Climate change and causes in the Yuanmou dry-hot valley of Yunnan, China

Journal of Arid Environments (2002) 51: 153–162doi:10.1006/jare.2001.0851, available online at http://www.idealibrary.com on

Climate change and causes in the Yuanmou dry-hotvalley of Yunnan, China

Zhang Jianpingw*, Yang Zhongz, Wang Daojiez & Zhang Xinbaoz

wDepartment of Geography, Lanzhou University, Lanzhou, 730000, ChinazInstitute of Mountain Hazards and Environment, Chinese Academy ofSciences & Ministry of Water Conservancy, Chengdu, 610041, China

(Received 25 July 2000, accepted 27 April 2001)

This paper analyses climate change in the Yuanmou dry-hot valley area sincethe 1950s and compares it with that on the Yunnan plateau (Chuxiong City,Wuding County and Dongchuan City). Annual average temperature, thecoldest month average temperature, annual evaporation, annual sunshinehours and annual mean wind speed, all have declined, while the annualrainfall and the average relative humidity have slightly increased. The causesof climate change are chiefly the result of large-scale irrigation in thecultivated land and the restoration of vegetation, and also related to localenvironmental change.

# 2002 Elsevier Science Ltd.

Keywords: Yuanmou dry-hot valley; climate change; irrigation

Introduction

Dry-hot valleys are an outstanding geoecological phenomenon in the HengduanMountains. It is here that climate change has considerable impact on the localagricultural production. The Yuanmou dry-hot valley, situated in the middle and lowreaches of the Longchuan River (a tributary of the Jinsha River), extends between eastlongitude 1011350–1021060 and north latitude 251230–261060. Owing to high altitude,three vertical zones can be identified. Between 900 and 1350 m is the typical dry-hotarea, between 1350 and 1600 m are middle and low mountains, and between 1600and 2283?5 m the high and middle mountains. The dry-hot valley below 1350 m hasan area of 112,600 hm2, making up 55?7% of the total land area. The valley areaenjoys abundant thermal energy and adequate sunshine. Rainfall is little butevaporation is much. The annual average temperature is 21?71C (data of theYuanmou Meteorological Station at 1113 m). The average temperature in the hottestmonth (May) is 27?11C, and the maximum temperature 431C. The averagetemperature of the coldest month (December) is 14?91C and the minimum airtemperature �2?1–0?11C. The accumulated temperature during the period of X101Cis 79961C. Frost-free days are 350–365 and annual sunshine is 2550–2744 h. Theannual rainfall is 623?95 mm, mainly between June and October. Rainfall in the rainyseason accounts for 90% of the total annual. The annual evaporation is 3507?2 mm,

*Correspondence to: Zhang Jianping. E-mail: [email protected]

0140-1963/02/010153 + 10 $35.00/0 # 2002 Elsevier Science Ltd.

154 ZHANG JIANPING ET AL.

which is 5?6 times the annual rainfall. The vegetation in the dry-hot valley areas below1600 m is chiefly composed of shrubs and grass with few trees. Above 1600 m areshrub and grass landscapes, but with patches of montane forest. Thanks to theconstruction of hydrological engineering, the irrigated farmland has increased from1287 hm2 in 1956 to 10,900 hm2 in 1999 in the dry-hot valley, making up 10% of thevalley areas.

This paper deals with climate change in the study region and comparison with thatin neighboring areas (Chuxiong City, Wuding County on the Yunnan Plateau) (seeFig. 1). The causes leading to the climate change have also been discussed.

The trend of climate change in the Yuanmou dry-hot valley since the 1950s

The global average temperature has increased by 0?5–0?71C (Liu, D., 1990; Follandet al., 1990; Xiao Xiang, 2000) in the past 100 years, and the climate has turnedwarmer. Many studies in China show a trend similar to the global trend. ZhangMingqing & Liu Guilian (1999), Ling Chaoning (1998), Liu Jinluan & Li Diqing(1999), Chen Kedong & Jian Jun (2000), Zhao Chunyu et al. (2000), studied,respectively, temperature change in different regions of China. Nanning District of theGuangxi Autonomous Region, Guangdong province, Zedang of Tibet, and Liaoningprovince all show a temperature-increasing trend.

Being affected by the local physical conditions, the climate in the Yuanmou dry-hotvalley has not changed the same way as the global climate. On the contrary, thetemperature in the Yuanmou dry-hot valley has declined since the 1950s. The averageannual temperature from 1956 to 1999 was 21?71C. In the 1950s, 1960s, 1970s,1980s and 1990s, it was, respectively, 22?31C, 22?01C, 27?71C, 21?71C and 21?21C.

Figure 1. Locality map.

Figure 2. The comparison of temperature changing trend of Yuanmou and the neighboringdistricts: ( ) Yuanmou; ( ) Wuding; ( ) Chuxiong; ( ) Dongchuan.

CLIMATE CHANGE IN YUANMOU VALLEY 155

The temperature has decreased by 1?11C from the 1950s to the 1990s. Temperaturehas a marked decline in winter, e.g. the average temperature of the coldest month(December) from 1956 to 1989 was 14?91C, while it was 13?61C from 1990 to 1997, adecrease of 1?31C. Prior to 1990, temperature in the plain areas rarely declined below01C. But in recent years, it has declined below zero degree for a number of days. In thesecond half of December 1999, for instance, the night temperature declined below01C incessantly for a number of days and even the ground temperature declined to�4?71C. The winter vegetable and fruits (tomato, maize, banana and longan seedling,etc.) in the south sub-tropical area have suffered a great damage from a spell of lowtemperature and frost. The economic loss reached over 100 million RMB yuan. Thetrend of annual average temperature is shown in Fig. 2.

Some references (Zhang, J., 2000) show that the global rainfall in the past fewdecades remained at nearly the same level. Zhao Chunyu, et al. (2000) showed thatthe rainfall has decreased in the recent 48 years. MA Jingxian and DAI Caidi’s (2000)study showed that annual rainfall declined in the eastern part of northwest Chinasince the 1990s. Liu Yijun and Li Lin’s (1999) studies show that the rainfall decreasedin the Tibetan plateau since the 1950s. But rainfall in the Yuanmou dry-hot valleys,however, has slightly increased. The average annual rainfall is 623?95 mm. For the1950s 1960s, 1970s, 1980s, and 1990s, it was respectively, 538?7, 647?8(comparatively increased), 610?6, 627?6 and 670?5 mm. It has increased by131?8 mm from the 1950s to 1990 (Fig. 3).

Evaporation in the study areas has obviously decreased (Zhang, J., 1994). Theaverage annual evaporation is 3507?2 mm (1956–1997). For the 1950s, 1960s, 1970s,

Figure 3. The comparison of rainfall changing trend: ( ) Yuanmou; ( ) Wuding; ( )Chuxiong; ( ) Dongchuan.


1980s and 1990s it was respectively, 4370?6 (from 1956 to 1960), 4004?8, 3444?6,3131?3 and 2806?1 mm (from 1991 to 1997). It has decreased by 1564?5 mm in thelast 50 years (Fig. 4).

Since the 1950s, the relative humidity in the Yuanmou dry-hot valley has increased.It was 50?4%, 53?1%, 54?6%, 56?0% and 59?7% in the 1950s, 1960s, 1970s, 1980sand 1990s, respectively (Fig. 5).

Sunshine hours have slightly decreased. The average annual sunshine hours were2669 h from 1956 to 1980, and 2582 h from 1981 to 1997 (Fig. 6).

The annual wind speed has changed a lot (Fig. 6). The average annual wind speedin the 1950s, 1960s, 1970s, 1980s and 1990s was, respectively, 2?50, 2?45, 2?50, 1?93and 2?1 m s�1. A trend of decline can be identified.

It can be concluded that the climate has undergone a marked change since the1950s in the Yuanmou dry-hot valley and the change is not consistent with the changeof global climate.

The trend of climate change in the neighboring areas

Yunnan Plateau (Wuding County and Chuxiong City)

Since the Yunnan Plateau is an open and broad area, its climate is affected byextensive climatic systems and always under the control of global environmentalchange. Climate change is, therefore, consistent with the change of global climate.

The climate of the Wuding County and the Chuxiong City on the Yunnan Plateaudoes not change much and is inconsistent with that in the Yuanmou dry-hot valley. Inthe Wuding County the average annual temperature in the 1960s, 1970s, 1980s and1990s was, respectively, 15?91C, 15?81C, 15?91C and 15?91C (Fig. 2). The averageannual rainfall in the 1960s, 1970s, 1980s and 1990s was, respectively, 1035?1, 963?0,900?9 and 1014?7 mm (Fig. 3). The average annual evaporation in the 1950s, 1960s,1970s, 1980s and 1990s was, respectively, 1965?0, 2081?6, 1993?1, 2018?6 and1908?0 mm, showing normal fluctuation without remarkable decrease or increase(Fig. 4). The average relative humidity in the 1970s, 1980s and 1990s was,respectively, 71.7%, 68.9% and 68.1% (Fig. 5). It can be concluded that the climatein the Wuding County is rather stable.

Chuxiong City is situated in the middle of the Yunnan Plateau, between 1001350–1011490 east longitude and 241290–251150 north latitude. The average annual

Figure 4. The comparison of evaporation changing trend of Yuanmou and the neighboringdistricts: ( ) Yuanmou; ( ) Wuding; ( ) Chuxiong; ( ) Dongchuan.

Figure 5. The comparison of relative humidity changing trend of Yuanmou and theneighboring districts: ( ) Yuanmou; ( ) Wuding; ( ) Chuxiong; ( ) Dongchuan.


temperature in the 1960s, 1970s, 1980s and 1990s was, respectively, 15?51C, 15?51C,16?01C and 16?51, showing a trend of increase, consistent with the trend of the globalclimate change (Fig. 2). The average annual rainfall in the 1960s, 1970s, 1980s and1990s was, respectively, 824?0, 803?5, 830?8 and 901?8 mm, showing a trend of slightincrease (Fig. 3). The average annual evaporation in the 1950s, 1960s, 1970s, 1980sand 1990s was, respectively, 2445?4, 2093?1, 2153?0, 1970?2 and 1867?7 mm,showing a trend of decrease (Fig. 4). The average annual air relative humidity is69?7%, and it shows a trend of decrease (Fig. 5).

Dongchuan dry-hot valley area

Dongchuan City is situated in the northern Yunnan Plateau, between 1021470–1031990 east longitude and 251470–261330 north latitude. Its average annualtemperature in the 1960s, 1970s, 1980s and 1990s was, respectively, 21?91C,21?71C, 21?61C and 21?41C, showing a trend of decrease (Fig. 2). The average annualrainfall in the 1960s, 1970s, 1980s and 1990s was, respectively, 708?6, 697?7, 734?5and 802?4 mm, showing a trend of slight increase (Fig. 3). The average annualevaporation in the 1950s, 1960s, 1970s, 1980s and 1990s was, respectively, 3281?8,3910?7, 3459?8, 3004?4 and 2504?2 mm, showing a trend of marked decrease (Fig. 4).But the decrease is not as big as in the Yuanmou dry-hot valley areas. The averageannual relative humidity in the 1970s, 1980s, 1990s was, respectively, 66?6%, 56?9%,and 59?2%, showing a trend of decrease (Fig. 5). Generally, climate change inDongchuan City is remarkable.

Causes of climate change in the Yuanmou dry-hot valley

On the Yunnan plateau, climate is chiefly under the control of macro-climatic systems,and the change of local environment has little impact on the climate. Climate changein the Wuding and Chuxiong Cities represents the trend of climate change on theYunnan Plateau. In the river valleys of the Yunnan Plateau, which is comparativelyclosed from outside, the change of local environment in Dongchuan and Yuanmouhelps us to know the causes of climate change in the Yuanmou dry-hot valley. Thelocal environment change that may have an impact on the local climate is chiefly theresult of human activities. Afforestation, for instance, has increased the forestcoverage. Thanks to the development of hydrological engineering, both irrigated andcrop sowing areas have expanded.


The enlargement of irrigated area

Owing to deeply cut valleys and the effect of foehn, the local climate is dry and hot.The average annual rainfall is 768, or 15?52 billion m3. Of the total rainfall, groundsurface runoff is 2?41 million m3, ground runoff 78?81 million m3, and the groundsurface reserve 122?14 million m3. Other parts of the rainfall return to the sky bysurface evaporation and plant transpiration. Before the construction of hydrologicalfacilities such as reservoirs, most of the runoff flowed into Jinsha River after they cameinto confluence. The limited water resource was therefore poorly utilized. After theconstruction of hydrological facilities, a considerable amount of runoff has beenreserved for agricultural irrigation and other uses. Since Yuanmou is situated in abasin surrounded by mountains, the rainfall from the vapor formed by ground surfaceevaporation accounts for a considerable part of the total rainfall (Zhang X, et al.,1995). The irrigation of large-area farmland has resulted in the increase of evaporationand leads to the change of climate.

Since the 1950s, four middle-sized reservoirs (Hewei, Bingjian, Menglian andMaliu reservoirs) with a total storage of 62?46 million m3, 73 small reservoirs with atotal storage of 29?51 million m3, and 1238 ponds with a total storage of 10?69million m3 have been finished in the Yuanmou County. Up to now 29 irrigation canalswith a discharge of 0?3 m3 s�1 and other hydrological facilities have been built. As aresult, the use rate of water resource has been enhanced, and the irrigated areaenlarged from 2187 hm2 in 1957 to 10,900 hm2 in 1999 (Fig. 6). If the water utilityrate is 90% (discounting the loss due to leakage and evaporation), the total volume ofwater effectively used amounts to about 113?00 million m3. The fact that the changeof climate resulted from the development of hydrological engineering can beillustrated from the following two aspects.

First, the increase of evaporation from the irrigated farmland leads to the increase ofhumidity in the air and further leads to the change of climate. The water foragricultural use has mainly been used for irrigation in the dry season. This part ofwater, accounting for 85% of all water for irrigation, is about 96 million m3. The dryseason falls into two periods: from November to March, the water used for irrigationof vegetables, wheat, maize and bean (also used as vegetable) is about 67 billion m3,making up 70% of all the water for irrigation in dry season, of which 50 million m3 isused for irrigation of vegetables (Table 1), and 17 million m3 is used for irrigation ofother crop land. From April to May the water for irrigation of rice land is about 29million m3. The water for irrigation in the rainy season is about 17 million m3, only15% of all water used throughout a year. This part of water is mainly used forirrigation of rice land.

All in all, water used for irrigation of vegetable land is 50 million m3. If 0?6 istaken as the coefficient of evaporation and transpiration, the amount of evaporatedand transpired water will reach 30 million m3. Water for cultivation of rice and othercrops is 63 million m3. If 0?7 is taken as the coefficient of evaporation andtranspiration, the amount of water evaporated and transpired from rice and othercropland will reach 44 million m3. The total volume of water coming from irrigation is74 million m3.

Second, surface water evaporation has increased air humidity. The constant area ofwater surface in Yuanmou, such as reservoirs and ponds, is about 900 hm2. If theevaporation from water surface is 2900 mm, the annual increment of evaporated waterdue to the increase of water surface will reach 26 million m3.

Humidity will increase and the average temperature and evaporation will fall, whichwill lead to the change of climate. With the increase of hydrological engineering, thecondition of irrigation in Yuanmou has improved. As the irrigated farmland enlarged,the cropping index also increased. Since the 1980s the production of sugarcane andwinter vegetable has been well developed (Table 1). The irrigation of large-area


farmland in the dry season has improved the condition of the ground surface, leadingto the improvement of climate in the plain areas (Zhang J., 2000). The process ofclimate change that resulted from irrigation and the increase of water surface is asfollows.

The increase of evaporation due to irrigation and the evaporation from watersurface - the increase of air humidity - the increase of rainfall, the decline oftemperature and the decrease of evaporation - the change of climate.

Third, a comparison between different hydrological data also indicates the impactof irrigation and water surface evaporation on the change of climate. Comparing therainfall and runoff data collected at Xiaohekou (XHK) Hydrological Station at theupper reach of the Longchuan River with those collected at the Xiaohuangguayuan(XHGY) Hydrological Station at the lower reach of Longchuan River, we can see thatrainfall and runoff at Xiaohekou Hydrological Station are more stable. Although thevariation of rainfall at Xiahuangguayuan Hydrological Station is very limited, therunoff has a trend of decline, which indicates that the decline of runoff atXiohuangguayuan Hydrological Station is due to the construction of irrigationsystem, the evaporation from ground-water surface and plant transpiration. It istherefore proved that the evaporation from water surface and the evaporation from theexpanded irrigated farmland may lead to the change of climate (Table 2).

In this paper, we use the theory of Grey relative degree to analyse the mutual impactdegree of climate factors in the Yuanmou dry-hot valley. Relative degree analysis isbased on the similarity or difference degree of the trend of a developing system toweigh the relative degree between the factors. It is a quantitative comparison tochanging trend of the developing system. The analysing method is to compare thegeometric relative degree of the data column and it is expressed as follows (DengJulong, 1987)

�ki

¼min

imin

kjX0ðkÞ � XiðkÞj � 0�5 max jX0ðkÞ � XiðkÞj

jX0ðkÞ � XiðkÞj þ 0�5 maxi

maxi

jX0ðkÞ � XiðkÞj

In the above formula �iðKÞ, the relative difference between comparative curve {Ki}and reference curve at K point, is called the relative coefficient of Ki to K0 at K point;

mini

mink

kjX0ðkÞ � XiðkÞj is the minimum difference of two levels:

mini

mink

kjX0ðkÞ � XiðkÞj is the maximum difference of two levels. The relative

coefficient was calculated by the following formula:

i ¼1

N

XN

K¼1

�iðkÞ

Y, is called relative degree between {Xi} (comparative curve) and {X0} (referencecurve); high Relative degree indicated that {Xi} has an impact on {X0} or a closerelation exists between them.

Here, we suppose {X0} as irrigated land area, {X1} as air relative humidity, {X2} asrainfall, {X3} as evaporation amount, {X4} as yearly temperature, {X5} as sunshinehour, {X6} as yearly average wind speed. The result is as follows:

The relative degrees of irrigated area with each climate factor are all above 0.6; thisshows that each climate factor has a close relation with the increase of irrigated landarea.

Y(0, 1)=0?9003107, Y(0, 2)=0?6726791, Y(0, 3)=0?8124863Y(0, 1)=0?9533985, Y(0, 5)=0?9140763, Y(0, 6)=0?6001014The compositor of relative degree is Y(0, 4)4Y(0, 5)4Y(0, 1)4Y(0, 3)4Y(0, 2)4

Y(0, 6). The order of climatic factors being affected by irrigation is yearly temperature,sunshine hours, air relative humidity, yearly evaporation, yearly rainfall and yearlyaverage wind speed.

Table 2. A comparison of rainfall and runoff between Xiaohekou Station andXiaohuangguayuan Station

Period Rainfall (mm) Runoff (100 million m3)

XHK XHGY Difference XHK XHGY Difference

1966–1969 850?4 654?2 196?2 3?84 10?67 6?831970–1979 842?1 635?9 206?2 2?98 7?80 4?821980–1989 772?7 631?3 141?4 2?23 5?73 3?51990–1996 966?0 635?8 330?2 3?01 8?595 5?585


The relative degrees of air relative humidity with each climate factor are all alsoabove 0?6; this shows close relation between each climate factor with the air relativehumidity.

Y(1, 2)=0?6685891, Y(1, 3)=0?8221054, Y(1, 4)=0?9592679Y(1, 5)=0?9191946, Y(1, 6)=0?6104039The compositor of relative degree is Y(1, 4)4Y(1, 5)4Y(1, 3)4Y(1, 2)4Y(1, 6).The relative degrees of rainfall with each climate factor are as follows:Y(2, 3)=0?8187455, Y(2, 4)=0?9635987, Y(2, 5)=0?9191654, Y(2, 6)=0?60070631The compositor of relative degree is Y(2, 4)4Y(2, 5)4Y(2, 3)4Y(2, 6)

The relative degrees of evaporation amount with other climate factors are as follows:Y(3, 1)=0?9646149, Y(3, 4)=0?9250681, Y(3, 4)=0?6174096

The compositor of relative degree is Y(3, 4)4Y(3, 4)4Y(3, 4).The relative degrees of yearly average temperature with other climate factors are as

follows:Y(4, 5)=0?9216917, Y(4, 5)=0?6128337The relative degree of sunshine hour with yearly average wind speed is

Y(4, 5)=0?6110584.

The increase of forest coverage

Forest is not only one of the main components of ecosystem, but also a chief protectorand an adjuster of the environment. The expansion of forestland and the increase of

Table 3. The afforestation area in Yuanmou from 1985 to 1995 unit: hm2

Year Afforestation area In which Tending of young trees

Shelterbreak

Timberforest

Cashforest

Fuelforest

1985 1380?0 133?3 880?0 366?7 0?0 0?01986 5815?9 2426?7 3232?8 150?0 6?5 0?01987 1995?3 769?7 986?3 239?2 0?0 0?01988 433?3 0?0 0?0 401?5 31?8 351?11989 796?3 66?7 460?0 186?7 83?0 133?31990 2095?6 25?4 1397?5 198?5 474?3 0?01991 1815?3 95?1 1223?4 66?6 430?1 20?51992 1166?1 193?0 445?5 101?7 425?8 100?01993 1658?3 1045?8 494?3 94?8 23?3 100?01994 1004?0 986?7 0?0 17?3 0?0 340?01995 2529?7 1945?5 210?8 22?1 351?3 0?0Total 20689?8 7687?9 9330?7 1845?1 1826?1 1044?9


forest coverage rate will improve the ecological condition, and bring about the changeof climate. Afforestation and the increase of vegetation coverage are also the causesleading to the climate change in the Yuanmou dry-hot valley areas. Since the 1950sthe area of forestland and vegetation has greatly increased in the Yuanmou County. Inthe early 1950s the mountain areas above 2000 m, where Pinus yunnanensis is thedominant species, enjoyed a forest coverage rate of 60–70% with luxuriant shrubs.There were also large areas of forests dominant with Pinus yunnanensis in themountainous areas at an altitude of 1500–2000 m. Although there were no large areasof forest in the hilly and plain areas below 1500 m, shrubs grew well and thick. It israre to see bald mountains with no vegetation. The forest coverage rate of the wholecounty is 12?8% since in the mid 1950s forests in Yuanmou had been severelydestroyed. In the late 1950s nearly all the trees in the Yuanmou basin and thesurrounding hills were felled. The forest coverage rate reduced from 12?8% in 1950 to6?3% in 1973. In 1977 many trees in Yuanmou were once again felled. In 1985 theforest coverage rate reduced to 5?2% (Liu, X., 1989). Since the 1980s an afforestationcampaign has been initiated in Yuanmou (see Table 3). Forest covered area began toexpand and forest coverage rate also increased. A survey done by the forest bureau ofthe Yuanmou county shows that in 1993 forest covered area had reached 11407 hm2

and forest coverage rate 5.8%. With the implementation of the national project of soilerosion control in the Yangtze River valley, the driving forces to plant trees have beenintensified and forest areas have been expanding at a faster speed. The increase ofvegetation cover reduced solar radiation to land surface, surface reflection andtemperature decrease. Based on the observation data (Wen Chuanjia & GaoShenghuai, 1989) of Wolong district in Sichuan Province, the reflection rate of pineforest, grassland and bare land is, respectively, 0?124, 0?181 and 0?101, but thesurface temperature is contrary to the reflection rate.

Conclusions

From the above analysis the following conclusions can be drawn:1. Climate change in the Yunamou dry-hot valley is caused by human activities, and

the trend of climate change is not consistent with that of global climate change. Theincrease of rainfall, the decline of evaporation and the decrease of wind speed arebeneficial to agricultural production. The increase of air humidity has provided thelocal people a more comfortable living environment. Nevertheless, light and thermalresources have been reduced.

2. The trend of climate change in Chuxiong City on the Yunnan Plateau isconsistent with that of global climate change, but climate in the Wuding County at ahigher altitude is much more stable. In Dongchuan City also in dry-hot valley, exceptrelative air humidity with a marked decline, the change trend of other climaticindicators such as the average annual temperature, rainfall and evaporation isconsistent with that of the corresponding indicators in the Yuanmou dry-hot valley.

3. Climate change in the Yuanmou dry-hot valley is chiefly the result of humanactivities.

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Climate change and causes in the Yuanmou dry-hot valley of Yunnan, China