Propagation characteristics from meteorological drought to agricultural drought over the Heihe River Basin, Northwest China

In the context of global warming,drought events occur frequently.In order to better understanding the process and mechanism of drought occurrence and evolution,scholars have dedicated much attention on drought propagation,mainly focusing on drought propagation time and propagation probability.However,there are relatively few studies on the sensitivities of drought propagation to seasons and drought levels.Therefore,we took the Heihe River Basin(HRB)of Northwest China as the case study area to quantify the propagation time and propagation probability from meteorological drought to agricultural drought during the period of 1981–2020,and subsequently explore their sensitivities to seasons(irrigation and non-irrigation seasons)and drought levels.The correlation coefficient method and Copula-based interval conditional probability model were employed to determine the drought propagation time and propagation probability.The results determined the average drought propagation time as 8 months in the whole basin,which was reduced by 2 months(i.e.,6 months)on average during the irrigation season and prolonged by 2 months(i.e.,10 months)during the non-irrigation season.Propagation probability was sensitive to both seasons and drought levels,and the sensitivities had noticeable spatial differences in the whole basin.The propagation probability of agricultural drought at different levels generally increased with the meteorological drought levels for the upstream,midstream,and southern downstream regions of the HRB.Lesser agricultural droughts were more likely to be triggered during the irrigation season,while severer agricultural droughts were occurred mostly during the non-irrigation season.The research results are helpful to understand the characteristics of drought propagation and provide a scientific basis for the prevention and control of droughts.This study is of great significance for the rational planning of local water resources and maintaining good ecological environment in the HRB.

Future meteorological drought conditions in southwestern Iran based on the NEX-GDDP climate dataset

Investigation of the climate change effects on drought is required to develop management strategies for minimizing adverse social and economic impacts.Therefore,studying the future meteorological drought conditions at a local scale is vital.In this study,we assessed the efficiency of seven downscaled Global Climate Models(GCMs)provided by the NASA Earth Exchange Global Daily Downscaled Projections(NEX-GDDP),and investigated the impacts of climate change on future meteorological drought using Standard Precipitation Index(SPI)in the Karoun River Basin(KRB)of southwestern Iran under two Representative Concentration Pathway(RCP)emission scenarios,i.e.,RCP4.5 and RCP8.5.The results demonstrated that SPI estimated based on the Meteorological Research Institute Coupled Global Climate Model version 3(MRI-CGCM3)is consistent with the one estimated by synoptic stations during the historical period(1990-2005).The root mean square error(RMSE)value is less than 0.75 in 77%of the synoptic stations.GCMs have high uncertainty in most synoptic stations except those located in the plain.Using the average of a few GCMs to improve performance and reduce uncertainty is suggested by the results.The results revealed that with the areas affected by wetness decreasing in the KRB,drought frequency in the North KRB is likely to increase at the end of the 21st century under RCP4.5 and RCP8.5 scenarios.At the seasonal scale,the decreasing trend for SPI in spring,summer,and winter shows a drought tendency in this region.The climate-induced drought hazard can have vast consequences,especially in agriculture and rural livelihoods.Accordingly,an increasing trend in drought during the growing seasons under RCP scenarios is vital for water managers and farmers to adopt strategies to reduce the damages.The results of this study are of great value for formulating sustainable water resources management plans affected by climate change.

Spatiotemporal Analysis of Meteorological Drought Using Standardized Precipitation Index (SPI) in Gabiley Region, Somaliland

Drought is a common natural disaster worldwide, with varying durations, severity levels, and spatial extents. This study aimed to model the spatiotemporal variation of meteorological drought events in the Gabiley region of Somaliland. The study utilized primary data collected from the meteorological station in Gabiley and CHIRPS (Climate Hazards Group InfraRed Precipitation with Station) data to develop the standardized precipitation index (SPI) at a 3-month timescale. The results of the study revealed that the study area was characterized by drought and received inadequate precipitation, resulting in catastrophic droughts that negatively impacted the socioeconomic situation of the community. Mild-to-severe meteorological drought events occurred every two to three years, with the most severe droughts occurring in 1998, 2002, 2009, 2015, and 2017. Specifically, the year 2015 experienced the most severe meteorological drought in the region during the studied period. The predominant type of drought was a mild year in the study area. The SPI was found to potentially identify meteorological drought, making it a useful tool for policymakers as they develop drought adaptation and mitigation policies. This study provides valuable information that can benefit local authorities and policymakers in creating drought mitigation and adaptation strategies in the Gabiley region.

Estimation of meteorological drought indices based on AgMERRA precipitation data and station-observed precipitation data

Meteorological drought is a natural hazard that can occur under all climatic regimes. Monitoring the drought is a vital and important part of predicting and analyzing drought impacts. Because no single index can represent all facets of meteorological drought, we took a multi-index approach for drought monitoring in this study. We assessed the ability of eight precipitation-based drought indices(SPI(Standardized Precipitation Index), PNI(Percent of Normal Index), DI(Deciles index), EDI(Effective drought index), CZI(China-Z index), MCZI(Modified CZI), RAI(Rainfall Anomaly Index), and ZSI(Z-score Index)) calculated from the station-observed precipitation data and the Ag MERRA gridded precipitation data to assess historical drought events during the period 1987–2010 for the Kashafrood Basin of Iran. We also presented the Degree of Dryness Index(DDI) for comparing the intensities of different drought categories in each year of the study period(1987–2010). In general, the correlations among drought indices calculated from the Ag MERRA precipitation data were higher than those derived from the station-observed precipitation data. All indices indicated the most severe droughts for the study period occurred in 2001 and 2008. Regardless of data input source, SPI, PNI, and DI were highly inter-correlated(R^2=0.99). Furthermore, the higher correlations(R^2=0.99) were also found between CZI and MCZI, and between ZSI and RAI. All indices were able to track drought intensity, but EDI and RAI showed higher DDI values compared with the other indices. Based on the strong correlation among drought indices derived from the Ag MERRA precipitation data and from the station-observed precipitation data, we suggest that the Ag MERRA precipitation data can be accepted to fill the gaps existed in the station-observed precipitation data in future studies in Iran. In addition, if tested by station-observed precipitation data, the Ag MERRA precipitation data may be used for the data-lacking areas.

Meteorological drought in semi-arid regions:A case study of Iran

Drought occurs in almost all climate zones and is characterized by prolonged water deficiency due to unbalanced demand and supply of water,persistent insufficient precipitation,lack of moisture,and high evapotranspiration.Drought caused by insufficient precipitation is a temporary and recurring meteorological event.Precipitation in semi-arid regions is different from that in other regions,ranging from 50 to 750 mm.In general,the semi-arid regions in the west and north of Iran received more precipitation than those in the east and south.The Terrestrial Climate(TerraClimate)data,including monthly precipitation,minimum temperature,maximum temperature,potential evapotranspiration,and the Palmer Drought Severity Index(PDSI)developed by the University of Idaho,were used in this study.The PDSI data was directly obtained from the Google Earth Engine platform.The Standardized Precipitation Index(SPI)and the Standardized Precipitation Evapotranspiration Index(SPEI)on two different scales were calculated in time series and also both SPI and SPEI were shown in spatial distribution maps.The result showed that normal conditions were a common occurrence in the semi-arid regions of Iran over the majority of years from 2000 to 2020,according to a spatiotemporal study of the SPI at 3-month and 12-month time scales as well as the SPEI at 3-month and 12-month time scales.Moreover,the PDSI detected extreme dry years during 2000-2003 and in 2007,2014,and 2018.In many semi-arid regions of Iran,the SPI at 3-month time scale is higher than the SPEI at 3-month time scale in 2000,2008,2014,2015,and 2018.In general,this study concluded that the semi-arid regions underwent normal weather conditions from 2000 to 2020.In a way,moderate,severe,and extreme dry occurred with a lesser percentage,gradually decreasing.According to the PDSI,during 2000-2003 and 2007-2014,extreme dry struck practically all hot semi-arid regions of Iran.Several parts of the cold semi-arid regions,on the other hand,only experienced moderate to severe dry from 2000 to 2003,except for the eastern areas and wetter regions.The significance of this study is the determination of the spatiotemporal distribution of meteorological drought in semi-arid regions of Iran using strongly validated data from TerraClimate.

Response of hydrological drought to meteorological drought in the eastern Mediterranean Basin of Turkey

The hydrographic eastern Mediterranean Basin of Turkey is a drought sensitive area.The basin is an important agricultural area and it is necessary to determine the extent of extreme regional climatic changes as they occur in this basin.Pearson’s correlation coefficient was used to show the correlation between standardized precipitation index(SPI)and standardized streamflow index(SSI)values on different time scales.Data from five meteorological stations and seven stream gauging stations in four sub-basins of the eastern Mediterranean Basin were analyzed over the period from 1967 to 2017.The correlation between SSI and SPI indicated that in response to meteorological drought,hydrological drought experiences a one-year delay then occurs in the following year.This is more evident at all stations from the mid-1990 s.The main factor causing hydrological drought is prolonged low precipitation or the presence of a particularly dry year.Results showed that over a long period(12 months),hydrological drought is longer and more severe in the upper part than the lower part of the sub-basins.According to SPI-12 values,an uninterrupted drought period is observed from 2002–2003 to 2008–2009.Results indicated that among the drought events,moderate drought is the most common on all timescales in all sub-basins during the past 51 years.Long-term dry periods with moderate and severe droughts are observed for up to 10 years or more since the late 1990 s,especially in the upper part of the sub-basins.As precipitation increases in late autumn and early winter,the stream flow also increases and thus the highest and most positive correlation values(0.26–0.54)are found in January.Correlation values(ranging between–0.11 and–0.01)are weaker and negative in summer and autumn due to low rainfall.This is more evident at all stations in September.The relation between hydrological and meteorological droughts is more evident,with the correlation values above 0.50 on longer timescales(12-and 24-months).The results presented in this study allow an understanding of the characteristics of drought events and are instructive for overcoming drought.This will facilitate the development of strategies for the appropriate management of water resources in the eastern Mediterranean Basin,which has a high agricultural potential.

Impact of Meteorological Drought on Streamflows in the Lobo River Catchment at Nibéhibé, Côte d’Ivoire

The management of water resources in watersheds has become increasingly difficult in recent years due to the frequency and intensity of drought sequences. The Lobo River catchment, like most tropical regions, has experienced alternating wet and dry periods. These drought periods have a significant impact on the availability of water resources in the basin. The objective of this study is to analyse the impact of meteorological drought on flows in the Lobo River catchment. Therefore, using the Normalized Precipitation and Evapotranspiration Index (SPEI) and the Drought Flow Index (SDI), the characteristics of droughts were studied. The results of this study show that meteorological droughts were more frequent than hydrological droughts in the Lobo River watershed. However, the hydrological drought was longer and more intense than the meteorological drought. The greater relationship between meteorological and hydrological drought was observed at the Daloa and Vavoua station (0.43 < r < 0.50) compared to the Zuenoula station (r < 0.5). In addition, there was a resumption of precipitation and runoff between 2007 and 2013 in the basin. The study of these climatic trends would be very useful in the choice of management and adaptation policies for water resources management.

Prediction of Meteorological Drought in the Lower Nu River by Statistical Model

Global climate change, temperature rise and some kinds of extreme meteorological disaster, such as the drought, threaten the development of the natural ecosystem and human society. Forecasting in drought is an important step toward developing a disaster mitigation system. In this study, we utilized the statistical, autoregressive integrated moving average (ARIMA) model to predict drought conditions based on the standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI) in a major tributary in the lower reaches of Nu River. We employed data from 2001 to 2010 to fit the model and data from 2011 to 2013 for model validation. The results showed that the coefficients of determination (R2) was over 0.85 in each index series, and the root-mean-square error and mean absolute error were low, implying that the ARIMA model is effective and adequate for this region.

Influence of varied drought types on soil conservation service within the framework of climate change:insights from the Jinghe River Basin,China

Severe soil erosion and drought are the two main factors affecting the ecological security of the Loess Plateau,China.Investigating the influence of drought on soil conservation service is of great importance to regional environmental protection and sustainable development.However,there is little research on the coupling relationship between them.In this study,focusing on the Jinghe River Basin,China as a case study,we conducted a quantitative evaluation on meteorological,hydrological,and agricultural droughts(represented by the Standardized Precipitation Index(SPI),Standardized Runoff Index(SRI),and Standardized Soil Moisture Index(SSMI),respectively)using the Variable Infiltration Capacity(VIC)model,and quantified the soil conservation service using the Revised Universal Soil Loss Equation(RUSLE)in the historical period(2000-2019)and future period(2026-2060)under two Representative Concentration Pathways(RCPs)(RCP4.5 and RCP8.5).We further examined the influence of the three types of drought on soil conservation service at annual and seasonal scales.The NASA Earth Exchange Global Daily Downscaled Projections(NEX-GDDP)dataset was used to predict and model the hydrometeorological elements in the future period under the RCP4.5 and RCP8.5 scenarios.The results showed that in the historical period,annual-scale meteorological drought exhibited the highest intensity,while seasonal-scale drought was generally weakest in autumn and most severe in summer.Drought intensity of all three types of drought will increase over the next 40 years,with a greater increase under the RCP4.5 scenario than under the RCP8.5 scenario.Furthermore,the intra-annual variation in the drought intensity of the three types of drought becomes smaller under the two future scenarios relative to the historical period(2000-2019).Soil conservation service exhibits a distribution pattern characterized by high levels in the southwest and southeast and lower levels in the north,and this pattern has remained consistent both in the historical and future periods.Over the past 20 years,the intra-annual variation indicated peak soil conservation service in summer and lowest level in winter;the total soil conservation of the Jinghe River Basin displayed an upward trend,with the total soil conservation in 2019 being 1.14 times higher than that in 2000.The most substantial impact on soil conservation service arises from annual-scale meteorological drought,which remains consistent both in the historical and future periods.Additionally,at the seasonal scale,meteorological drought exerts the highest influence on soil conservation service in winter and autumn,particularly under the RCP4.5 and RCP8.5 scenarios.Compared to the historical period,the soil conservation service in the Jinghe River Basin will be significantly more affected by drought in the future period in terms of both the affected area and the magnitude of impact.This study conducted beneficial attempts to evaluate and predict the dynamic characteristics of watershed drought and soil conservation service,as well as the response of soil conservation service to different types of drought.Clarifying the interrelationship between the two is the foundation for achieving sustainable development in a relatively arid and severely eroded area such as the Jinghe River Basin.

Spatiotemporal Variations of Meteorological Droughts in China During 1961–2014: An Investigation Based on Multi-Threshold Identification

As a major agricultural country, China suffers from severe meteorological drought almost every year.Previous studies have applied a single threshold to identify the onset of drought events, which may cause problems to adequately characterize long-term patterns of droughts.This study analyzes meteorological droughts in China based on a set of daily gridded(0.5° 9 0.5°) precipitation data from 1961 to 2014. By using a multi-threshold run theory approach to evaluate the monthly percentage of precipitation anomalies index(Pa), a drought events sequence was identified at each grid cell. The spatiotemporal variations of drought in China were further investigated based on statistics of the frequency, duration,severity, and intensity of all drought events. Analysis of the results show that China has five distinct meteorological drought-prone regions: the Huang-Huai-Hai Plain, Northeast China, Southwest China, South China coastal region,and Northwest China. Seasonal analysis further indicates that there are evident spatial variations in the seasonal contribution to regional drought. But overall, most contribution to annual drought events in China come from the winter. Decadal variation analysis suggests that most of China's water resource regions have undergone an increase in drought frequency, especially in the Liaohe, Haihe, and Yellow River basins, although drought duration and severity clearly have decreased after the 1960 s.

Comprehensive evaluation of hydrological drought characteristics and their relationship to meteorological droughts in the upper Tarim River Basin,central Asia

Comprehensive evaluation of the characteristics and mechanisms of droughts is of great significance to drought risk prediction and prevention.The 3-monthly scale Standardized Runoff Index(SRI-3)and 3-monthly scale Standardized Precipitation Evapotranspiration Index(SPEI-3)were employed herein to characterize hydrological and meteorological droughts,respectively,within the four upper subbasins of the Tarim River Basin(TRB)during 1961-2015.The propagation of droughts was also evaluated.The hydrological drought duration(Dd)and drought severity(Ds)were determined by Run theory,and Copula functions were adopted to investigate the hydrological drought probabilities and return periods.The propagation relationships of hydrological and meteorological droughts were assessed.The results indicated that:1)hydrological drought index(SRI-3)significantly increased in the TRB from 1961 to 2015;the increase was most significant in winter.Meteorological drought index(SPEI-3)exhibited a weak upward trend through time;2)the characteristics of hydrological droughts varied between the subbasins;increases in the SRI were most significant in the Yarkand and Hotan Rivers,whereas the Dd and Ds of hydrological droughts were higher in the Kaidu and Yarkand Rivers;3)Frank Copula was the most closely fitted Copula function in the four subbasins of the TRB and yielded average drought return periods of 4.86,4.78,3.72,and 5.57 years for the Kaidu,Aksu,Yarkand,and Hotan River Basins,respectively.The return periods in the four subbasins were generally less than 10 years from 1961 to 2015;4)a cross wavelet transform(XWT)exhibited a significant positive correlation between hydrological and meteorological droughts,except for the Yarkand River Basin,which exhibited a significant negative correlation.Besides,the propagation relationship of meteorological droughts to hydrological droughts showed remarkable seasonal variations.

Multiscale spatiotemporal meteorological drought prediction:A deep learning approach

Reliable monitoring and thorough spatiotemporal prediction of meteorological drought are crucial for early warning and decision-making regarding drought-related disasters.The utilisation of multiscale methods is effective for a comprehensive evaluation of drought occurrence and progression,given the complex nature of meteorological drought.Nevertheless,the nonlinear spatiotemporal features of meteorological droughts,influenced by various climatological,physical and environmental factors,pose significant challenges to integrated prediction that considers multiple indicators and time scales.To address these constraints,we introduce an innovative deep learning framework based on the shifted window transformer,designed for executing spatiotemporal prediction of meteorological drought across multiple scales.We formulate four prediction indicators using the standardized precipitation index and the standard precipitation evaporation index as core methods for drought definition using the ERA5 reanalysis dataset.These indicators span time scales of approximately 30 d and one season.Short-term indicators capture more anomalous variations,whereas long-term indicators attain comparatively higher accuracy in predicting future trends.We focus on the East Asian region,notable for its diverse climate conditions and intricate terrains,to validate the model's efficacy in addressing the complexities of nonlinear spatiotemporal prediction.The model's performance is evaluated from diverse spatiotemporal viewpoints,and practical application values are analysed by representative drought events.Experimental results substantiate the effectiveness of our proposed model in providing accurate multiscale predictions and capturing the spatiotemporal evolution characteristics of drought.Each of the four drought indicators accurately delineates specific facets of the meteorological drought trend.Moreover,three representative drought events,namely flash drought,sustained drought and severe drought,underscore the significance of selecting appropriate prediction indicators to effectively denote different types of drought events.This study provides methodological and technological support for using a deep learning approach in meteorological drought prediction.Such findings also demonstrate prediction issues related to natural hazards in regions with scarce observational data,complex topography and diverse microclimate systems.

Spatial Rainfall Variability and an Increasing Threat of Drought, According to Climate Change in Uttaradit Province, Thailand

This study presents the work commenced in northern Thailand on spatial and temporal variability of rainfall. Thirty years (1988-2017) rainfall data of eight meteorological stations were used for assessing temporal variability and trend analysis. The results showed decreasing trend in rainfall from its first half of the observed study period (1988-2002) to last half of the time period (2003-2017) in total average annual as well as monsoonal average rainfall by 14.92% and 15.50% respectively. It was predicted from linear regression results that by 2030 the average annual and monsoonal rainfall will drop by 35% and 34.10% respectively. All stations showed negative trend except Fakara met-station in annual rainfall. In the seasonal trend analysis, the results showed decreasing trend almost in all met-stations. Mann-Kendall trend test was applied to assess the trend. All met-stations show significant negative trend. To assess drought in the study area, Standardized Precipitation Index (SPI) was applied to 12-month temporal time period. The results predicted meteorological drought in the near future. The spatial distribution of rainfall presented changing phenomena in average annual, monsoonal, winter, and summer seasons in both analyzed periods.

Causes and Changes of Drought in China:Research Progress and Prospects

Drought is one of the most serious and extensive natural hazards in the world.Subject to monsoon climate variability,China is particularly influenced by drought hazards,especially meteorological drought.Based on a comprehensive understanding of the current status of international drought research,this paper systematically reviews the history and achievements of drought research in China since the founding of the People’s Republic of China,from four main perspectives:characteristics and spatiotemporal distribution of historical and recent drought events,drought formation mechanism and change trend,drought hazard risk,and the particular flash drought.The progress and problems of drought research in China are analyzed and future prospects are proposed,with emphasis on the multi-factor synergetic effect for drought formation;the effect of land-atmosphere interaction;identification,monitoring,and prediction of flash drought;categorization of drought and characteristics among various types of drought;the agricultural drought development;drought response to climate warming;and assessment of drought hazard risks.It is suggested that strengthening scientific experimental research on drought in China is imperative.The present review is conducive to strategic planning of drought research and application,and may facilitate further development of drought research in China.

A daily drought index based on evapotranspiration and its application in regional drought analyses

With climate warming, frequent drought events have occurred in recent decades, causing huge losses to industrial and agricultural production, and affecting people’s daily lives. The monitoring and forecasting of drought events has drawn increasing attention. However, compared with the various monthly drought indices and their wide application in drought research,daily drought indices, which would be much more suitable for drought monitoring and forecasting, are still scarce. The development of a daily drought index would improve the accuracy of drought monitoring and forecasting, and facilitate the evaluation of existing indices. In this study, we constructed a new daily drought index, the daily evapotranspiration deficit index(DEDI), based on actual and potential evapotranspiration data from the high-resolution ERA5 reanalysis dataset of the European Center for Medium-Range Weather Forecasts. This new index was then applied to analyze the spatial and temporal evolution characteristics of four drought events that occurred in southwest, north, northeast, and eastern northwest China in the spring and summer of 2019. Comparisons with the operationally used Meteorological Drought Composite Index and another commonly used index, the Standardized Precipitation Evapotranspiration Index, indicated that DEDI characterized the spatiotemporal evolution of the four drought events reasonably well and was superior in depicting the onset and cessation of the drought events,as well as multiple drought intensity peaks. Additionally, DEDI considers land surface conditions, such as vegetation coverage,which enables its potential application not only for meteorological purposes but also for agricultural drought warning and monitoring.

Role of Intraseasonal Oscillation in a Compound Drought and Heat Event over the Middle of the Yangtze River Basin during Midsummer 2018

In late July 2018, a compound drought and heat event(CDHE) occurred in the middle of the Yangtze River basin(MYRB) and caused great damage to the national economy. The CDHE over the MYRB has been documented to be linked with intraseasonal oscillations(ISOs) from different regions. However, specific roles of different ISOs on the development of the CDHE cannot be separated in the observational analysis. By using partial lateral forcing experiments driven by ISO in the Weather Research and Forecasting(WRF) model, we found that the midlatitude ISO generated by a westerly wave train in the upper troposphere played an important role in this heatwave and drought event in the northern MYRB, causing a regional average temperature rise of 1.65°C and intensification of drought over23.49% of the MYRB area. On the other hand, the ISO associated with the Pacific-Japan(PJ)-like teleconnection wave train in the lower troposphere induced a more pronounced impact on the event, causing an average temperature rise of 2.44°C, intensifying drought over 29.62% of the MYRB area. The MYRB was mainly affected by northward warm advection driven by the westward extension of the western North Pacific subtropical high in the early period of the CDHE development. In the late period, because of the establishment of a deep positive geopotential height field through the troposphere leading to intensive local subsidence, there was a remarkable temperature rise and moisture decrease in the MYRB. The results will facilitate a better understanding of the occurrence of CDHE and provide empirical precursory signals for subseasonal forecast of CDHE.

1960-2016年中国黄土高原气象干旱趋势的归因

This study uses two forms of the Palmer Drought Severity Index(PDSI),namely the PDSI_TH(potential evapotranspiration estimated-by the Thornthwaite equation)and the PDSI_PM(potential evapotranspiration estimated by the FAO Penman-Monteith equation),to characterize the meteorological drought trends during 1960–2016 in the Loess Plateau(LP)and its four subregions.By designing a series of numerical experiments,we mainly investigated various climatic factors'contributions to the drought trends at annual,summer,and autumn time scales.Overall,the drying trend in the PDSI_TH is much larger than that in the PDSI_PM.The former is more sensitive to air temperature than precipitation,while the latter is the most sensitive to precipitation among all meteorological factors.Increasing temperature results in a decreasing trend(drying)in the PDSI_TH,which is further aggravated by decreasing precipitation,jointly leading to a relatively severe drying trend.For the PDSI_PM that considers more comprehensive climatic factors,the drying trend is partly counteracted by the declining wind speed and solar radiation.Therefore,the PDSI_PM ultimately shows a much smaller drying trend in the past decades.