A GIS-based Study on Grassland Degradation and. Increase of Dust Storms in China

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1 1 A GIS-based Study on Grassland Degradation and Increase of Dust Storms in China Qinxue WANG* and Kuninori OTSUBO* * National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki , Japan Abstract: Serious dust storms and fly dust often occur continuously throughout spring in northern and northwestern China. In the spring of 2000 and of 2001, dust storms occurred so often more frequently than at any other time in the last 50 years that they attracted worldwide attention. In this paper, we describe the characteristics of dust storms and outline the damage they have caused over the last 2 decades. Using GIS technology, we analyzed the causes of the increasing trend of dust storms and fly dust and found that the major causes of the increase of strong dust storm were human-driven factors, such as grassland degradation, over-cultivation, misuse of water resources, and unclear land ownership rights. Keywords : fly dust, desertification, over-cultivation, land use change, GIS 1. Description of Dust Storms During the spring of 2000 and of 2001, serious fly dust and dust storms occurred very often in northern and northwestern China. Dust storms occurred more frequently than at any time in the last 50 years so often that they were brought to worldwide attention. During a dust outbreak in the spring of 2000, the Xinhua News Agency in China 1),2) reported that visibility in Beijing had been reduced to the point where buildings were barely visible across city streets, and airline schedules were significantly disrupted. The dust has also been implicated in adverse health effects, such as respiratory discomfort and eye irritation. Dust storms are a severe weather condition characterized by strong winds and dust-filled air over a large area. In China they are classified as fly dust in cases where visibility is between 1 and 10 km, and dust storms in cases where visibility is less than 1 km (Figure 1). Dust storms are further classified into 3 levels: dust storms, strong dust storms, and very strong dust storms (so-called black wind in China) according to visibility, wind speed, continuation, and amount of damage. Conditions for dust storms to occur are : (1) Land surface properties: dry land surface, soft soil texture, and low vegetation or snow coverage. (2) Weather conditions: strong winds with a velocity larger than 10 m/s, unstable stratification, and no precipitation. Figure 1 Dust storm and fly dust

2 2 The objectives of this study were to: (1) investigate the occurrence, change, and influence of dust storms ; (2) monitor affected areas by remote sensing; and (3) use GIS and remote sensing technology to analyze the reasons that strong dust storms are increasing in China. 2. Increase in Numbers of Strong Dust Storms Strong dust storms that occurred during the last several decades and the damage caused are documented in this study. It was found that the number of strong dust storms has been increasing since the 1950s (Figure 2). Although varying across China, the scale of dust storms has increased, affecting a wider area and the damage caused has increased year by year. Records show that during the 17th century there were from 0.3 to 1.0 dust storms in Inner Mongolia per year, but by 1990 the annual rate of occurrence had risen to 3.0 to 5.0 times per year. At the same time, the rate of dust storms occurring in Beijing has also increased, and the number of violent dust storms that occurred in the spring of 2000 in Beijing was more than 3 times the average for the same period in the 1990s. Number of strong dust storm s 1960s 1970s 1980s 1990s Desertified area (km 2 /yr.) Strong dust storm Desertified area Figure 2 Decadal variations in the number of strong dust storms and desertified areas in China Typical strong dust storms and the damage caused over the last 20 years are listed in Table 1. From the table, we see that the most serious damage, such as people dead, stock dead, and economic loss, mainly occurred in Gansu Province, and the Xinjiang and Inner Mongolia Autonomous Regions. Satellite remote sensing can efficiently monitor areas being assailed by dust storms. The Earth Probe TOMS (Total Ozone Mapping Spectrometer) is the first instrument to allow observation of aerosols as the particles cross the land/sea boundary. It is possible to use these data to observe a wide range of phenomena such as desert dust storms, forest fires, and biomass burning. The TOMS data can distinguish between different types of aerosol particles on the basis of their size (dust tends to have larger particles than smoke) and absorbance properties. Although dust from the Sahara desert is routinely transported across the Atlantic to the east coast of the US, Asian dust does not very often make it across the Pacific to the west coast. However, the TOMS data have shown that dust originating in China's Gobi desert has been transported across the Pacific, where it was observed in 1998 as a significant event in the western US 3). We used the Earth Probe TOMS Aerosol Index data set of NASA to identify areas being assailed by dust storms. Figures 3 and 4 show the aerosol index distributions in April of 2000 and 2001, respectively. We can see that dust storms or fly dust occurred almost every day during April in northern China.

3 3 Table 1 Places affected and damage inflicted by some typical strong dust storms over the last 20 years 1),2) ID* Month Year Place Damage 1 April 1979 Xinjiang Dust fall: t/km 2 2 May 1983 Xinjiang 3 May 1986 Xinjiang 4 May April 1994 Farmland damaged: 167 km 2 Economic loss: 3 million RMB Yuan Farmland damaged: 133 km 2 Economic loss: 50 million RMB Yuan Gansu People dead: 380 Xinjiang Stock dead: Inner Mongolia Farmland damaged: 3367 km 2 Gansu Inner Mongolia Economic loss: 540 million RMB Yuan Dust fall: Tremendous (no values recorded) 6 May 1995 Gansu Dust fall: t 7 November Shandong People dead: 156 Economic loss: 1 billion RMB Yuan 8 May 1996 Gansu Economic loss: 200 million RMB Yuan 9 April 1998 Gansu Stock dead: Ningxia Farmland damaged: 307 km 2 Inner Mongolia Economic loss: 800 million RMB Yuan 10 April Xinjiang People dead: 50 May Gansu Xinjiang People injured: 256 Wind speed: 32 m/s Dust fall: Tremendous (no values recorded) 11 April 1999 Inner Mongolia Wind speed: 32 m/s Dust fall: Tremendous (no values recorded) 12 March 2000 Northwestern China 4 times of dust storm occurred, and wind speed ranges 10 m/s to 25 m/s. April Northwestern China, 6 times of dust storm occurred, and wind speed Inner Mongolia, and ranges 10 m/s to 30 m/s. North China Plain May * ID shows the areas damaged by dust storms Inner Mongolia, Northeastern China 3 times of dust storm occurred, and wind speed ranges 10 m/s to 25 m/s. Figure 3 Distribution of aerosol index in April 2000

4 Figure 4 Distribution of aerosol index in April Causes of the Increased Numbers of Strong Dust Storms 3.1 Climate change In order to discover the cause of the increased number of strong dust storms, we need to study the problem from the aspects of both climate change and land use/cover change. To analyze the relationship between climate change and the increase in strong dust storms, we examined changes in climatic factors, such as the maximum and average values of daily mean wind speeds from 1961 to 1998, and precipitation, both annually and in spring, at 8 meteorological stations (Ürümqi, Jiuquan, Lanzhou, Yinchuan, Xi an, Taiyuan, Huhehot, and Zhengzhou) in northern China (Figures 5 and 6). However, we were unable to recognize any increase in wind speed or decrease in precipitation at the 8 stations; therefore, we realized that it is difficult to attribute the recent increasing t rend of strong dust storms to climatic factors wind speed (m/s) Maximum wind speed Wind speed Figure 5 Changes in the maximum and average values of daily mean wind speed

5 5 Precipitation (mm) May Apr Mar Feb Jan Annual Figure 6 Changes in precipitation both annually and in the period from January to May 3.2 Land use/cover change Owing to the development of agriculture, the desertified area has gradually increased 4). During the 1950s and 1960s, such areas were expanding by an average of 1600 km 2 /y but by the 1990s, these areas were expanding by 2500 km 2 /y. This kind of situation is particularly serious in many regions in northern and northwestern China. To analyze the relationship between land use/cover change and the increased number of strong dust storms, we produced a set of 1-km grid scale digital maps of land u se/cover in China. We found that grassland degradation in arid and semi -arid areas greatly corresponded to the occurrence of strong dust storms both temporally (Figure 2) and spatially (Figure 7), so we concluded that the main cause of the increased numbers of strong dust storms was the widespread degradation of grassland. Figure 7 Areas where grassland degradation has occurred and sites where devastatingly strong dust storms occurred during the last 2 decades

6 6 Grassland degradation is considered mainly caused by human-driven factors, such as over-cultivation, over-grazing, deforestation, misuse of water resources, and unclear land ownership rights. 1) Over-cultivation Due to the rapid increase of population and the pursuit of short-term profit, land unsuitable for cultivation has been cultivated without a rational plan. For instance, in the past there have twice been large-scale reclamation projects in Inner Mongolia, which caused the desertification of over km 2. In recent years, over km 2 of land in Inner Mongolia, Gansu, Heilongjiang, and Xinjiang were cultivated, of which half was soon devastated. Local farmers say: Grasslands were cultivated in the 1st year, crops were planted in the 2nd year, and deserts came in the 3rd year. 2) Over-grazing In recent years, most of the grasslands in China have been over-grazed. Especially in the desertified areas, in particular, the animal population is 50% to 120%, sometimes even up to 300%, above sustainable carrying capacity. The average height of the grass in Inner Mongolia was 70 cm in the 1970s, but is now lower than 25 cm. 3) Deforestation The forest has been cut down recklessly in some arid and semi-arid areas. For example, about 200 km 2 of the forest around Yanhu Lake, Inner Mongolia, has disappeared over the last 2 decades. 4) Mis use of water resources Owing to inefficient irrigation systems, large amounts of water resources are wasted, causing serious water shortages, salinization of soils, and vanishing vegetation in some regions of northwestern China. 5) Unclear land ownership rights The Chinese government has launched a land use policy asking farmers to stop cultivation and plant trees on the slopes of mountainous regions in northwestern China. However, it has not been effectively carried out, because the long-term land ownership right is not clearly determined. It is therefore very difficult for farmers to conduct a planned and sincere land management strategy for effective land protection. References 1) Zhao, C., and Li, J. Strong winds and dust storms occurring in recent years. Xinhua News Agency, Apr. 6, ) Li, J., and Zhao, C. Occurrence regions and situation of fly dust and dust storms in Xinhua News Agency, Apr. 9, ) Husar, R. B., Tratt, D. M., Schichtel1, B. A., et al. The Asian dust events of April J. Geophys. Res., 105 (12), ) Dong, G., Wu, B., Ci, L., Zhou, K., Lu, Q., and Luo, B. Current situations, causes and protecting strategies of degradation in China. Desertification in China, 1999: 19 (4).