Study area
Based on relevant literature regarding the countries participating in the Belt and Road initiative and the sensor collection range from the TRMM precipitation remote sensing data, this knowledge service selected 61 countries along the Belt and Road area, south of N50°. These 61 countries are distributed in Asia, Europe, and Africa, with a total land area of 62.3776 million square kilometers. Only parts of Mongolia, Russia, China, Poland, Czech Republic, Ukraine and Kazakhstan were included.
Data
The TRMM 3B43 data were used to study the spatiotemporal distribution of meteorological drought caused by lack of precipitation. The TRMM satellite, jointly developed by the US and Japan, is the first meteorological satellite dedicated to the quantitative measurement of rainfall in tropical and subtropical regions. Its main purpose is to further our understanding of global circulations of energy and water by studying precipitation and latent heat in the tropics. The main sensors on the satellite equipped for precipitation observation are: Precipitation Radar (PR), TRMM Microwave Imager, and Visible/Infrared Sensor. PR is the first spaceborne precipitation radar and it can observe the three-dimensional structure of rainfall. The TRMM 3B43 data were obtained from the website of Goddard Earth Sciences Data and Information Services Center. The dataset ranges from January, 1998 to December, 2015 in HDF format with a spatial resolution of 0.25° × 0.25°. There are 1440 × 400 grids globally ,which the corresponding precipitation value of the grid is the average within the grid with the units of mm/h. The monthly spatiotemporal distribution of precipitation can be summarized after some correction processing of rotation, georeferencing, and resampling.
Methods
The spatiotemporal distribution of drought was calculated by using the Precipitation Abnormity Percentage drought model. The Precipitation Abnormity Percentage (Pa) is one indicator used to measure variations of precipitation and compared to the normal value in a certain period. It is defined as the ratio of the difference between the actual precipitation and historical average precipitation to the historical average precipitation. It can directly reflect the drought caused by abnormal precipitation .It is suitable for periods when the average temperature is above 10°C in semihumid, semiarid areas. In this knowledge service, the descriptions, such as extremely dry, severely dry, moderately dry, mildly dry, and no drought, are based on the standards given by the following drought classification table of Precipitation Abnormity Percentage.
Results
The distribution of monthly Precipitation Abnormity Percentages in the Belt and Road area from 1998 to 2015 was calculated according to the method for calculating the Precipitation Abnormity Percentage given in section 2.3. Subsequently, we got the spatiotemporal distribution of drought in the study area.
According to the monthly spatiotemporal distribution of drought in the Belt and Road area during 1998 to 2015, there were significantly more arid regions in winter than in summer.Large areas of extreme drought and severe drought occurred in Northeast Asia, Southeast Asia, South Asia, Central Asia, and West Asia. In Northeast Asia, areas of northeast China, northern China, the southwest and central parts of Mongolia suffered the most severe drought followed by the northwest region of China. However, the southeastern coastal areas experienced relatively light drought. In Southeast Asia, the most severe drought mainly occurred in Myanmar, Laos, Thailand, and Cambodia. In South Asia, the most serious drought took place in India, Pakistan, Nepal, Bhutan, and Bangladesh. In Central Asia, the severely dry areas in Uzbekistan and Turkmenistan were the largest. In West Asia, large areas of severe drought arose in Saudi Arabia, Oman, and southern Egypt. Due to low precipitation, decrease of river flow in winter, and the monsoon climate caused by the Siberian cold, severe drought usually occurs in large areas of Southeast and South Asia in winter. In the summer, the drought area became relatively smaller and the severe drought was mainly concentrated in Central Asia and West Asia, especially in Uzbekistan, Turkmenistan, Afghanistan, Saudi Arabia, Egypt, Syria, Lebanon, Jordan, Israel, Iraq, Iran, Kuwait, Bahrain, and Qatar. From the perspective of meteorology, it is known that the ecological environment is fragile near the Caspian Sea, the amount of water vapor is relatively small, and the rivers are mostly inland relying on the iceberg-melt water,the high pressure of the summer Azores leads to low precipitation and high evaporation in the East Coast area. The Red Sea area and the Persian Gulf region are usually controlled by the subtropical high pressure or the low latitude wind.Therefore, subsidence airflow usually prevails, leading to scarce precipitation and a mostly tropical desert climate hot and dry throughout the year.
Regional variation patterns of drought levels
The Belt and Road area can be divided into six major economic corridors: China-Mongolia-Russia, New Asia-Europe land bridge, China-Central Asia-West Asia, China-Indo China Peninsula, China-Pakistan, and Bangladesh-China-India-Myanmar. Based on the spatial distribution of these six economic corridors, regional differences of drought variations were studied in the sub-regions of Northeast Asia, Southeast Asia, South Asia, West Asia, and North Africa, Central and Eastern Europe, and Central Asia.
Drought in Northeast Asia mainly occurred from November to March, and the drought conditions, characterized by the average of Precipitation Abnormity Percentages fall within the level of moderate drought and below. Only in February, 1999 did the drought conditions reach above severe drought. There were 50 months with the drought conditions in the moderate drought level. The droughts in December, 1999 and December, 2003 became very close to severe droughts except other months, and the amplitude of the variation of Precipitation Abnormity Percentage in the Northeast Asia region was –0.81 to 2.37. The variation of the average drought condition in northeast Asia is very stable over 18 years, showing a slight decreasing trend, and the increase rate of the Precipitation Abnormity Percentage is 0.012/10yr.
The drought condition in the Southeast Asia region was less severer and the drought mostly occurred in the winter. The drought condition classified based on the monthly mean value of the Precipitation Abnormity Percentages was mostly mildly drought. Only in February, 2014, the drought reached the level of moderately dry. There were 16 mildly dry months, in which the drought condition in February, 2014 and March, 2015 were at the brink of mild drought and no drought; The other months were at the level of no drought. Overall Southeast Asia indicated a more humid environment compared to North-east Asia and the variation amplitude of the Precipitation Abnormity Percentages in Southeast Asia ranged from -0.62 to 0.72. The average drought level in Southeast Asia from January, 1998 to August, 2002 has a decreasing trend with the Precipitation Abnormity Percentage having an increment rate of 1.5/10yr. From September, 2002 to December, 2015, the average drought level in Southeast Asia became increasing severe; However, there were two breakpoints in June, 2005 and June, 2010. The decreasing rates of Precipitation Abnormity Percentages during the three time periods were 0.384/10yr, 0.24/10yr, and 0.552/10yr respectively.
The drought in South Asia was severe and mostly accurred in the winter, also the drought conditions as evaluated by the monthly mean values of the Precipitation Abnormity Percentages, were severely dry and moderately dry. In December, 1999, the drought conditions reached extremely dry. There were 30 months with the drought conditions reached the severe drought level and 38 months at the moderate drought level, in which the drought levels in January, 2002 and January, 2013 were close to severe drought. There were 43 months at mildly dry and the other months experienced no drought. The amplitude of the variations of Precipitation Abnormity Percentage in South Asia was –0.95to 2.96. The change of the average drought level in South Asia is relatively stable over 18 years and the overall level shows a decreasing trend in a small range. Additionally, the Precipitation Abnormity Percentage has a slight increasing trend of 0.036/10yr.
The drought in West Asia and North Africa was quite severe and occurred during the summer and autumn. The drought conditions classified by the monthly mean value of the Precipitation Abnormity Percentage were mostly in the moderate dry and mildly dry levels. In June, 1999, June, 2000, July, June and September, 2001, June, 2002, June, 2003, June, 2004, September, 2005, June, 2006, September, 2007, July, 2008, September, 2010, and September, 2013, the drought conditions for a total of 14 months were severely dry. Fifty months of drought were in the moderately dry level. Twenty-two months of drought were at the level of mild drought and the other months experienced no drought. The amplitude of the variation of the Precipitation Abnormity Percentage in West Asia and North Africa was –0.9 to 2.92. The change in the average drought level in West Asia and North Africa over 18 years is relatively stable, showing a decreasing trend over a small range and the rate of increase of the Precipitation Abnormity Percentage was 0.072/10yr.
The average drought condition in Central and Eastern Europe was mild and mostly distributed during the spring and winter and the drought conditions as classified by the monthly average of the Precipitation Abnormity Percentages were mostly in the mildly dry level. The drought level in November, 2011 reached severely dry. In October, 2000, October, 2001, March, 2003, April, 2007, February, 2008, April, 2009, September, 2011, March, 2012, December, 2013, and December, 2015, for a total of 10 months, the drought conditions reached moderately dry. There were 18 months at the mildly dry level and the other months experienced no drought. The amplitude of the variation of the Precipitation Abnormity Percentage in the Central and Eastern Europe was –0.9 to 0.83. The average drought level in Central and Eastern Europe was stable over 18 years and the overall trend was decreasing over a small range and the percentage of precipitation abnormity increased at the rate of 0.024/10yr.
The drought in the Central Asia region was mild and occurred mainly during autumn and the drought conditions as classified by the monthly mean value of the Precipitation Abnormity Percentage were mostly moderately dry and mildly dry. In September, 2005 and August, 2006, the drought conditions reached severely dry. Below the severely dry level, there were 24 months in total that had a moderately dry conditions. In August, 2002 (the 57th month), September, 2004, August, 2007, and August, 2008, the drought level was close to severely dry. A total of 31 months of drought were at the mildly dry level and the other months experienced no drought. The amplitude of variation of the Precipitation Abnormity Percentage in Central Asia was –0.89 to 1.68. It can be seen from Fig. 6f that the average drought level in Central Asia was stable over 18 years with an overall small increase in a small range; The rate of reduction of the Precipitation Abnormity Percentage was 0.036/10yr.
Bai Yongqing, WANG Juanle * , WANG Yujie, Han Xuehua, Bair Z. Tsydypov, Altansukh Ochir, Davaadorj Davaasuren, Spatio-temporal Distribution of Drought in the Belt and Road Area during 1998–2015 Based on TRMM Precipitation Data, Journal of Resources and Ecology, 2017, 8(6): 559–570.
The information is provided by Disaster Risk Reduction Knowledge Service.
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