Shandong province is located in the northern part of China and tends to be a drought-prone region. This study is dedicated to making a comprehensive and quantitative analysis of the spatial patterns of drought frequency and its climate trend coefficient, drought grades, and temporal characteristics of drought coverage area, drought duration, and drought intensity from 1961 to 2008 by using the meteorological drought composite index (CI). The results indicated that the occurrence frequency of meteorological drought in Shandong province was generally high and some part of this region such as Jiaodong peninsula had suffered drought pressure with an evident ascending trend. The drought extent and influence were very severe in 1980s and 1990s but very slight in 1964; large-area drought mainly occurred after the 1970s and the yearly and seasonally interdecadal drought duration both showed a fluctuation of 10a periodic cycle approximately. Furthermore, the slight drought mainly appeared in the northwest and southwest while the other grades of drought exhibited much significant spatial and temporal variability. Besides, drought in spring was more serious than that in winter. This study is anticipated to support the mitigation of drought hazards and to improve the management practices of environment system in Shandong province.
According to the Intergovernmental Panel on Climate Change (IPCC) report [
The American Meteorological Society classifies drought into four categories, including meteorological, hydrological, agricultural, and socioeconomic drought [
In this study, we made a thorough and quantitative analysis of the spatial-temporal dynamics of the meteorological drought that occurred in Shandong province from 1961 to 2008 by adopting CI which has not been employed in this region. Specifically, we analyzed the spatial patterns of drought frequency and its climate trend coefficient, drought grades, and temporal characteristics of drought coverage area, drought duration, and drought intensity in the expectation of providing reasonable reference and scientific support for alleviating and mitigating the impacts of meteorological drought.
Shandong province is situated in the downstream area of the Yellow River; it lies between E 114°36′~E 122°43′ and N 34°25′~N 38°23′, respectively. Being located in the eastern coast area of China, Bohai Sea borders its north and Yellow Sea borders its northeast and southeast (Figure
Location map of Shandong province and the meteorological stations.
The meteorological data records with excellent quality through a strict quality control were selected from the Chinese Terrestrial Climate Information of Daily Datasets. There are 32 representative meteorological stations in these datasets in Shandong province; however, due to the limitation of inconsistency observation records in some stations, we finally choose 18 meteorological stations’ daily climate data including measured average temperature and precipitation data from 1961 to 2008. In order to satisfy the requirements of CI calculation, the meteorological records in nearly 50 years were readily preprocessed according to the data processing document.
In this study, CI was employed to analyze the spatial and temporal characteristics of meteorological drought in Shandong province at time scales of seasonal and interannual. CI is calculated based on standardized precipitation index (SPI) and relative moisture index (MI) as follows [
SPI was developed for the purpose of defining and assessing drought by McKee et al. [
The meteorological data including the daily average temperature and daily precipitation were employed to calculate the CI according to formula (
Classification standard of the meteorological drought composite index (CI).
Level | Class | CI |
---|---|---|
1 | No drought | −0.6 < CI |
2 | Slight drought | −1.2 < CI ≤ −0.6 |
3 | Moderate drought | −1.8 < CI ≤ −1.2 |
4 | Heavy drought | −2.4 < CI ≤ −1.8 |
5 | Extreme drought | CI ≤ −2.4 |
We adopted CI to determine the meteorological drought process. A meteorological drought process is initiated when CI in 10 consecutive days is above slight drought level (CI10 ≤ −0.6), and this process is terminated until another 10 consecutive nondrought days appear (CI10 > −0.6). The drought duration refers to the period between the first date and last date of the whole process. A drought event for a specific time scale (month, season, and year) appears when there is at least one occurrence of a drought process and the length of cumulative drought duration exceeds a quarter of the timescale [
In this paper, the annual and seasonal drought duration in Shandong province were obtained by calculating the arithmetic mean of the total number of days of all drought processes for total meteorological stations within the corresponding periods. And the annual drought intensity was defined as the arithmetic mean of the drought intensity for total meteorological stations for each year. The definition of season is as follows: winter covers January, February, and December of last year, spring is from March to May, summer is from June to August, and autumn covers from September to November.
The drought frequency is calculated as follows:
The climate trend coefficient is used to describe the temporal characteristics of climate trend change intensity quantitatively [
In this study, the climate trend coefficient of drought frequency was calculated based on the occurrence frequency of drought in five decades for each meteorological station at time scales of year and season. The positive and negative values of
The probability of drought coverage area (PDCA) is defined as the area ratio of drought events that occur for a given timescale in the study region and is calculated as follows:
There are very significant spatial differences of meteorological drought frequency from 1962 to 2008 in Shandong province. It can be seen that the annual drought frequency decreased both from north to south and from west to east (Figure
Spatial distribution of annual (a), spring (b), summer (c), autumn (d), and winter (e) drought frequency in Shandong province.
Furthermore, the temporal trend analysis of drought frequency showed that the meteorological drought in Shandong province tended to be aggravating from 1962 to 2008 (Figure
Spatial distribution of annual (a), spring (b), summer (c), autumn (d), and winter (e) climate trend coefficient of drought frequency in Shandong province.
The spatial patterns of different drought grades from 1962 to 2008 in Shandong province are shown in Figure
Spatial patterns of annual ((a)~(c)), spring ((d)~(f)), summer ((g)~(i)), autumn ((j)~(l)), and winter ((m)~(o)) different drought grades in Shandong province.
In spring, the annual days of slight drought, moderate drought and severe and extreme drought were 16~25 d (Figure
In summer, the annual days of slight drought, moderate drought, and severe and extreme drought were 10~17 d (Figure
In autumn, the annual days for the three drought grades were 14~22 d (Figure
In winter, the annual drought days for different grades were 14~22 d (Figure
To sum up, each grade of drought that lasts a relatively long time has occurred in Shandong province from 1962 to 2008. And the slight drought was generally observed in the northwest and southwest both for annual and seasonal, but the other grades of drought were observed in different areas for different timescales.
The temporal sequence of PDCA in Shandong province from 1962 to 2008 is shown in Figure
Annual, spring, summer, autumn, and winter probability of drought coverage area in Shandong province.
Moreover, time-continuous large-area drought events have been observed in a few years in Shandong province; in specific, transseasonal large-area droughts that covers spring, summer, and autumn occurred in 1981, 1988, and 2001, respectively. Autumn-winter continuous large-area droughts happened in 1990 and 2002, respectively. Serious spring-summer large-area drought occurred in 1992 and summer-autumn large-area drought occurred in 1997, respectively. It can be seen that time-continuous large-area droughts mainly took place after the 1970s in Shandong province, revealing that time-continuous droughts were steadily increasing with time during the study period. However, it is also found that the linear changing tendencies were not obvious in the annual and seasonal PDCA in Shandong province by carrying out the
As is shown in Figure
Annual, spring, summer, autumn, and winter drought duration in Shandong province.
It reveals that the periodic variation of the annual and seasonal interdecadal drought duration was roughly similar, showing a cycle of 10 a approximately (Table
Annual and seasonal interdecadal drought duration (d).
Inter-decadal | Timescales | ||||
---|---|---|---|---|---|
Year | Spring | Summer | Autumn | Winter | |
60 s | 124 | 34 | 32 | 37 | 21 |
70 s | 141 | 44 | 32 | 25 | 39 |
80 s | 180 | 55 | 42 | 50 | 32 |
90 s | 159 | 42 | 35 | 43 | 43 |
Early 21 s | 153 | 51 | 32 | 39 | 34 |
Over the past nearly 50 years, the annual drought intensity, which is referred to the arithmetic mean of the sum of all the days of CI value that are all above slight drought level for total meteorological stations for each year in Shandong province, showed a fluctuation tendency (Figure
The annual drought intensity in Shandong province.
The linear tendencies of PDCA and drought duration for annual and seasonal and annual drought intensity were all insignificant; however, it is still obvious that the temporal drought parameters were salient in the 1980s and 1990s than in any other decades, which gives an evidence of the highlighting drought extent and influence in these two decades in Shandong province. In addition, the three temporal drought parameters were all the palest in 1964, demonstrating that the drought was lightest in this year in Shandong province. Moreover, taking into account the annual drought duration and intensity, it is found that the drought duration in 1999 was 291 d which was longer than 288 d in 1988, whereas the annual drought intensity was −333.56 in 1999, which was lower than −369.93 in 1988, indicating that the drought cumulative effects in 1988 were more significant than in 1999. In general, the three temporal parameters of drought, namely, PDCA, drought duration, and drought intensity, effectively depict the drought from different aspects and all of them are of great importance to the evaluation of the meteorological drought.
Owing to various factors such as global warming, abnormity of East Asia monsoon wind, and local precipitation decreasing, the drought hazard in Shandong province is becoming more and more serious. Therefore, on the basis of drought analysis by adopting and calculating the CI for 18 meteorological stations, we comprehensively evaluated the spatial-temporal characteristics of meteorological drought in Shandong province from 1962 to 2008. Main conclusions are as follows.
The frequency of meteorological drought was generally high in Shandong province, and high-frequency regions are mainly concentrated in the southwest and north areas whereas low-frequency regions are generally centered in the central and southeast. The spatial pattern of drought frequency is closely related to the distribution of precipitation which decreases from southeast to northwest and temperature which increases from northeast to southwest.
The seasonal drought frequency followed such an order of spring > autumn > summer > winter. The precipitation is rare in spring but the evaporation is high due to the land surface warming rapidly, which aggravates the spring drought. In contrast, abundant rain falls in summer and low evaporation in winter owing to low temperature result in relatively low drought frequency in these two seasons.
The slight drought, moderate drought, and severe and extreme drought were all observed in Shandong province from 1962 to 2008. And the slight drought generally occurred in the northwest and southwest, but drought of other grades was observed in different areas at different timescales. Additionally, various drought grades were longest in spring but shortest in winter which was consistent with the statistics of the calculated drought parameters and, therefore, further revealed that the spring drought was much more serious than that in the other three seasons.
With respect to the temporal trend of drought frequency, it demonstrated that regions including north, south, and Jiaodong peninsula were all suffering an ascend drought over the past nearly 50 years, but the upward trend in other regions was much evident. Besides, temporal trends of multiple drought parameters including PDCA, drought duration, and drought intensity all indicated that the drought extent and influence were very severe in the 1980s and 1990s but much mild in 1964; the time-continuous large-area droughts mainly occurred after the 1970s, and the interdecadal drought duration at annual and seasonal scales both showed a fluctuation tendency of 10 a periodic cycle approximately.
In brief, this study proves that Shandong province is a sensitive area to global climate change from the perspective of meteorological drought occurrence. What is more, the much complicated spatial-temporal characteristics of meteorological drought in Shandong province detected by this study do confirm that the regional response to climate change exhibits very significant spatial-temporal differences. The findings summed up above will in turn give us essential knowledge and guidelines on adaptation and mitigation options to address climate change. In specific, in Shandong province, critical works to be carried out are as follows:
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was financially supported by the CAS Strategic Priority Research Program (Grant no. XDA05130703), the Key Research Program of the Chinese Academy of Sciences (Grant no. KZZD-EW-14), and the Research Program of Shandong Association for Science & Technology (Grant no. 20130202). The authors appreciate the reviewers and editors for the positive remarks and insightful comments as well as suggestions.