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Water in the Mogao Grottoes, China: where it comes from and how it is driven

Authors:
  • 敦煌研究院

Abstract

The Dunhuang Mogao Grottoes in China was designated as a world heritage site by UNESCO in 1987 and is famous for its cultural relics. Water is the most active factor that harms the relics in the caves as it damages the grotto murals and painted sculptures. Thus, determining the water sources and driving forces of water movement is a key issue for protecting these cultural relics. These issues have troubled relics protectors for a long time. In this study, the authors chose a representative cave in the Mogao Grottoes and, by completely sealing the cave to make a closed system, measured the water vapor from the surrounding rock. This was accomplished by installing a condensation-dehumidification temperature-humidity control system for the collection of water vapor. The results show that there is continuous evaporation from the deep surrounding rock into the cave. The daily evaporation capacity is determined to be 1.02 g/(d·m2). The water sources and driving forces of water movement were further analyzed according to the character of the water evaporation and by monitoring the temperature and humidity of the surrounding rock. It was found that the water vapor in the cave derives from phreatic water. Moreover, the yearly fluctuation of temperature in the surrounding rock and geothermal forces are the basic powers responsible for driving phreatic evaporation. Under the action of the yearly temperature fluctuations, decomposition and combination of bound water acts as a “pump” that drives phreatic water migration and evaporation. When the temperature rises, bound water decomposes and evaporates; and when it falls, the rock absorbs moisture. This causes the phreatic water to move from deep regions to shallow ones. Determining the source and dynamic foundation of the water provides a firm scientific basis for protecting the valuable cultural relics in the caves.
J Arid Land (2015) 7(1): 37–45
doi: 10.1007/s40333-014-0072-y
jal.xjegi.com; www.springer.com/40333
Water in the Mogao Grottoes, China: where it comes
from and how it is driven
HongShou LI1,2, WanFu WANG1,2*, HongTao ZHAN1, Fei QIU1, QingLin GUO1,2, GuoBin ZHANG1,2
1 The Conservation Institute of Dunhuang Academy, State Administration for Cultural Heritage, Dunhuang 736200, China;
2 Key Scientific Research Base of Conservation for Ancient Murals, Dunhuang Academy, State Administration for Cultural
Heritage, Dunhuang 736200, China
Abstract: The Dunhuang Mogao Grottoes in China was designated as a world heritage site by UNESCO in 1987
and is famous for its cultural relics. Water is the most active factor that harms the relics in the caves as it damages
the grotto murals and painted sculptures. Thus, determining the water sources and driving forces of water move-
ment is a key issue for protecting these cultural relics. These issues have troubled relics protectors for a long time.
In this study, the authors chose a representative cave in the Mogao Grottoes and, by completely sealing the cave to
make a closed system, measured the water vapor from the surrounding rock. This was accomplished by installing a
condensation–dehumidification temperature–humidity control system for the collection of water vapor. The results
show that there is continuous evaporation from the deep surrounding rock into the cave. The daily evaporation
capacity is determined to be 1.02 g/(dm2). The water sources and driving forces of water movement were further
analyzed according to the character of the water evaporation and by monitoring the temperature and humidity of the
surrounding rock. It was found that the water vapor in the cave derives from phreatic water. Moreover, the yearly
fluctuation of temperature in the surrounding rock and geothermal forces are the basic powers responsible for
driving phreatic evaporation. Under the action of the yearly temperature fluctuations, decomposition and combina-
tion of bound water acts as a “pump” that drives phreatic water migration and evaporation. When the temperature
rises, bound water decomposes and evaporates; and when it falls, the rock absorbs moisture. This causes the
phreatic water to move from deep regions to shallow ones. Determining the source and dynamic foundation of the
water provides a firm scientific basis for protecting the valuable cultural relics in the caves.
Keywords: the Silk Road; cultural heritage; surrounding rock; bound and phreatic water; thermodynamics
Citation: HongShou LI, WanFu WANG, HongTao ZHAN, Fei QIU, QingLin GUO, GuoBin ZHANG. 2015. Water in the Mogao Grottoes, China:
where it comes from and how it is driven. Journal of Arid Land, 7(1): 37–45. doi: 10.1007/s40333-014-0072-y
With the opening of the Silk Road and development of
cultural communication in ancient times, numerous
temples were excavated in the form of caves along the
Silk Road. These caves are representative examples of
cave-style Buddhist temples. Today, these ancient
caves remain well preserved and have become an im-
portant part of Buddhist heritage. Their good pre-
servation is mainly thanks to the inland arid climate
conditions in their location. However, under global
climate warming and intensified human interference,
sites of cultural heritage in arid lands, including the
Mogao Grottoes, are suffering unprecedented losses
(Ci, 2011).
Due to its long history, grand scale, rich content
and state of preservation, the Dunhuang Mogao Grot-
toes was listed into the Directory of World Cultural
Heritage by UNESCO in 1987 (Agnew, 2004). How-
ever, a survey found that about 50% of the wall paint-
ings have suffered deteriorating diseases since their
creation ca. 1600 years ago (Wang, 2005). Long-term
Corresponding author: WanFu WANG (E–mail: wwanfu@hotmail.com)
Received 2013-11-01; revised 2014-04-20; accepted 2014-08-10
© Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Science Press and Springer-Verlag Berlin Heidelberg 2015
38 JOURNAL OF ARID LAND 2015 Vol. 7 No. 1
studies on the mechanisms of the diseases have shown
that water is the most active factor in the deterioration
of the murals and that most diseases involve the par-
ticipation of water (Zhang et al., 1995; Guo, 2009).
Due to the action of water, salts are activated in
the rock surrounding the caves and the plaster layers
of the murals. This leads to the formation of various
sorts of disorder and disease in the murals. For in-
stance, due to the impact of humidity fluctuations,
salts repeatedly recrystallize. This changes the struc-
ture of the plaster layer, causing particles to become
loose and fall off and resulting in delamination
(Zhang et al., 1995). Mural salt activity can also di-
rectly cause alkali efflorescence, which is also
known as “mural cancer”. Once this occurs, it is very
hard to control (Cheng et al., 2005). Meanwhile, hu-
midity and air temperature changes can cause the
cave murals to chap and flake. In addition, the high
humidity in the caves may cause the multiplication of
a large number of bacteria, fungi, and other microbes
(Yardım and Tunçoku, 2008; Sterflinger, 2010). This
results in the structure deterioration of the plaster
layers and painted sculptures (Herrera et al., 2008).
Some microbes can even take organic pigments di-
rectly for nutrition and this leads to their mass prop-
agation on the mural surface, forming serious mildew
pollution (Wang et al., 2010).
Moisture problems have been the focus of efforts
for the protection of cultural relics in caves (Liu et al.,
2011). Wang et al. (2000) analyzed the geological
environment of the Mogao Grottoes. Zheng et al.
(2001) subsequently investigated the infiltration of
irrigation water in the caves and measured transp-
iration from the tree-belt in front of the caves (Qin et
al., 2002). In 2005, the current authors studied the
water consumption and evapotranspiration mechanism
in the tree-belt, and analyzed the influence of the wa-
ter system on the microclimate in the Mogao Grottoes
(Li, 2005, 2006; Li et al., 2009a, b). Guo et al. (2009)
applied a high-density resistivity technique to detect
the moisture status of the rock surrounding the Mogao
Grottoes. Yang et al. (2009) also measured the wa-
ter-salt spatial distribution in the rock. They conclud-
ed that some of the moisture came from the surround-
ing rock in the caves. Zhang et al. (2005) found that
the breath of visitors also has a certain influence on
the humidity in the caves. Zhang et al. (2006) investi-
gated the environmental characteristics of the Daquan
River which passes in front of the Mogao Grottoes.
Yan et al. (2008) analyzed the moisture absorp-
tion–desorption pattern in the caves. More recently,
Wang et al. (2010) studied the permeability of the
rock surrounding the caves.
Although the water in the caves has been extens-
ively studied by experts, there is confusion over the
source of the moisture and the active mechanisms re-
sponsible for its presence. At present, a consistent
conclusion has not been reached. To briefly sum up,
there are four possible sources: (1) rainfall infiltration,
(2) lateral seepage of irrigation water from the Daquan
River, (3) atmospheric moisture (Guo, 2009), and
(4) phreatic water (Li et al., 2011). However, each of
these inferred sources is not supported by systematic
and concrete evidence. Therefore, it is necessary to
obtain a scientific understanding of the activity of wa-
ter in the whole of the surrounding rock. Further re-
search is clearly required to fundamentally determine
the source of the water and the associated driving
forces for water movement.
For the above objective, we conducted a study on
the peripheral environment of the Mogao Grottoes to
investigate moisture problems in 2005. At the top of
the Mogao Grottoes, repeated experiments involving a
greenhouse condensation method to simulate precipit-
ation proved that precipitation could evaporate com-
pletely (Li et al., 2009c, 2010a, b, 2013). Also, it was
found that there was a stable amount of water evapo-
ration except for precipitation sources. Thus, it can be
concluded that phreatic water is one of the important
sources of moisture in the Gobi soil (Li et al., 2010a,
2011). In 2009, a greenhouse/air-conditioning method
was used to measure the phreatic evaporation capacity
of the Gobi, producing a result of 0.0219 mm/d (Li et
al., 2010c). This new discovery on phreatic evapora-
tion provides an important reference for the water
source in the caves of the Mogao Grottoes.
Studies on phreatic evaporation and its related
factors show that the environment is dynamic and
depends on factors such as the intensity of the solar
radiation, the temperature, air humidity gradient and
geothermal activity. Of these, temperature change
(which is affected by solar radiation) has the greatest
HongShou LI et al.: Water in the Mogao Grottoes, China: where it comes from and how it is driven 39
influence on phreatic evaporation (Li et al., 2011). In
relation to the caves, which largely avoid the effects
of solar radiation, evaporation should be affected
only by the air temperature and geothermal activity.
However, conclusive evidence of whether there is
phreatic evaporation has not yet been obtained. In this
paper, with reference to the previously employed
greenhouse experimental method, a closed system and
artificial condensation method were used to investi-
gate a chosen representative cave in the Mogao Grot-
toes. The research aims at exploring the source of wa-
ter and the driving force of water movement, and
providing scientific support for the protection of the
precious cultural relics in the Mogao Grottoes. The
work may also provide a reference for the protection
of cultural relics in other grottoes along the Silk Road.
1 Study area and cave
The Mogao Grottoes (40°02'13"N, 94°47'38"E) is loc-
ated in the southern margin of the Dunhuang basin,
between the Sanwei and Mingsha mountains, and in
the valley of the Daquan River. The caves, excavated
30–40 m high in the conglomerate cliff on the west
bank of Daquan River, can be roughly divided into
three layers. The conglomerate belongs to the Jiuquan
group, Pleistocene series (Q3). The surrounding rock
is dry and the water content is 0.5%–1.5% (Guo,
2009). The depth of phreatic water is over 200 m. The
climate is very dry with an annual precipitation of
42.2 mm; the potential evaporation is 4,347.9 mm.
The solar radiation intensity can reach 1.1 kW/m2 and
the sunshine rate is 71%. The annual average temper-
ature is 11.3ºC. Also, the relative humidity is 38% and
the annual average wind speed is 4.1 m/s (Li et al.,
2009b).
The chosen experimental cave is Cave 72 (Fig. 1).
It is located in the middle of the southern district and
situated in the ground layer of the Mogao Grottoes.
The cave is 9.6 m in depth. The cave type is arch
crown and it was excavated during the Five Dynasties
Period (AD 907–960). The length and width of the
main room (Fig. 2) are both 6.6 m. The height of the
cave is 6.0 m, the indoor surface area is 218 m2, and
the volume is 233 m3. It is a representative of medi-
um-sized caves in the Mogao Grottoes.
Fig. 1 A sketch highlighting the locations of Cave 72 and Cave 98 in the Mogao Grottoes
2 Methods
We applied a closed system and artificial conde-
nsation method, and collected a stable amount of eva-
poration coming from the surrounding rock in a closed
cave. Then, according to the evaporation charac-
teristics, temperature and humidity, we further deter-
mined the final source and dynamic foundation of the
water in the cave.
Cave 72, as a representative cave of the Mogao
Grottoes, was sealed on both sides of its aisle, which
is a channel to the main room, by using plastic film
(Fig. 2). Before the cave was closed, a specially de-
signed condensation–dehumidification temperature–
humidity control (CDTHC) system was installed for
this experiment. This system consists of an
air-conditioner (Gree KFR–120LW 12568L AL–HN5)
and a dehumidifier (DH–890C), as shown in Fig. 2.
40 JOURNAL OF ARID LAND 2015 Vol. 7 No. 1
Fig. 2 A stylistic sketch map of Cave 72 and the specially de-
signed condensation–dehumidification temperature–humidity
control (CDTHC) system. 1, air conditioner; 2, dehumidifier; 3, air
conditioning compressor; 4, balance; 5, condensate drain pipe; 6,
cooling circulation tube; 7, sealing film.
The system was used to control the upper limits of
temperature (16 ºC–18ºC) and humidity (35%). It also
condensed and collected water vapor every day (at 8:
30 am) over a continuous period of 2 years. The re-
sults can be used to describe the basic features of the
evaporation in the cave and help to analyze the main
source of water.
The dynamic mechanism, which is responsible for
moisture activity, has been the research focus of the
water in the caves. For aiding the analysis, it is neces-
sary to monitor temperature, relative humidity and
absolute humidity in the surrounding rock. As there
are precious murals on the walls of Cave 72, steps
were taken to reduce the experimental effects on the
ancient cultural relics as far as possible. To reduce
unnecessary repetition, the authors employed micro-
environmental monitoring results obtained in 2006
from the rock surrounding Cave 98 (Guo et al., 2009).
These were combined with the condensed water data
collected in Cave 72 to study the moisture activity in
the surrounding rock and act as the basis of an ana-
lysis of the environment that causes the evaporation.
Cave 98 is located in the ground layer in the middle
of the Mogao Grottoes (Fig. 1). It was excavated dur-
ing the period of AD 914–935. The cave is 40 m from
the top of the cliff surface and about 130 m from Cave
72. It has the same style but is bigger than Cave 72,
with a depth of 21.6 m. The large mural on the west-
ern wall has fallen off and been lost. At this location,
the temperature was more stable and the range of an-
nual temperature changes is lower than that in Cave
72. On the south end of the western wall, a series of
holes were drilled using a drilling rig (TYQEJ100D).
The holes were 90 cm high from the ground and lo-
cated 138 cm away from the south wall at 0, 10, 30,
62, 95 and 125 cm, respectively. In these holes we put
temperature and humidity detection probes (produced
by Vaisala, Finland), which were used to regularly
determine the temperature and humidity on a weekly
timescale. The temperature and humidity of the at-
mosphere outside the cave were recorded by weather
stations in the Mogao Grottoes.
3 Results
3.1 Evaporation characteristics
After a continual collection of 2 years (2010–2012),
140.748 kg of condensed water vapor was taken in
total. The quantity of condensate and the trend in an-
nual change is shown in Fig. 3. The result reveals wa-
ter evaporation from the surrounding rock into the
caves. The average daily evaporation over a year is
222 g/d, which, in per unit area, equates to 1.02 g/(dm2).
Fig. 3 The quantity of condensate collected on an annual basis (2010–2012)
HongShou LI et al.: Water in the Mogao Grottoes, China: where it comes from and how it is driven 41
As it is affected by climate processes, evaporation
is subject to a certain fluctuation. Therefore, evapor-
ation along with the changes in temperature of the
surrounding rock fluctuated on a yearly timescale.
Evaporation occurred from April to November. Dur-
ing the evaporation period, there was stable contin-
uous evaporation in the cave, which was uninterrupted
at night. This was in contrast to the Gobi surface,
where, on a daily timescale, the evaporation stopped
in the night. Water evaporation in the cave can be
characterized using functions such as the polynomials.
A sinusoidal variation consistent with the phreatic
evaporation characteristics found in the Gobi primari-
ly suggests that the cave water comes from phreatic
water (Li and Wang, 2014), which, of course, needs to
be determined by analysis of the mechanisms respon-
sible for the water dynamics of the surrounding rock.
3.2 The diving force and activity mechanism
The annual changes in the temperature, absolute hum-
idity and relative humidity of the rock surrounding
Cave 98 are shown in Fig. 4.
When the temperature is stable, the water vapor
moves from areas of high humidity to areas of lower
humidity (Li et al., 2011). The distribution of the ab-
solute and relative humidity in the rock surrounding
the cave decreases progressively from deep to shallow
depths (Fig. 4). Therefore, if only humidity is consid-
ered, water vapor should move outwards and evapora-
tion should exist throughout the year. However, ac-
cording to the monitoring results, there is evaporation
only from April to December. This is mainly a result
of the thermal activity of the water in the surrounding
rock, which is dominated by temperature. The behav-
ior is closely related to the temperature of the sur-
rounding rock in the different strata and its changes in
an orderly manner.
It is well known that on a microscopic scale, when
the temperature is stable, decomposition of the bound
water in the surrounding rock (i.e. hygroscopic water,
film water and water of crystallization) is dynamically
balanced by recombination due to adsorption of the
surrounding air moisture. In the surrounding rock,
when the temperature from the outside gradually rises,
the bound water decomposes in progressively greater
quantities, which breaks the original dynamic equilib-
rium of the moisture. This implies that decomposition is
greater than combination, which results in vapor for-
mation. Conversely, when the temperature falls, moisture
adsorption is greater and bound water is formed.
At the end of January, the deep surrounding rock
has a relatively high temperature and humidity comp-
ared to the shallow layer (Fig. 4), that is, the conditi-
ons are sufficient for water to move outwards. How-
ever, the water vapor that moves outwards contin-
uously replaces the water that was lost last year by
evaporation from the shallow surrounding rock. As
the surface temperature of the surrounding rock begins
to rise, decomposition of the bound water starts.
However, the water vapor produced will not enter into
the surrounding rock but instead directly evaporate
into the atmosphere. Since the temperature rise at this
time is small, the amount of water evaporated is very
limited. Thus, even the CDTHC system could not de-
tect the amount involved. However, according to the
continuous monitoring of the absolute humidity in the
sealed-up cave, the vapor could be observed to be
slowly increasing.
In April, the pattern of the temperature changes in
the surrounding rock at 0–125 cm appears to begin to
reverse. That is, the temperatures of the shallower
layers become higher than those of the deeper layers.
In each layer, evaporation increases along with the
continuous rise in temperature. So, more water vapor
is produced through decomposition by heating in the
shallow layer, and it will attempt to enter the deeper
layers where the temperature is lower. However, as
the water absorption capacity and water transportation
in the soil are limited, a small amount of water vapor
will be emitted outwards, forming vapor. Therefore,
evaporation in the cave rises along with the gradually
increasing temperature from April to August.
In the surrounding rock, the humidity of the deep
layers is always larger than that of the shallower lay-
ers (Figs. 4a and b). This inhibits the migration of
moisture into the rock to a certain extent, which is
beneficial to the outward evaporation of water. After
all, higher temperature and humidity are both suffi-
cient conditions for water vapor to move to those are-
as which are cooler and less humid, but these are not
necessary conditions (Li et al., 2011). The actual di-
rection in which the water moves depends on the rela-
tive differences in temperature gradient compared to
42 JOURNAL OF ARID LAND 2015 Vol. 7 No. 1
Fig. 4 Annual changes in the temperature, absolute humidity and relative humidity at different depths of the rock surrounding Cave 98 in
2006
humidity gradient. Higher temperature and slightly
lower humidity, or higher humidity and slightly lower
temperature, could make the moisture move to the
outside. During April–August, the temperature and
humidity conditions follow the former pattern and
there should be much water vapor moving to the deep
layers.
From August to December, the temperature gradu-
ally decreases and a positive gradient is set up from
the shallow surrounding rock to the deep. According
to the rock–soil properties, once the temperature drops,
the decomposing of bound water in the soil is no
longer faster than the binding of vapor on soil at the
microscopic level, preventing the formation of vapor.
HongShou LI et al.: Water in the Mogao Grottoes, China: where it comes from and how it is driven 43
Instead, moisture adsorption will immediately begin,
which is the reverse of the process of water decompo-
sition (Li et al., 2011). However, at that point, due to
the delay in the heat (temperature) conduction to the
deep surrounding rock (which is still in the increasing
temperature phase), the “trapped” heat releases water
vapor to supply the shallow layer.
The shallow rock layer is controlled by the falling
temperature range. The quantity of rock that absorbs
moisture is small and evaporation from the deep layers
of the surrounding rock is relatively abundant. This
can not only meet the needs of shallow absorption, but
also form evaporation through the shallow surrou-
nding rock.
From November to December, transformations in
the temperature field within the layers have finished
(Fig. 4) and the temperature in all layers starts to fall.
The rock layers also start to absorb moisture and there
is no evaporation. However, considering the tempera-
ture delay in the deeper surrounding rock, as indicated
in the former analysis, the temperature is still rising
(the deepest monitoring is too shallow for this process
to be apparently detected in Fig. 4). Thus, the water
vapor from decomposition is relatively abundant and
it can still pass through the layer whose temperature is
falling to form evaporation.
Generally, the depth of the yearly heterotherm-
ozone here is 20–30 m (Li et al., 2010c). The vertical
position of the cave is beyond this depth but consi-
dering the lateral influence of temperature and solar
radiation acting on the cliff surface, the cliff body
20–30 m inwards should still be in the heterother-
mozone, as suggested by Fig. 4c. In any depth of the
heterothermozone there are two transformations in a
year, in which the temperature of the upper layer is
higher than that of the lower layers, and then this al-
ternates, so that the lower layer has higher temperature
than the upper one. According to the rock surrounding
the cave, the relationship between the shallow and
deep rock is clear. Near the cliff surface, temperature
changes present greater amplitudes. Even if the com-
munication between the heat of the cave air and the
outside atmosphere is not considered, the temperature
of the surrounding rock also changes alternately.
However, the opening of the cave and air communica-
tion will inevitably increase the change in temperature
and therefore increase evaporation.
The rising and falling of the temperature directly
relates to water decomposition (evaporation) and
moisture absorption in the surrounding rock. These
two properties relate to two completely different states.
Water evaporation and absorption in the layers change
in order with the temperature variation from the out-
side to the inside in an annual cycle. From April to
December, there is a larger amount of evaporation on
the whole. From January to March, although the
moisture in the deep surrounding rock is constantly
migrating outwards, all it does is supplementing the
water lost by evaporation from the shallow surrou-
nding rock. This absorbed water then forms vapor
when it warms up the next year. Year after year, this
cycle repeats itself. Under the alternating action of
temperature, water decomposition and recombination
in the surrounding rock acts as a “pump”. It transships
water from the deep surrounding rock to outer parts,
and this deep moisture originates from phreatic water,
just as in the Gobi soil (Li et al., 2009c). Therefore,
the cave water derives from phreatic water. As there is
an evaporation mechanism, over a long period of time
evaporation of the phreatic water (of which a certain
part is in the form of film water that can dissolve salt)
leads to a higher salt content in the shallow surrou-
nding rock (Gou et al., 2009).
On a macroscopic level, geothermal effects cannot
be ignored. Below the heterothermozone, where there
is high temperature and humidity, there are sufficient
conditions for phreatic water vapor to flow upwards
under the action of the geothermal temperature field.
A strong thermal effect can produce a saturated state
2.50 m under the Gobi surface throughout the year (Li
et al., 2011). Such a state is also apparent 1.25 m in-
side the rock surrounding the cave (Fig. 4). Therefore,
a general upward movement of vapor exists and this
water vapor converts to film water. However, tests on
the deep surrounding rock show that the moisture
content is no more than 1.5%, a quantity that is not
enough to form free water (Guo, 2009). The annual
temperature variation in the heterothermozone affects
the speed of moisture movement and the amount of
evaporation, but the deep geothermal forces provide
the most basic guarantee that water will migrate up-
wards. Therefore, the cave, its surrounding rock (het-
44 JOURNAL OF ARID LAND 2015 Vol. 7 No. 1
erothermozone) and the deep underground layers form
an “organic continuum”, which forms the moisture
continuously through its operation and evaporation.
4 Discussion
4.1 Phreatic water continuum
Many experiments show that in the Gobi soil which
connects with the surrounding rock of caves, the main
moisture comes from phreatic water (Li et al., 2009c,
2010a, b, 2013), but the buried depth of phreatic water
is over 200 m, and so the phreatic water cannot di-
rectly move to the soil surface as evaporation. Under
the heterothermozone, the temperature and humidity
gradients meet the conditions required for the applica-
tion of Fick’s Law (Fayer, 2000), and so moisture un-
dergoes upward diffusive migration. The situation is
relatively simple in this area. In the heterothermozone,
the Fick formula is still suitable on a small scale.
However, on a larger scale, the amount of moisture
migration changes with temperature as a result of the
yearly heat (temperature) conduction and migration in
the heterothermozone. The soil itself is both a
“source” and also a “sink”. In related studies on un-
saturated soil with generally and relatively higher wa-
ter contents (i.e. liquid water is present), it is assumed
that the amount of water studied does not change. In
the heterothermozone, the temperature field changes
with the seasons. This leads to the change in the direc-
tion of water migration and the presence of a small
amount of measured evaporation moisture, which
overflows on the land surface. This is contrary to the
major direction of water migration. Due to the annual
variation of the temperature field, suction in the sur-
rounding rock also inevitably changes. At the same
time, this causes vapor pressure and potential hydrau-
lic fluctuations. The situation is thus more complicat-
ed in this area. In the shallow 125 cm of the sur-
rounding rock or soil depths of less than 250 cm, the
relative humidity is 100%. Therefore, soil water is a
direct source of vapor diffusion here.
4.2 Atmospheric moisture
On 25 August 2006, there was a precipitation of 1.52 mm
as recorded by weather stations in the Mogao Grottoes
(Fig. 4). The humidity in the cave increased as a result
of the precipitation. Actually, there are more humid
weather processes which do not appear in the form of
precipitation in the Dunhuang region. The outside
humidity fluctuations also cause humidity changes
inside the cave (Fig. 4). According to incomplete sta-
tistics, there is a humid weather process every 15 days
or so in Dunhuang. However, precipitation and humid
weather processes influence the humidity inside the
surrounding rock only very slightly—they do not
cause the inside humidity to rise significantly (Fig. 4).
The outside humidity is lower than that inside or on
the surface (0 cm) on the whole (see the polynomials
in Fig. 4a). Therefore, no atmospheric water enters the
surrounding rock as a result of the normal climate. As
for the closed Cave 72, in which the effects of the at-
mosphere were eliminated, there still existed a stable
amount of evaporation. Hence, the measured water
evaporation came from phreatic water rather than at-
mospheric water or precipitation (Li et al., 2014).
5 Conclusions
By sealing up Cave 72 of the Mogao Grottoes and
collecting condensation in an artificial method, we
collected 140.748 kg of evaporated water within two
years. The evaporation characteristics, temperature
and humidity in the surrounding rock show that water
vapor in the cave derives from phreatic water. Fur-
thermore, the amount of phreatic evaporation is 1.02
g/dm2 and there must be a mechanism for its move-
ment. The research on the water source and the forces
driving water movement provided a scientific refer-
ence for the protection of the Mogao Grottoes and
other cultural relics in the caves along the Silk Road.
Acknowledgments
We gratefully acknowledge funding for this work from the
National Natural Science Foundation of China (41363009), the
Gansu Province Science and Technology Plan (1308RJZF290)
and a project of the Dunhuang Academy (201306).
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In the Mogao Grottoes in Dunhuang, China, temperature and relative humidity (RH) fluctuations are the main causes of the observed deterioration. In this paper, we experimentally and numerically analyze hygrothermal transfer in the cave wall of the Mogao Grottoes. A coupled heat and moisture transfer 1D model is developed to simulate the hygrothermal behavior of the cave wall of the Mogao Grottoes, which adopts the temperature and water vapor pressure as driving potentials. The model is implemented in a programming. By constructing the numerical model of the wall under boundary conditions and comparing the measured data and simulation results, we validate the numerical model and analyze the hygrothermal transfer characteristics of the wall. The temperature and moisture profiles of the cave wall are experimentally measured and numerically determined. The simulation results are compared to experimental temperature and RH values, with good agreement despite the simplifying hypotheses adopted during modeling. By analyzing the heat and moisture distributions, we find that the heat and moisture transfer process in the cave wall changes in space and time. The complicated heat and moisture transfer directions vary cyclically. Additionally, moisture transfer in the cave wall reaches the steady state more rapidly than heat transfer. Hygrothermal cycling plays an essential role in the development of mural deterioration. The developed model provides theoretical support and a scientific method for the conservation of cave sites and a quantitative analysis approach to study mural relics.
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Dunhuang Mogao Grottoes, the very famous world culture heritage, was chiseled in the cliff face of alluvial grit etching by the Daquan River in the Quaternary Period. Undergone natural and human factors, caves of Mogao Grottoes have many diseases and deteriorations including flaking, detachment and disruption; especially the caves of low-layer are most serious. Cave 98, one of the most valuable and representative diseases of low-layer caves in the Five Dynasties(AD914-AD935), is selected as an example. Like all other caves in Mogao Grottoes, it has been affected by the natural environment. From the hole drilled at the lower portion of the western wall without painting, samples obtained for salinity analysis. The temperature and humidity are measured inside the hole. The analysis of electrical resistivity data from south to north of the lower portion of the western wall indicates that the soluble salt contents of the rock mass are mainly vitriolic and chloride, mainly concentrated between the surface and the depth of 35 cm. Water condensation occurs in the rock mass at depth of 125 cm. Inside the rock mass, there is close correlation between relative humidity and salinity. When moisture content rises, salinity drops. Due to the high humidity and abundant supply of salt, the salinity of the western wall, which is backed by the main cliff structure, is higher than the other walls, and it is more sensitive to changes in the environment. These achievements have provided a scientific basis for conservation wall paintings.
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Influenced by the material and natural environment, the wall paintings of Dunhuang Mogao grottoes have many diseases: flaking, disruption, detachment and loss; and many researches have proved that these diseases are caused by moving of salts; however the moisture is the main factor for the activation and moving of the salt. So, research on rock permeability of the Mogao Grottoes can explain regulations of water migrations and help us to learn the mechanism of wall painting deterioration. This article adopted the test of designated water level in situ to get the permeation coefficient of different groups of surrounding rocks, the test of high-intensively resistivity permeation monitoring in situ experiments; X-Ray CT account the void ratio to study the permeability of the surrounding rocks. We know that different rock strata of Mogao Grottoes have different permeabilities; even the rocks from the same stratum have very different permeability coefficients. Some stratum with good permeability makes it possible that the water permeates inside the cave through the rock layers. Due to the particularity of culture relics and some other reasons, high-density electric resistivity method and X-Ray CT scanning are efficient ways to learn the permeability of surrounding rocks under the situation that we can't measure it directly.