Radial growth in Qinghai spruce is most sensitive to severe drought in the Qilian Mountains of Northwest China

Global warming and frequent extreme drought events lead to tree death and extensive forest decline,but the underlying mechanism is not clear.In drought years,cambial development is more sensitive to climate change,but in different phenological stages,the response rela-tionship is nonlinear.Therefore,the dynamic relationship between tree radial growth and climatic/environmental fac-tors needs to be studied.We thus continuously monitored radial growth of Qinghai spruce(Picea crassifolia Kom.)and environmental factors from January 2021 to November 2022 using point dendrometers and portable meteorological weather stations in the central area of the Qilian Mountains.The relationship and stability between the radial growth of Qinghai spruce and environmental factors were compared for different levels of drought in 2021 and 2022.The year 2022 had higher temperatures and less precipitation and was drier than 2021.Compared with 2021,the growing period in 2022 for Qinghai spruce was 10 days shorter,maximum growth rate(Grmax)was 4.5μm·d^(-1) slower,and the initiation of growth was 6 days later.Growth of Qinghai spruce was always restricted by drought,and the stem radial increment(SRI)was more sensitive to precipitation and air relative humidity.Seasonal changes in cumulative radial growth were divided into four phenological stages according to the time of growth onset,cessation,and maximum growth rate(Grmax)of Qinghai spruce.Stability responses of SRI to climate change were stronger in Stage 3 and Stage 4 of 2021 and stronger in Stage 1(initiation growth stage)and Stage 3 of 2022.The results provide important information on the growth of the trees in response to drought and for specific managing forests as the climate warms.

Severe Drought Events as Revealed in the Climate Records of China and Their Temperature Situations over the Last 1000 Years

The total 15 severe droughts are discovered with the aid of the 'RetrievalSystem of Chinese Historical Climate Records' for the last 1000 years. The droughts are extensive toenvelope more than 4 provinces and persistent to cover 3 yr or more, and their severity isequivalent to or in excess of that in the 1930s in China. According to the documentary records andrestorations it can be inferred that most droughts are more severe than those in the last 50 years.The 15 droughts may either occur warm or in a cold climate background, with 11 of the 15 cases inthe cold phase. This indicates the difference in climate correspondence between China and northernAmerica, showing the severity of the events in China to be in a cold instead of a warm climatesituation. That is likely to relate to the monsoon climate in eastern Asia.

Features of the Extremely Severe Drought in the East of Southwest China and Anomalies of Atmospheric Circulation in Summer 2006

The spatial-temporal features of the extremely severe drought and the anomalous atmospheric circulation in summer 2006 are analyzed based on the NCEP/NCAR reanalysis data,the characteristic circulation indices given by the National Climate Center of China,and the daily precipitation data of 20 stations in the east of Southwest China（ESC） from 1959 to 2006.The results show that the rainless period started from early June and ended in early September 2006 with a total of more than 80 days,and the rainfall was especially scarce from around 25 July to 5 September 2006.Precipitation for each month was less than normal,and analysis of the precipitation indices shows that the summer precipitation in 2006 was the least since 1959.The extremely severe drought in the ESC in summer 2006 was closely related to the persistent anomalies of the atmospheric circulation in the same period,i.e.,anomalies of mid-high latitude atmospheric circulation,western Pacific subtropical high（WPSH）,westerlies,South Asian high,lower-level flow,water vapor transport,vertical motion,and so on.Droughts usually occur when the WPSH lies anomalously northward and westward,or anomalously weak and eastward.The extreme drought in summer 2006 was caused by the former.When the WPSH turned stronger and shifted to the north and west of its normal position,and the South Asian high was also strong and lay eastward,downdrafts prevailed over the ESC and suppressed the water vapor transfer toward this area.At the same time,the disposition of the westerlies and the mid-high latitude circulation disfavored the southward invasion of cold air,which jointly resulted in the extremely severe drought in the ESC in summer 2006.The weak heating over the Tibetan Plateau and vigorous convective activities over the Philippine area were likely responsible for the strong WPSH and its northwestward shift in summer 2006.

Spatio-temporal variations in trends of vegetation and drought changes in relation to climate variability from 1982 to 2019 based on remote sensing data from East Asia

Studying the significant impacts on vegetation of drought due to global warming is crucial in order to understand its dynamics and interrelationships with temperature,rainfall,and normalized difference vegetation index(NDVI).These factors are linked to excesses drought frequency and severity on the regional scale,and their effect on vegetation remains an important topic for climate change study.East Asia is very sensitive and susceptible to climate change.In this study,we examined the effect of drought on the seasonal variations of vegetation in relation to climate variability and determined which growing seasons are most vulnerable to drought risk;and then explored the spatio-temporal evolution of the trend in drought changes in East Asia from 1982 to 2019.The data were studied using a series of several drought indexes,and the data were then classified using a heat map,box and whisker plot analysis,and principal component analysis.The various drought indexes from January to August improved rapidly,except for vegetation health index(VHI)and temperature condition index(TCI).While these indices were constant in September,they increased again in October,but in December,they showed a descending trend.The seasonal and monthly analysis of the drought indexes and the heat map confirmed that the East Asian region suffered from extreme droughts in 1984,1993,2007,and 2012among the study years.The distribution of the trend in drought changes indicated that more severe drought occurred in the northwestern region than in the southeastern area of East Asia.The drought tendency slope was used to describe the changes in drought events during 1982–2019 in the study region.The correlations among monthly precipitation anomaly percentage(NAP),NDVI,TCI,vegetation condition index(VCI),temperature vegetation drought index(TVDI),and VHI indicated considerably positive correlations,while considerably negative correlations were found among the three pairs of NDVI and VHI,TVDI and VHI,and NDVI and TCI.This ecological and climatic mechanism provides a good basis for the assessment of vegetation and drought-change variations within the East Asian region.This study is a step forward in monitoring the seasonal variation of vegetation and variations in drought dynamics within the East Asian region,which will serve and contribute to the better management of vegetation,disaster risk,and drought in the East Asian 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.

Analysis of spatio-temporal evolution of droughts in Luanhe River Basin using different drought indices

Based on the monthly precipitation and air temperature from 1960 to 1989 in the Luanhe River Basin, the standardized precipitation evapotranspiration index （SPEI） and standardized precipitation index （SPI） at three- and six-month time scales and the self-calibrating Palmer drought severity index （sc-PDSI） were calculated to evaluate droughts in the study area. Temporal variations of the drought severity from 1960 to 1989 were analyzed and compared based on the results of different drought indices, and some typical drought events were identified. Spatial distributions of the drought severity according to the indices were also plotted and investigated. The results reveal the following： the performances of different drought indices are closely associated with the drought duration and the dominant factors of droughts; the SPEI is more accurate than the SPI when both evaporation and precipitation play important roles in drought events; the drought severity shown by the sc-PDSI is generally milder than the actual drought severity from 1960 to 1989; and the evolution of the droughts is usually delayed according to the scPDSI. This study provides valuable references for building drought early warning and mitigation systems in the Luanhe River Basin.

Performance of different drought indices for agriculture drought in the North China Plain

The Palmer drought severity index （PDSI）, standardized precipitation index （SPI）, and standardized precipitation evapotranspiration index （SPEI） are used worldwide for drought assessment and monitoring. However, substantial differences exist in the performance for agricultural drought among these indices and among regions. Here, we performed statistical assessments to compare the strengths of different drought indices for agricultural drought in the North China Plain. Small differences were detected in the comparative performances of SPI and SPEI that were smaller at the long-term scale than those at the short-term scale. The correlation between SPI/SPEI and PDSI considerably increased from 1- to 12-month lags, and a slight decreasing trend was exhibited during 12- and 24-month lags, indicating a 12-month scale in the PDSI, whereas the SPI was strongly correlated with the SPEI at 1- to 24-month lags. Interestingly, the correlation between the trend of temperature and the mean absolute error and its correlation coefficient both suggested stronger relationships between SPI and the SPEI in areas of rapid climate warming. In addition, the yield-drought correlations tended to be higher for the SPI and SPEI than that for the PDSI at the station scale, whereas small differences were detected between the SPI and SPEI in the performance on agricultural systems. However, large differences in the influence of drought conditions on the yields of winter wheat and summer maize were evident among various indices during the crop-growing season. Our findings suggested that multi-indices in drought monitoring are needed in order to acquire robust conclusions.

Spatiotemporal analysis of drought variability based on the standardized precipitation evapotranspiration index in the Koshi River Basin, Nepal

Drought is an inevitable condition with negative impacts in the agricultural and climatic sectors,especially in developing countries.This study attempts to examine the spatial and temporal characteristics of drought and its trends in the Koshi River Basin(KRB)in Nepal,using the standardized precipitation evapotranspiration index(SPEI)over the period from 1987 to 2017.The Mann-Kendall test was used to explore the trends of the SPEI values.The study illustrated the increasing annual and seasonal drought trends in the KRB over the study period.Spatially,the hill region of the KRB showed substantial increasing drought trends at the annual and seasonal scales,especially in summer and winter.The mountain region also showed a significant increasing drought trend in winter.The drought characteristic analysis indicated that the maximum duration,intensity,and severity of drought events were observed in the KRB after 2000.The Terai region presented the highest drought frequency and intensity,while the hill region presented the longest maximum drought duration.Moreover,the spatial extent of drought showed a significant increasing trend in the hill region at the monthly(drought station proportion of 7.6%/10 a in August),seasonal(drought station proportion of 7.2%/10 a in summer),and annual(drought station proportion of 6.7%/10 a)scales.The findings of this study can assist local governments,planners,and project implementers in understanding drought and developing appropriate mitigation strategies to cope with its impacts.

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.

Drought fluctuations based on dendrochronology since 1786 for the Lenglongling Mountains at the northwestern fringe of the East Asian summer monsoon region

The Lenglongling Mountains （LLM） located in northeastern part of the Tibet Plateau, belong to a marginal area of the East Asian summer monsoon （EASM） and are sensitive to monsoon dynamics. Two tree-ring width chronologies developed from six sites of Picea crassifolia in the LLM were employed to study the regional drought variability. Correlation and temporal correlation analyses showed that relationships between the two chronologies and self-calibrated Palmer Drought Severity Index （sc_PDSI） were significant and stable across time, demonstrating the strength of sc_PDSI in modeling drought conditions in this region. Based on the relationships, the mean sc_PDSI was reconstructed for the period from 1786 to 2013. Dry conditions prevailed during 1817-1819, 1829-1831, 1928-1931 and 1999-2001. Relatively wet periods were identified for 1792-1795 and 1954-1956. Spatial correlations with other fourteen precipitation/drought reconstructed series in previous studies revealed that in arid regions of Northwest China, long-term variability of moisture conditions was synchronous before the 1950s at a decadal scale （1791-1954）. In northwestern margin of the EASM, most of all selected reconstructions had better consistency in low-frequency variation, especially during dry periods, indicating similar regional moisture variations and analogous modes of climate forcing on tree growth in the region.

Assessment of drought and its impact on winter wheat yield in the Chinese Loess Plateau

Drought has pronounced and immediate impacts on agricultural production,especially in semi-arid and arid rainfed agricultural regions.Quantification of drought and its impact on crop yield is essential to agricultural water resource management and food security.We investigated drought and its impact on winter wheat(Triticum aestivum L.)yield in the Chinese Loess Plateau from 2001 to 2015.Specifically,we performed a varimax rotated principal component analysis on drought severity index(DSI)separately for four winter wheat growth periods:pre-sowing growth period(PG),early growth period(EG),middle growth period(MG),and late growth period(LG),resulting in three major subregional DSI dynamics for each growth period.The county-level projections of these major dynamics were then used to evaluate the growth period-specific impacts of DSI on winter wheat yields by using multiple linear regression analysis.Our results showed that the growth period-specific subregions had different major DSI dynamics.During PG,the northwestern area exhibited a rapid wetting trend,while small areas in the south showed a slight drying trend.The remaining subregions fluctuated between dryness and wetness.During EG,the northeastern and western areas exhibited a mild wetting trend.The remaining subregions did not display clear wetting or drying trends.During MG,the eastern and southwestern areas showed slight drying and wetting trends,respectively.The subregions scattered in the north and south had a significant wetting trend.During LG,large areas in the east and west exhibited wetting trends,whereas small parts in south-central area had a slight drying trend.Most counties in the north showed significant and slight wetting trends during PG,EG,and LG,whereas a few southwestern counties exhibited significant drying trends during PG and MG.Our analysis identified close and positive relationships between yields and DSI during LG,and revealed that almost all of the counties were vulnerable to drought.Similar but less strong relationships existed for MG,in which northeastern and eastern counties were more drought-vulnerable than other counties.In contrast,a few drought-sensitive counties were mainly located in the southwestern and eastern areas during PG,and in the northeastern corner of the study region during EG.Overall,our study dissociated growth period-specific and spatial location-specific impacts of drought on winter wheat yield,and might contribute to a better understanding of monitoring and early warning of yield loss.

A Comparative Analysis of Primary and Extreme Characteristics of Dry or Wet Status between Asia and North America

In this study, the Palmer Drought Severity Index （PDSI） was used to analyze the average and extreme dry/wet states of Asia and North America from 1953 to 2003. The results indicate that the two continents underwent drying trends during this period. Compared with North America, Asia showed more severe drought trends. However, more significant and regular seasonal variation for drought was found in North America. The driest regions in Asia were located in the northern region of China, Mongolia, and eastern mid-Siberian plateau. Most regions in central North America were relatively wetter than other regions. The northern and southwestern regions of North America, as well as the Atlantic and Pacific coastal areas, experienced the most drought during this period. A sharp increase of the drought area and the number of extreme drought events took place from 1997 to 2003 in both Asia and North America. Severe drought events were more likely to occur during the summer on both continents. Asia had the most extreme drought events during July, but North America reached its highest drought frequency from June to September. In Asia, a persistent increasing trend of extreme drought emerged throughout the studied period. However, a more complex evolution of drought emerged in North America： a decreasing trend appeared before the mid-1960s and an increasing trend appeared after the late 1970s. A relatively steady dry/wet status was observed between the mid-1960s and the late 1970s. The role of exceptional, extreme drought events with respect to the La Nin？a event was considered during 1997–2003.