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

February 2015
Journal of Arid Land 7(1)

DOI:10.1007/s40333-014-0072-y

Authors:

Hongshou Li

Wanfu Wang

Fei Qiu

敦煌研究院

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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.

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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/(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 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

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/(d•m2).

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/d•m2 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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Article

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Article

Sep 2009

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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Article

Oct 2010

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.

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Article