the non‐growing
season (DTirr−ngs = 0), when there is no irrigation, and non‐
zero only in the growing season (DTirr−gs≠0) when there is
irrigation. Figure 2 shows that such seasonality is indeed
observed in the regional temperature change, with the
regional average temperature increase of 0.81°C in the
growing season being much smaller than the increase of
1.67°C in the non‐growing season.
[10] Assuming that the seasonality of the continuous long‐
term temperature change components DTcl from the global
climate change and DTshr from the Aral Sea shrinkage is
much smaller than that of the clearly seasonal irrigation
component DTirr implies that the average seasonal T changes
can be expressed as DTgs = DTcl − DTshr + DTirr−gs for
the growing season, and DTngs = DTcl + DTshr for the
non‐growing season. Subtracting the latter from the former
yields DTirr−gs = DTgs−DTngs, which can be estimated
directly from the observed seasonal T changes in Figure 2
as DTirr−gs = (0.81−1.67) °C = −0.86 °C, a result that is
insensitive to the exact season length definition. The annual
average T change component due to irrigation in the ASDB
is then DTirr = −0.86·(8/12) °C = −0.57 °C.
[11] Furthermore, assuming that the latent heat flux
change due to the ASDB irrigation, DFASDB−irr, affects
more or less the same regional air mass as the latent heat
flux change due to the Aral Sea shrinkage, DFAS−shr, the
temperature change component DTshr can be estimated as
DTshr = DFAS−shrDTirr/DFASDB−irr (see Text S1, section AI‐2).
From these estimates and the observed DTa = 1.1°C, the
temperature change component due to the regional manifes-
tation of global climate change can be estimated as DTcl =
DTa − DTirr − DTshr. Note that DTcl then quantifies the total
climate‐driven surface temperature change and not only the
latent heat‐related change contribution, whereas DTirr and
DTshr are entirely due to the latent heat flux changes implied
by the regional irrigation development and associated water
diversions and Aral Sea shrinkage.
3. Results and Discussion
[12] Figure 3 summarizes the changes from the pre‐1950
period to 1983–2002 in the regional annual average water
flux balance P‐(ET+E), runoff R from the ASDB into the Aral
Sea, ET from the original ASDB area, and E from the original
Aral Sea area, all normalized by the fixed total regional area.
Figure 3a shows the changes in absolute terms, and Figure 3b
in relation to the pre‐1950 water fluxes. The pre‐1950 value
of P‐(ET+E) was essentially zero, i.e., the regional incoming
and outgoing water fluxes were balanced. The relative change
in P‐(ET+E) is therefore very large (infinite for entirely bal-
anced pre‐1950 conditions of P‐(ET+E) = 0) and not shown
in Figure 3b.
[13] Figure 3 shows that the regional manifestation of
global climate change had not affected the regional water
balance much by 1983–2002. In absolute terms, the ET
increase due to the irrigation development in the ASDB
was the largest regional water flux change. In relation to the
pre‐1950 conditions, however, the largest relative change
was the decrease of more than 80% of the river runoff R, due
to the engineered water diversions for irrigation.
[14] In total, the pre‐1950 water balance of P‐(ET+E)≈0
had by 1983–2002 shifted to a net water outflow of about
Figure 2. Changes in average seasonal surface temperature in the growing season (March–October, DT = 0.81°C) and the
non‐growing season (November‐February, DT = 1.67°C) from the pre‐1950 to the 1983–2002 period, based on data from the
CRU TS 2.1 database by Mitchell and Jones [2005].
1
Auxiliary materials are available in the HTML. doi:10.1029/
2010GL044153.
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4. 5 mm year−1
. This implies that the regional water system was
in transience, so that the Aral Sea had to continue to shrink
also after 1983–2002 (as it has) in order for this regional water
imbalance to decrease to zero by decreasing E from the
shrinking Aral Sea.
[15] Figure 4 summarizes the changes in total outgoing
water flux due to ET and E from the whole regional system,
and the associated regional latent heat flux changes. The
latent heat flux change includes an increase component of
about 1.6 W m−2
from the irrigation development in the
ASDB, and a decrease component of about −1.3 W m−2
from
the Aral Sea shrinkage associated with the water diversion for
this irrigation. Individually, these changes are greater than the
latent heat flux increase of 0.8 W m−2
from global climate
change. Also their combined net effect, amounting to an
increase of 0.3 W m−2
, is notable relative to the 0.8 W m−2
increase from global climate change.
[16] Separately, the water diversion and irrigation effects
on the regional average surface temperature are also consid-
erable, yielding warming and cooling components of about
0.5°C and −0.6°C, respectively, but their combined net effect
of −0.1°C is small relative to the regional temperature
increase of 1.2°C from global climate change. However,
the relatively small combined net temperature change from
water diversion and irrigation depends on the endorheic
(land‐internal) nature of the ASDB [Vörösmarty et al., 2000],
combined with the relatively large runoff diversion and loss
of water in this basin.
Figure 3. Water flux changes from the pre‐1950 to the 1983–2002 period: (a) in absolute terms, and (b) in relation to the pre‐
1950 water fluxes. Changes are shown for the regional annual average precipitation P minus evapotranspiration ET and evap-
oration E, runoff R from the Aral Sea Drainage Basin (ASDB) into the Aral Sea (AS), ET from the pre‐1950 ASDB area, and E
from the pre‐1950 Aral Sea area, all normalized with the total regional area of both the ASDB and the Aral Sea.
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5. [17] Specifically, the river runoff R from the ASDB
discharges into the terminal Aral Sea, whereas R from an
exorheic (ocean‐contributing) basin discharges into the
ocean. In an exorheic basin, the same absolute R decrease as
in the ASDB would not affect the ocean level much, and
hence the warming effect of surface water area shrinkage
would be negligible. In analogy, for an endorheic basin with
much larger R into its surface waters than in the ASDB, the
same absolute R decrease would imply much smaller relative
shrinkage of surface water area and regional average warming
effect relative to the total pre‐change water and basin area.
[18] In the ASDB, the consumptive human water use for
irrigation and associated decrease in runoff to the Aral Sea
were large relative to pre‐1950 conditions: a whole 80% of
the original R into the Aral Sea was diverted from the rivers
and lost from the regional water system by the increase of ET
due to irrigation. It was this large relative water loss that led to
the dramatic Aral Sea desiccation and associated warming
effect. For the whole regional system, this warming effect
counteracted much of the cooling effect of irrigation, which
can explain why the irrigation‐accounting climate simulation
results by Lobell et al. [2009] exhibited a cooling bias for this
region, i.e., their results did not account for the relatively large
regional warming effect of the Aral Sea shrinkage.
[19] The present study has been based on the relatively
fine‐resolution, spatially distributed hydrological model
results of Shibuo et al. [2007]. We extended these results
based on direct observations of total (DTa) and irrigation‐
driven (DTirr) temperature changes, and straightforward
physical relations between these changes and changes in
average ET, E and latent heat flux over the whole ASDB and
Aral Sea regional system. The remaining temperature change
(DTa−DTirr) was distributed between the global warming
component DTcl and the Aral Sea shrinkage component
DTshr by use of a rough physical assumption regarding
the regional air mass affected by DTshr . The error and
uncertainty associated with this assumption can be resolved
by climate modeling that explicitly accounts for all involved
atmospheric processes. Even with this uncertainty, how-
ever, the present hydrological results provide some important
insights into the influence of human water use on regional
climate.
4. Conclusions
[20] This study has distinguished three main factors that
control the effects of human water use on regional climate
change: a) the water amount used for irrigation, determining
the irrigation cooling effect, which was on average −0.6 °C
in the studied regional system; b) the endorheic or exorheic
nature of the drainage basin, determining if the consumptive
water use for irrigation will primarily affect inland water or
the ocean, which for the endorheic ASDB implied effect
convergence on the terminal Aral Sea; and c) the relative
fraction of consumptive water use from the endorheic runoff,
determining the relative water area shrinkage and its regional
warming effect, which was on average 0.5 °C for the ASDB
and Aral Sea system, where more than 80% of the original R
into the Aral Sea was diverted for irrigation and lost by
increased ET.
[21] For the same irrigation and absolute R decrease as in
the ASDB, the warming effect (b, c) would be negligible in an
exorheic drainage basin with negligible ocean level decrease,
and smaller in an endorheic basin with smaller relative
decrease in R yielding smaller relative surface water area
shrinkage. In such cases, the irrigation cooling effect (a)
would dominate the regional climate effect of human water
use and mask the regional effect of global warming to larger
degree than in the studied Central Asian region.
[22] Acknowledgments. The Swedish Research Council (VR) and the
Swedish International Development Cooperation Agency (SIDA) have pro-
vided financial support for this work. The Bert Bolin Centre for Climate
Research is supported by a Linnaeus grant from VR and The Swedish
Research Council Formas.
Figure 4. Changes in the sum of evapotranspiration ET and evaporation E (D(ET+E)), latent heat flux (DF), and surface
temperature (DT) from the pre‐1950 to the 1983–2002 period for the whole Aral Sea Drainage Basin (ASDB) and Aral
Sea (AS) system.
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