Spatiotemporal drought analysis by the standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI) in Sichuan Province, China

Drought refers to a meteorological disaster that causes insufficient soil moisture and damage to crop water balance due to long-term lack of precipitation. With the increasing shortage of water resources, drought has become one of the hot issues of global concern. The standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI) can effectively reflect the changes in drought characteristics of different geomorphologies in Sichuan on time and space scales, to explore the difference in drought characteristics between different physiognomy types in Sichuan Province, We calculated the SPI and SPEI values based on the data of 44 meteorological stations in Sichuan Province from 1961 to 2019 and used Mann–Kendall trend test and multivariable linear regression method (MLR) to quantify the significance of the drought characteristic trends at different time and space scales. The results as follow: (1) The SPEI drought trend in plain and hilly regions was greater than that in plateau and mountain regions on all time scales (− 0.039 year−1 for 1-month in hilly, − 0.035 year−1 for 1-month in plain, − 0.14 year−1 for 1-month in plateau, − 0.026 year−1 for 1-month in mountain) and the magnitude of trend of eastern (− 4.4 to 0.1 year−1) was lager than western (− 2.1 to 2.7 year−1), means that the drought trends transfer from northwest to east. (2) The drought intensity in the western region gradually increased (0.54–1.05) and drought events mainly occurred in the southwest plateau and central mountainous regions (24–47 times), means that drought meteorological hotspots were mainly concentrated in the Sichuan basin. (3) The MLR indicated altitude (H) is not the main influencing factor that causes the spatial unevenness of precipitation in Sichuan Province, but altitude (H), temperature (T), longitude (Lo) and latitude (La) can co-determined the precipitation. The results of this study are instructive and practical for drought assessment, risk management and application decision-making in Sichuan Province, and have guiding significance for agricultural disaster prevention, mitigation and agricultural irrigation in Sichuan Province.


Scientific Reports
| (2021) 11:1280 | https://doi.org/10.1038/s41598-020-80527-3 www.nature.com/scientificreports/ topography, that is, in the transition zone between the first-level Qinghai-Tibet Plateau (3000-5000 m) and the second-level plain of the middle (1000-2000 m) and lower reaches of the Yangtze River, there are obvious characteristics of high west and low east 23 . The physiognomy type map of Sichuan Province is formed after classification and recoding function and five best factors (elevation, slope change rate, cumulative ground curve height, elevation coefficient of variation and relative elevation difference) clustering (Fig. 1). Eastern Sichuan is mainly plateaus and mountains, which altitude is above 3000 m and covers 3.2 × 10 5 km 2 approximately (67.3%). The eastern Sichuan basin is one of the four major basins in China, covering an area of 1.65 × 10 5 km 2 and located between alternating mountain areas. The eastern of Sichuan is hilly and plain and covering an area of 1.5 × 10 5 km 2 (32.7%), the elevation of the terrain decreases from west to east. There are large regional differences in rainfall distribution in Sichuan Province. The annual rainfall can reach more than 1700 mm at most and less than 400 mm at least. The annual rainfall in the middle and lower reaches of the Yangtze River is relatively uniform, reaching 1000 ~ 1800 mm 24 .
Climate data. The data in this study are from China Meteorological Science Data Sharing Service System (http://cdc.cma.gov.cn), and the monthly precipitation data of 44 meteorological stations with long data series from 1961 to 2019 are selected, and which include precipitation (mm), air temperature (℃), wind speed (m s −1 ), relative humidity (%), and sunshine duration (h). The 44 data series is well represented, with few missing records and detection period longer than 50 year (44 series have full data during the period 1961 ~ 2019). There were 3 data absence sites, accounting for 6.82% of the total data, and the absence time was part of the months between 1969 and 1970. Based on the locations of the meteorological stations, we interpolated the data at a resolution of 1 km by means of inverse distance weighted (IDW) interpolation to obtain the spatial distribution of drought and passed the homogeneity test. The research scope and the distribution of each site are shown in Fig. 1.

Standardized precipitation index (SPI). Standardized precipitation index (SPI) is an indicator repre-
senting the probability of rainfall occurrence in a certain period of time in a region. It has the advantages of simple calculation and stability, and eliminates the temporal and spatial difference of rainfall. It is sensitive to drought change and applicable to drought monitoring and assessment of climatic conditions above the monthly scale. Mckee 10 proposed the SPI in 1993 and used it to assess climate and drought change, an incomplete gamma probability density function is firstly fitted to a given frequency distribution of precipitation series: where α is a parameter about shape, β is a parameter about scale, x is the amount of precipitation, and the gamma function is presented as: the best values of α and β are estimated by the maximum likelihood method.  where x is the amount of precipitation and G(x) is Γ function related precipitation probability distribution, S is the positive and negative coefficient of cumulative probability distribution, when G(x) > 0.5, S = 1 and when G(x) ≤ 0.5, S = -1. c 0 = 2.5155, c 1 = 0.8028, c 2 = 0.0103, d 1 = 1.4327, d 2 = 0.1892, d 3 = 0.0013.

Standardized precipitation evapotranspiration index (SPEI).
Standardized precipitation evapotranspiration index (SPEI) replaces the monthly rainfall in SPI with the difference between monthly rainfall and monthly potential evapotranspiration, and takes into account the temperature factor, and introduces the influence of surface evaporation changes, which is more sensitive to the drought reaction caused by global temperature rise. In order to estimate the value of SPEI, the difference of the water balance is normalized as log-logistic probability distribution. The following equation expresses the probability density function: where parameters α, β, and γ represent scale, shape and origin, respectively. Therefore, the probability distribution function can be expressed as: Vicente-Serrano 25 calculated the SPEI as follow: When P ≤ 0.5, W = √ −2 ln(P) , and when P > 0.5, W = √ −2 ln(1 − P) , C 0 = 2.5155,C 1 = 0.8028, C 2 = 0.0203, d 1 = 1.4327, d 2 = 0.1892, d 3 = 0.0013. The categorization of drought classified by the SPI and SPEI is show in Table 1. www.nature.com/scientificreports/ Runs theory and conditional probability. Drought characteristics include drought duration, drought intensity and drought frequency. When calculating the absolute value of SPI/SPEI, the value of SPI/SPEI under normal conditions (SPI/SPEI ≥ − 1) is also calculated in general method, which will greatly affect the assessment of drought. So we used the runs theory proposed by Yevjevich 26 to define the drought intensity and drought frequency, the runs theory defines a part of the drought variable time series in which all values are lower or higher than the selected threshold, called negative or positive run, the drought intensity calculation formula is as follows: where S is drought intensity, S SPI/SPEI is SPI or SPEI value below the threshold, K is drought threshold, set to be less than or equal to -1 in this study, means the drought level is greater than moderate drought, T is the duration of the drought process. Drought frequency is used to assess the frequency of drought in the area, the formula is as follows: where N represents the time period of site detection, n represents the number of droughts at the site during the time period. Conditional probability (Cp) refers to the probability of occurrence of a given event A under the condition that another event B has occurred, which means Cp (A/B). In this study, Cp refers probability of the event probability of SPI drought in another event probability of SEPI drought , which record Cp (SPI), on the contrary record Cp (SPEI), the formula is as follows: where T SPI and T SPEI represents the the times of droughts in an area in a period of time based on the SPI/SPEI value, T SPI/SPEI and T SPEI/SPI represents based on the SPEI/SPI assessment that drought has occurred, SPI/SPEI re-evaluate the times of droughts in the area.

M-K trend test and multivariable linear regression method. Theil-Sen Median method was used
to calculate the trend value, which was usually combined with MK non-parametric test, the MK method was used to determine the significance of the trend. Theil-Sen Median method was a robust non-parametric statistical trend calculation method, which had high computational efficiency and was not sensitive to measurement error and outlier data, and was often used in trend analysis of long time series data, the formula is as follows: where, x j and x i are time series data, β greater than 0 means the time series presents an upward trend, while less than 0 means the time series presents a downward trend.
The M-K trend test is a non-parametric statistical test method. Its advantage is that the measured values do not need to follow normal distribution and are not affected by missing values and outliers. It is widely used in the trend significance test of long time series data, and its statistical method is as follows 27 : where, x j and x i are time series data, When n ≥ 8, the test statistic S is approximately normally distributed, and its mean value and variance are as follows: at a given significance level α, if |z|> z 1-α/2 , means assumption that there is no trend is rejected, The time series data have obvious trend change, z 1−α/2 is the value corresponding to the www.nature.com/scientificreports/ standard normal function distribution table at the confidence level α. When |z| is greater than 1.65, 1.96 and 2.58, it indicates that the trend has passed the significance test with a reliability of 90%, 95% and 99% respectively. In this study, the M-K trend test was used to determine the drought characteristic trend of different physiognomy on different time scales (1 month, 3 month, 6 month and 12 month) and the drought trend changes of precipitation at different time periods of each station. The multivariable linear regression method (MLR) is to apply mathematical statistics to establish a multiple regression model in which meteorological elements influence the spatial interpolation factors of meteorological elements. We set precipitation (Y) as the dependent variable, and the four independent variables that affect precipitation are altitude (H), temperature (T), longitude (L o ) and latitude (L a ) respectively. We assume that the influence of each independent variable on the dependent variable is linear, the mean value of precipitation varies uniformly with the change of the independent variable when the other independent variables remain unchanged. In this study, a multiple regression model of precipitation (Y) for altitude (H), temperature (T), longitude (L o ) and latitude (L a ) was established, and the residuals were calculated, the expression is as follows: 4 is the undetermined coefficient, ɛ is the residual value and b 0 is the constant 28 .

Result
Multi-scale patterns of the drought. The monthly SPI and SPEI was calculated at 4 time scales (1 month, 3 months, 6 months and 12 months) for different physiognomy (Plain elevation range was 0-200 m, hilly elevation range was 200-500 m, fluctuation was not more than 200 m, mountain elevation was 500-1000 m, fluctuation is not more than 200 m, plateau 1000 m above, fluctuation was more than 200 m) during the period 1961 to 2019 (Fig. 2). Then, these SPI and SPEI value at 4 time scales were averaged to characterize the drought conditions in Sichuan Province, China. There are significant differences in the sensitivity of SPI and SPEI values at different time scales, the smaller the time scale, the more obvious the wet and dry changes. The SPEI value of the Sichuan plateau, mountain, hill and plain all shows an downward trend on various time scales, but SPI value of Sichuan Plateau and mountain shows a upward trend on various time scales, which are calculated by M-K test (Table. 2). Because SPEI takes into account evapotranspiration, it is more sensitive to changes in drought, so there are some differences with SPI. This study used conditional probability (Cp) to analyze the difference between SPI and SPEI, we defined the degree of moderate drought above (SPI/SPEI ≤ − 1) as an event of drought, Cp refers probability of the event probability of SPI drought in another event probability of SPEI drought , which record Cp (SPI), on the contrary record Cp (SPEI), The Cp (SPI) and Cp (SPEI) of different landforms at each time scale are shown in Table 3 The results showed that SPEI was more sensitive to drought assessment than SPI, and could more accurately reflect dry/wet alternations in more complex regions.
To study the overall trends of SPEI and SPI in Sichuan Province, the non-parametric M-K test method was used to analyze the drought trends of different landform types on different time scales from 1961 to 2019 (Table 2). Base on the trend results, we can see that the drought conditions of hilly and plain areas continued to aggravate from 1961 to 2019, and their SPEI showed an extreme significant downward trend on all time scales (P < 0.01), and partly SPI trends indicated a significant downward trend (P < 0.05), the drought trend was gradually increasing when SPI/SPEI calculated with more lagged time scales. The SPEI drought trend in plain and hilly regions was greater than that in plateau and mountain regions on all time scales (− 0.039 year −1 for 1-month in hilly, − 0.035 year −1 for 1-month in plain, − 0.14 year −1 for 1-month in plateau, − 0.026 year −1 for 1-month in mountain), which means that the degree of drought in the eastern part of Sichuan Province is increasing, and the drought trend is gradually shifting from the southwest to the east.

Drought precipitation trend.
To further analyze the trend of drought characteristics in Sichuan Province from 1961 to 2019, as shown in Fig. 3, we analyzed the meteorological changes of 44 stations in Sichuan Province in different periods (1961-1970s, 1971-1980s, 1981-1990s, 1991-2000s, 2001-2019s) through the nonparametric MK test method based on precipitation data. In the period 1961-1970s, 47.7% of stations presented decreasing trends and only 16.7% of the sites showed significance (P < 0.05), 55.5% plateau, 75.5% hilly and 65.5% plain sites all show negative trends and the highest magnitudes trend in plateau areas was − 4.37 year −1 . In the period 1971-1980s, the drought trend in the all area gradually alleviated, and only 29.5% of the sites showed a negative trend, which was not significant (P > 0.05), the magnitude of trend varied between − 2.2 to 2.0 year −1 . However, during 1981-1990s, the rainfall in the eastern part of Sichuan gradually decreased, average monthly rainfall at the site is only 95.5 mm, and the drought trend showed a significant negative trend, and 80% of the stations in the eastern region showed a negative growth, the magnitude of trend varied between − 4.4 to 0.1 year −1 in eastern region, partly plateau areas (11%) have also begun to show negative trends. During the period from 1991 to 2000s, the drought trend in the eastern plain of Sichuan gradually slowed, and 34% of the stations showed a negative growth trend but none was significant (P > 0.05), and the magnitude of trend varied between − 3.    www.nature.com/scientificreports/ The percentages of stations with negative rainfall trends in each period (1961-1970s, 1971-1980s, 1981-1990s, 1991-2000s, 2001-2009s   Drought intensity. Based on SPEI was more sensitive to drought assessment than SPI on characterize the temporal and spatial differences of droughts, meanwhile Li 29 proved the 1-month times scale for SPEI was adopted as a good indicator of changes in drought, the spatial distribution diagram of the inter-annual variation of drought intensity was calculated by the run theory and 1-month SPEI (Fig. 4). In the 1960s, drought areas were mainly distributed in the southwest of Sichuan plateau region and the central Sichuan basin region, the maximum drought intensity was 0.72 and 0.68, respectively. In the 1970s and 1980s, drought mainly occurred in the northwest plateau region, the drought intensity range was 0.52-0.7 and 0.49-0.64 respectively, but the degree of drought gradually relieved in the central Sichuan basin, and its intensity ranged from 0.28 to 0.33. In the eastern plains and hilly regions, the degree of drought increased significantly in the 1990s, with the range of drought intensity rising from 0.28-0.45 to 0.46-0.66. However, from 2000 to 2009, the drought region shifted from the eastern plain region to the western plateau region, and the drought intensity in the eastern part of Sichuan gradually weakened (0.36-0.51), while the drought intensity in the western region gradually increased (0.54-1.05). By 2019, the southwest and northwest fringe areas of Sichuan have become drought-prone areas, with the drought intensity being 0.74 and 0.62 respectively. Sichuan Province is surrounded by the Qinghai-Tibet Plateau, Qinling Mountains and Yunnan-Guizhou Plateau, it is difficult for external air flow to enter and release internal hot air 30 . At the same time, the western plateau area has caused the southeast monsoon to sink in central Sichuan, causing frequent droughts in the Yangtze River coast. The southeast monsoon from east to west gradually sink into the Sichuan basin, its rainfall from the east to the west in turn decline, as result, there are frequent occurrences of drought in plateau and mountainous regions. The drought between 2000 and 2009 mainly occurred in eastern Sichuan because of the extreme drought in Sichuan Province in 2006 19 . The South Asian High and the subtropical regions of the Western Pacific were active, which caused the eastern Sichuan to be controlled by the continental high temperature. The eastern plain area of Sichuan province is a dense area of human activities, over-exploitation of resources, lack of water conservancy construction, and natural and human factors have caused drought to shift to the east. The trend of drought in the early twentieth century corresponded to rising levels of greenhouse gas emissions. Aerosols formed by the atmosphere and suspended solid and liquid particles affect rainfall and change cloud cover, so human economic activities are closely related to the risk of drought.  (1961-1970s, 1971-1980s, 1981-1990s, 1991-2000s, 2001-2010s, 2011-2019s), red represents a negative trend and blue represents a positive trend. (The figure was generated by ArcGIS 10.6 software, https ://deskt op.arcgi s.com/en/).   (1961-1970s, 1971-1980s, 1981-1990s, 1991-2000s, 2001-2010s, 2011-2019s) (1961-1970s, 1971-1980s, 1981-1990s, 1991-2000s, 2001-2010s, 2011-2019s)  www.nature.com/scientificreports/ occurred in northwestern plateau, especially in areas such as Dege, Seda, and Dujiangyan, the drought frequencies were 26, 27, and 23 times respectively. In the period 1991-2000, the drought frequency in the northwest plateau dropped to 10-24 times, while the drought frequency in the eastern basin rose to 34 times in Yongxu county. After 2000, the frequency of drought increased significantly compared with 1961-1999, which mainly occurred in the southwest plateau and central mountainous regions, the drought frequency ranged from 24 to 47 times. According to the results of drought frequency in 1961-2019, we found that the Drought meteorological hotspots were mainly concentrated in plateau and mountainous areas. The drought frequency has been between 7 and 27 times before 2000, and the drought frequency has increased significantly after 2000, reaching the highest in Muli (47 times). Comparing the drought duration data in Southwest China, we found that there were extreme drought events in Southwest China in 2006 and 2010. Statistics show that the disaster area of crops in Sichuan Province reached 5.1 × 10 5 km, the disaster-affected area was 2.5 × 10 5 km, and the no-harvest area was 5.7 × 10 4 km. The affected population was 8.28 million and the economic loss was 1.38 billion yuan 21 . In the early 1960s, due to backward water conservancy projects and over-deforestation, serious soil erosion was caused, and the ecosystem and water circulation system were disordered and rainfall decreased. The extreme drought event in southwest China in 2006 was due to the weakening of the westerly circulation effect due to the abnormal subtropical high in the western Pacific, which affected the significant decrease in the intensity and frequency of southward cold air activity.
For the analysis of drought frequency factors in Sichuan Province, synoptic climatology was a major factor. we found that the anomalies of the Western Pacific Subtropical High and Continental Subtropical High, the northerly of the Western Pacific Subtropical High and the easterly of the South Asian High have made the downdraft in Sichuan stronger and inhibited the transportation of water vapor in the Bay of Bengal, it was the main reason for the severe drought in Sichuan in 2006. Sea temperature also has a certain impact on rainfall in Sichuan Province. The tropical western Pacific and Indian Ocean are warming up, causing an abnormal anticyclonic circulation over the tropical western Pacific, causing anomalous southwestern airflow and strengthening along the southeast coast of China, and sinking in the east of the plateau by the northwest. Airflow control has made it difficult for the water vapor in the Bay of Bengal to reach Sichuan Province, causing long-term lack of rainfall in the area. At the same time, the reason of drought prone areas of Sichuan province in addition to the geographical factors, Sichuan province is located in the basin, surrounded by the Tibetan plateau, Qinling and Yunnan-Guizhou plateau, the air is not easy to enter, and the internal air also is not easy to release, forming climate effect of bamboo steamer, south of the Yunnan-Guizhou plateau of Sichuan province made the southeast monsoon in the basin subsidence, along the Yangtze river region drought less rain, which making the drought aggravated.

Discussion
Some studies have analyzed the causes of frequent droughts in Sichuan Province, and concluded that three major factors are the special climate caused by topography, the uneven temporal and spatial distribution of rainfall, and the abnormal atmospheric circulation 8,12,16,22 . Li using the unary linear regression trend analysis method to analyze the drought in Sichuan Province, it is found that the drought trend in the northwest region has gradually eased in the past ten years, while the drought in the Sichuan Basin and the western plateau area has gradually increased, and the drought trend changes between different terrains are significantly different 16 . The study found that the monthly average temperature of Sichuan for many years is negatively correlated with the latitude. At the same time, the longitude of the average temperature distribution in Sichuan is much greater than the latitude zonality, and the monthly average temperature for many years is negatively correlated with the altitude, indicating that the temperature, latitude, longitude and altitude are main factors affecting drought and rainfall. In addition to the factors caused by the internal topography of Sichuan Province, the atmospheric circulation is also a direct cause of abnormal weather and climate. Antonio found that active convective activities in the subtropical region of the western Pacific can cause the western continental sinking airflow to prevail, rainfall is suppressed, and the temperature is abnormally high 31 .
Sichuan Province has significant regional differences in climate and is dominated by vertical climate. The eastern part of Sichuan Province belongs to the subtropical climate region, while the western high principle belongs to the Qinghai-Tibet Plateau climate region 32 . The subtropical climate is mainly concentrated in the mountainous river region and the bottom area of the basin, both of which are dominated by the vertical climate 33 . Above the mountain, there is a temperature or even a frigid climate, with large vertical differences in space. The climatic area of the Qinghai-Tibet Plateau is dominated by sub-frigid climate, and the altitude difference causes the vertical distribution to be more obvious than that of mountainous areas 34 . The complex spatial distribution of rainfall in Sichuan Province, which is not only restricted by atmospheric conditions but also affected by topography and altitude. We use multivariable linear regression method (MLR) to establish a multiple regression model of average annual precipitation in different periods based on altitude (H), temperature (T), longitude (L o ) and latitude (L a ), and calculate its significance coefficients ( Table 4). The results show that altitude, temperature, longitude and latitude can significantly affect the average annual precipitation in each period. Precipitation and altitude, temperature and longitude all show significant negative trends (P < 0.05), means that the higher the altitude and temperature caused less precipitation. But the previous study showed that the temperature decreases with the increase in altitude, and the temperature decreases 0.65℃ for every 100 m increase in altitude 35 , this can explain why the coefficient of influence of altitude on rainfall is lower than that of temperature and longitude. Therefore, altitude is not the main influencing factor that causes the spatial unevenness of precipitation in Sichuan Province, the precipitation is determined by multiple factors of geographical features. According to the regression coefficient, annual rainfall decreases by 15.1-25.8 mm for every 100 m of elevation increase, and annual rainfall www.nature.com/scientificreports/ decreases by 20.7-71.5 mm for every degree of temperature rise, but the largest effect is the change in latitude and longitude, the more eastern part of the Sichuan Province receives less annual rainfall. Calculate the trend changes of SPEI of different geomorphological type on different time scales according to MK test, we found that the four types all showed negative trends from 1961 to 2019, and the hills and plains regions showed extreme significant negative trends (P < 0.01), the trend coefficient of hills and plains is also greater than that of plateaus and mountainous regions, means that the degree of drought in the eastern part of Sichuan Province is gradually aggravated. At the same time, the drought precipitation trend demonstrated decreasing trends transfer from northwest to east and gather in central Sichuan Province. Some previous studies on Southwest China have shown that 65% of the regional droughts in parts of Southwest China have increased significantly in drought intensity and duration, especially in Sichuan Province, and extreme drought indicate that the trend coefficient from east to west in the southwest region is decreasing 14,15 . Therefore, it is particularly important to study the distribution of drought characteristics in Sichuan Province.
Although the degree of drought in the eastern part of Sichuan Province continues to increase, but according to the spatial distribution trends of drought intensity and drought frequency in different periods from 1961 to 2019, we found that the western plateau area of Sichuan Province is still a drought-prone area, and the drought meteorological hotspots are mainly in the plateau and intermountain basins. Wu based on the principle of vegetation water supply index (VWSI) test the Sichuan drought spatial distribution and in-site time series analysis, the results show that drought meteorological hotspots are mainly concentrated in the Sichuan basin, because this area is surrounded by mountains, which is difficult for the continental monsoon to release after the basin sinks, causing this area to become a hotspot with frequent droughts 19 . According to the distribution characteristics of drought in different time and space, it can be helpful to build a targeted drought control strategy, such as strengthening the construction of farmland water conservancy infrastructure regionally, popularizing farmland water-saving technology locality, and adjusting agricultural planting structure periodically.

Conclusions
In this study, the meteorological and geographical drought characteristic in Sichuan Province of China were evaluated with SPEI, SPI, drought precipitation trend, drought intensity and drought intensity respectively on different time and space scales. The main conclusions are listed as follows: (1) There are significant differences in drought characteristics between SPI and SPEI in some regions and time periods, We used conditional probability (Cp) to analyze the difference between SPI and SPEI, the Cp (SPI) decreases continuously while the Cp (SPEI) increases continuously in the plateau region from 1961 to 2019, and Cp (SPEI) was greater than Cp (SPI) on the time scale of all regions indicated that SPEI was more sensitive to drought assessment than SPI, and could more accurately reflect dry/wet alternations in more complex regions. (2) The SPEI drought trend in plain and hilly regions was greater than that in plateau and mountain regions on all time scales (− 0.039 year −1 for 1-month in hilly, − 0.035 year −1 for 1-month in plain, − 0.14 year −1 for 1-month in plateau, − 0.026 year −1 for 1-month in mountain), and 80% of the stations in the eastern region showed a negative growth, the magnitude of trend varied between − 4.4 to 0.1 year −1 in eastern region, means that the degree of drought in the eastern part of Sichuan Province is gradually aggravated and the drought trends transfer from northwest to east. (3) Based on drought intensity and drought frequency, we found the drought intensity in the eastern part of Sichuan gradually weakened (0.36-0.51), while the drought intensity in the western region gradually increased (0.54-1.05) and drought events mainly occurred in the southwest plateau and central mountainous regions (24-47 times), means that drought meteorological hotspots was mainly concentrated in the Sichuan basin. (4) By using multivariable linear regression method (MLR) to establish a multiple regression model of average annual precipitation in different periods, the correlation coefficient of influence of altitude on rainfall is lower than that of temperature and longitude, indicated altitude is not the main influencing factor that causes the spatial unevenness of precipitation in Sichuan Province. According to the correlation coefficient of each factor, the difference of geographical location and temperature is the main influencing factor of the uneven rainfall. www.nature.com/scientificreports/ (5) The main reasons for the frequent drought in Sichuan province can be attributed to the synoptic climatology, the uneven spatial and temporal distribution of rainfall and the anomaly of atmospheric circulation. The subsidence is prevalent in Sichuan province all the year round, and the surrounding mountains form a special steamer effect, which exacerbates the climatic drought events.