Precipitation and vegetation transpiration variations dominate the dynamics of agricultural drought characteristics in China

Agricultural drought posing a significant threat to agricultural production is subject to the complex influence of ocean, terrestrial and meteorological multi-factors. Nevertheless, which factor dominating the dynamics of agricultural drought characteristics and their dynamic impact remain equivocal. To address this knowledge gap, we used ERA5 soil moisture to calculate the standardized soil moisture index (SSI) to characterize agricultural drought. The extreme gradient boosting model was then adopted to fully examine the influence of ocean, terrestrial and meteorological multi-factors on agricultural drought characteristics and their dynamics in China. Meanwhile, the Shapley additive explanation values were introduced to quantify the contribution of multiple drivers to drought characteristics. Our analysis reveals that the drought frequency, severity and duration in China ranged from 5-70, 2.15-35.02 and 1.76-31.20, respectively. Drought duration is increasing and drought intensity is intensifying in southeast, north and northwest China. In addition, potential evapotranspiration is the most significant driver of drought characteristics at the basin scale. Regarding the dynamic evolution of drought characteristics, the percentages of raster points for drought duration and severity with evapotranspiration as the dominant factor are 30.7 % and 32.7 %, and the percentages with precipitation are 35.3 % and 35.0 %, respectively. Precipitation in northern regions has a positive effect on decreasing drought characteristics, while in southern regions, evapotranspiration dominates the dynamics in drought characteristics due to increasing vegetation transpiration. Moreover, the drought severity is exacerbated by the Atlantic Multidecadal Oscillation in the Yangtze and Pearl River basins, while the contribution of the North Atlantic Oscillation to the drought duration evolution is increasing in the Yangtze River basin. Generally, this study sheds new insights into agricultural drought evolution and driving mechanism, which are beneficial for agricultural drought early warning and mitigation.

Monitoring vegetation sensitivity to drought events in China

The frequency and severity of drought events have increased over the decades under the influence of global warming. Continued drought increases the risk of vegetation degradation. Many studies have investigated the responses of vegetation to drought but rarely from the perspective of drought events. Moreover, the spatial distributions of vegetation sensitivity to drought events are not well understood in China. Thus, the spatiotemporal patterns of drought events were quantified based on the run theory at different time-scales in this study. The relative importance of drought characteristics for vegetation anomalies during drought events were calculated by using the BRT model. Then, the sensitivity of vegetation anomalies and vegetation phenology was quantified by dividing standardized anomalies of vegetation parameters (NDVI and phenological metrics) and SPEI during drought events for different regions in China. The results show that Southern Xinjiang and Southeast China experienced relatively higher values of drought severity, especially at the 3-month and 6-month scales. Most arid areas experienced more drought events but of low severity, while some humid zones underwent few drought events but of high severity. Notable negative NDVI anomalies appeared in the Northeast China and Southwest China, while positive NDVI anomalies were observed in Southeast China and Northern central region. Drought interval, intensity and severity contributed approximately 80 % of the model's explained vegetation variance in most regions. The sensitivity of vegetation anomalies to drought events (VASD) varied regionally in China. The Qinghai-Tibet Plateau and Northeast China tended to exhibit higher sensitivity to drought events. Vegetation in these regions with high sensitivity faced a high risk of degradation and could function as warning signals of vegetation degradation. Drought events at high timescales had a greater impact on vegetation sensitivity in dry zones, while they had a smaller impact on humid areas. With the decrease in drought degree of climate zones and the increase in vegetation coverage, VASD showed a gradual increase. Furthermore, a strong negative correlation between VASD and the aridity index (AI) was observed in all vegetation types. The change in VASD for sparse vegetation was the largest with the change in AI. For vegetation phenology, drought events in most regions delayed the end of the growing season and extended the length of growing season, especially for sparse vegetation. The start of the growing season was advanced in most humid areas, while being delayed in most dry areas during drought events. Knowledge of vegetation sensitivity to drought events will be beneficial to provide decision-making references for the prevention and control of vegetation degradation, especially in the ecological fragile regions.

Characterizing drought events occurred in the Yangtze River Basin from 1979 to 2017 by reconstructing water storage anomalies based on GRACE and meteorological data

The extreme change of water storage in the Yangtze River Basin (YRB) have a significant impact on identifying the characteristics of drought events in the basin. To quantify the historical hydrological drought characteristics, we put forward new framework to reconstruct the pre-2003 total water storage anomaly (TWSA) through the nonlinear autoregressive with exogenous input (NARX) model. The NARX model is developed by the Gravity Recovery and Climate Experiment (GRACE) based TWSA and the hydrometeorological data after removing the trend and seasonal signals from 2003 to 2017, then the full pre-2003 reconstructed TWSA signals were obtained by synthesizing hydrometeorological data driven NARX model results from 1979 to 2002 and GRACE-estimated seasonal cycle. We combined the reconstructed TWSA with GRACE observed TWSA to characterize the historical hydrological drought events (onset, end, duration, magnitude, intensity, and recovery) in the YRB. The results show that the drought-related extreme anomalies in total water storage can be captured successfully. From 1979 to 2017, 23 hydrological drought events were identified in the YRB with an average recovery time of 4.7 months. The longest drought lasted 28 months spanning from July 2006 to October 2008. The exceptional drought occurred in September 2011 reached to the largest deficit with a magnitude of -48.5 mm and minimum drought severity index (DSI) of -2.3. Comparing to the period of 1979-1999, the frequency, duration, and average recovery time of drought events increased significantly since 2000 in the YRB. Furthermore, we found that the duration and average recovery time of the drought events have an exponential relationship with the severity, which could help us to estimate the potential recovery time when drought events occur and predict water resources dynamic in the future.

Tree characteristics and drought severity modulate the growth resilience of natural Mongolian pine to extreme drought episodes

Global climate change and the increase in the frequency and intensity of drought have led to widespread forest decline and tree mortality. Studying the resilience components of tree growth to drought, including resistance (R_t), recovery (R_c), and resilience (R_s) and the influencing factors, helps assess forests' production and ecological stability under a changing climate. This study analyzed the responses of three resilience components of natural Mongolian pine (Pinus sylvestris var. mongolica) to drought events by examining individual-tree characteristics in two sites of Hulunbuir using the linear mixed effect model. The result showed that drought severity, diameter at breast height (dbh), pre-drought growth, and growth variability prior to drought had significant effects on the three resilience components of Mongolian pine growth. Specifically, as drought severity, dbh and growth variability increased, the R_t and R_s decreased, but R_c increased, showing a trade-off relationship with R_t. However, the R_t, R_c, and R_s decreased with pre-drought growth. Inter-tree competition and tree age also significantly impacted two resilience components. Besides, the interaction term between tree competition and tree age negatively affects R_t and R_s but positively affects R_c. Our findings highlight the influence of drought severity and individual-tree characteristics on drought resilience components, which can serve the adaptive management of natural Mongolian pine forests in the future.

Short-term forest resilience after drought-induced die-off in Southwestern European forests

Drought-induced die-off in forests is becoming a widespread phenomenon across biomes, but the factors determining potential shifts in taxonomic and structural characteristics following mortality are largely unknown. We report on short-term patterns of resilience after drought-induced episodes of tree mortality across 48 monospecific forests from Morocco to Slovenia. Field surveys recorded plants growing beneath a canopy of dead, defoliated and healthy trees. Site-level structural characteristics and management legacy were also recorded. Resilience was assessed with reference to forest composition (self-replacement), structure, and changes in the climatic suitability of the replacing community relative to the climatic suitability of the dominant pre-drought species. Species climatic suitability was estimated from species distribution models calculated for the baseline 1970-2000 period. Short-term resilience decreased under higher levels of drought-induced damage to the dominant species and with evidences of management legacy. Greater resilience of structural features (fewer gaps, greater canopy height) was observed overall in forests with a larger basal area. Less gaps were also associated with greater woody species richness after drought. Overall, Fagaceae-dominated forests exhibited greater structural resilience than conifer-dominated ones. On those sites that were more climatically suited to the dominant pre-drought species, replacing communities tended to exhibit lower climatic suitability than pre-drought dominant species. There was a greater loss of climatic suitability under a legacy of management and drought intensity, but less so in the replacing communities with higher woody species richness. Our study reveals that short-term forest resilience is determined by pre-drought stand characteristics, often reflecting previous management legacies, and by the impact of drought on both the dominant pre-drought species and post-drought replacing species in terms of their climatic suitability.

Implementation of initial emission mitigation measures for 1,4-dioxane in Germany: Are they taking effect?

Since our comprehensive investigation of finished drinking water in Germany obtained from managed aquifer recharge systems in the period 2015-2016, which revealed widespread contamination with 1,4-dioxane, mitigation measures (integration of AOP units, shutdown or alteration of production processes) have been implemented at some sites. In this study, we conducted follow-up tests on surface water concentrations and associated finished drinking water concentrations in 2017/2018, to evaluate the effectiveness of these measures. Our findings demonstrate that the emission mitigation measures had considerably reducing effects on the average 1,4-dioxane drinking water concentrations for some of the previously severely affected areas (Lower Franconia: -54%, Passau: -88%). Conversely, at notoriously contaminated sites where neither monitoring nor mitigation measures were introduced, the drinking water concentrations stagnated or even increased. Drinking water concentrations determined via a modified US EPA method 522 ranged from below LOQ (0.034 μg/L) up to 1.68 μg/L in all drinking water samples investigated. In river water samples, the maximum concentration exceeded 10 μg/L. Effluents of wastewater treatments plants containing 1,4-dioxane (5 μg/L-1.75 mg/L) were also analyzed for other similar cyclic ethers by suspected target screening. Thus, 1,3-dioxolane and three other derivatives were tentatively identified in effluents from the polyester processing or manufacturing industry. 1,3-Dioxolane was present in concentrations >1.2 mg/L at one site, exceeding up to sevenfold the 1,4-dioxane concentration found there. At another site 2-methyl-1,3-dioxolane was still found 13 km downstream of the discharge point, indicating that ethers analogous to 1,4-dioxane should be further considered regarding their occurrence and fate in wastewater treatment and the aquatic environment.

A July-August relative humidity record in North China since 1765 AD reconstructed from tree-ring cellulose δ18O

Since the late 1970s, East Asian summer monsoon (EASM) has shown a significant weakening trend, and sustained drought has occurred across North China. Placing recent climate changes in the paleoclimatic context can better understand the EASM variations. Four δ¹⁸O sequences based on tree-ring cellulose of Chinese pine were developed from Mt. Beiwudang, North China, covering a period from 1700 to 2013. Based on a climatic response analysis, a transfer function was designed to reconstruct the relative humidity from July to August (RH_JA hereafter). The RH_JA spans from 1765 to 2013 and explains 49% (R²_adj = 48%) of the instrumental variance during the calibration period (1961-2013, r = - 0.70, p < 0.0001). The RH_JA is mainly influenced by precipitation in the summer rainy season and reflect EASM variations. Spatial representation analysis indicates that RH_JA represents the dry/wet variations across North China. At the interannual scale, RH_JA records many extreme dry/wet events, among which the events in 1876-1878, 1900, and the 1920s are extensive droughts. Those events correspond well to ENSO events, plus further correlation and periodicity analysis indicate that RH_JA contains ENSO signals. At the interdecadal scale, RH_JA shows a decreasing trend and unprecedented low values from 1981 to 2013, suggesting that the weakening of EASM since the late 1970s is unprecedented in the past 249 years. Similarly, the significantly correlating region in the spatial correlation analysis, covering the Meiyu/Baiu/Changma rainfall belt and India, have also undergone a climatic shift since the late 1970s according to previous papers.

Remote sensing strategies to characterization of drought, vegetation dynamics in relation to climate change from 1983 to 2016 in Tibet and Xinjiang Province, China

Due to various land cover changes, vegetation dynamics, and climate, drought is the most complex climate-related disaster problem in Tibet and Xinjiang, China. The purpose of the present study is to analyze the performance of the AVHRR Normalized Vegetation Index (NDVI) and the temporal and spatial differences of seasonal vegetation dynamics by correlating the results with rainfall and temperature data of NASA's MERRA to examine the vegetation dynamics and droughts in Tibet and the Xinjiang Province of China. Our method is based on the use of AVHRR NDVI data and NASA MERRA temperature and precipitation during 1983-2016. Due to the dryness and low vegetation, NDVI is more useful to describe the drought conditions in Tibet and Xinjiang of China. The NDVI, TCI, VHI, NVSWI, VCI, TVDI, and NAP from April to October increased rapidly. While the NDVI, TCI, VHI, NVSWI, NAP, TVDI, and VCI are stable every month in September, again improve in October, and then confirm downward trend in December. The NDVI, TCI, VHI, NVSWI, NAP, VCI, and TVDI monthly values indicate that Tibet and Xinjiang province of China suffered from severe drought in 2006, 2008, and 2012 which were the most drought years. For monitoring drought in Tibet and Xinjiang province of China, the NDVI, TVDI, NAP, VCI, and NVSWI values were selected as a tool for reporting drought events during different growing seasons. Seasonal values of TVDI, NDVI, NAP, NVSWI, and VCI confirmed that Tibet and Xinjiang province of China suffered from severe drought in 2006, 2008, and 2012 and led the durations of severe drought. The correlation between NDVI, TCI, VHI, NAP, TVDI, and VCI showed a significantly positive correlation, while the significantly negative correlation between NVSWI and NDVI showed a good indication for the assessment of drought, especially for the agricultural regions of Tibet and Xinjiang province of China. This shows that the positive sign to support NAP, NVSWI, and TVDI is good monitoring of the drought indexes in Tibet and the Xinjiang province of China.

A new global database of meteorological drought events from 1951 to 2016

STUDY REGION

This study has three spatial scales: global (0.5°), macro-regional, and country scale. The database of drought events has specific entries for each macro-region and country.

STUDY FOCUS

We constructed a database of meteorological drought events from 1951 to 2016, now hosted by the Global Drought Observatory of the European Commission's Joint Research Centre. Events were detected at macro-regional and country scale based on the separate analysis of the Standardized Precipitation-Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) at different accumulation scales (from 3 to 72 months), using as input the Global Precipitation Climatology Centre (GPCC) and Climatic Research Unit (CRU) Time Series datasets. The database includes approximately 4800 events based on SPEI-3 and 4500 based on SPI-3. Each event is described by its start and end date, duration, intensity, severity, peak, average and maximum area in drought, and a special score to classify 52 mega-droughts.

NEW HYDROLOGICAL INSIGHTS FOR THE REGION UNDER STUDY

We derived trends in drought frequency and severity, separately for SPI and SPEI at a 12-month accumulation scale, which is usually related to hydrological droughts. Results show several drought hotspots in the last decades: Amazonia, southern South America, the Mediterranean region, most of Africa, north-eastern China and, to a lesser extent, central Asia and southern Australia. Over North America, central Europe, central Asia, and Australia, the recent progressive temperature increase outbalanced the increase in precipitation causing more frequent and severe droughts.

A new global database of meteorological drought events from 1951 to 2016

STUDY REGION

This study has three spatial scales: global (0.5°), macro-regional, and country scale. The database of drought events has specific entries for each macro-region and country.

STUDY FOCUS

We constructed a database of meteorological drought events from 1951 to 2016, now hosted by the Global Drought Observatory of the European Commission's Joint Research Centre. Events were detected at macro-regional and country scale based on the separate analysis of the Standardized Precipitation-Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) at different accumulation scales (from 3 to 72 months), using as input the Global Precipitation Climatology Centre (GPCC) and Climatic Research Unit (CRU) Time Series datasets. The database includes approximately 4800 events based on SPEI-3 and 4500 based on SPI-3. Each event is described by its start and end date, duration, intensity, severity, peak, average and maximum area in drought, and a special score to classify 52 mega-droughts.

NEW HYDROLOGICAL INSIGHTS FOR THE REGION UNDER STUDY

We derived trends in drought frequency and severity, separately for SPI and SPEI at a 12-month accumulation scale, which is usually related to hydrological droughts. Results show several drought hotspots in the last decades: Amazonia, southern South America, the Mediterranean region, most of Africa, north-eastern China and, to a lesser extent, central Asia and southern Australia. Over North America, central Europe, central Asia, and Australia, the recent progressive temperature increase outbalanced the increase in precipitation causing more frequent and severe droughts.

Effects of rain shortage on carbon allocation, pools and fluxes in a Mediterranean shrub ecosystem - a 13C labelling field study

Hydrological cycle is expected to become the primary cause of ecosystem's degradation in near future under changing climate. Rain manipulation experiments under field conditions provide accurate picture on the responses of biotic processes to changed water availability for plants. A field experiment, mimicking expected changes in rain patterns, was established in a Mediterranean shrub community at Porto Conte, Italy, in 2001. In November 2011 Cistus monspeliensis, one of the dominating shrub species in the Mediterranean basin, was ¹³C labelled on plots subjected to extended rain shortage period and on control non manipulated plots. Carbon (C) allocation was traced by ¹³C dynamics in shoots, shoot-respired CO₂, roots, microbial biomass, K₂SO₄-extractable C and CO₂ respired from soil. Most of the recovered ¹³C (60%) was respired by shoots within 2weeks in control plots. In rain shortage treatment, ¹³C remained incorporated in aboveground plant parts. Residence time of ¹³C in leaves was longer under the rain shortage because less ¹³C was lost by shoot respiration and because ¹³C was re-allocated to leaves from woody tissues. The belowground C sink was weak (3-4% of recovered ¹³C) and independent on rain manipulation. Extended rain shortage promoted C exudation into rhizosphere soil in expense of roots. Together with lowered photosynthesis, this "save" economy of new C metabolites reduces the growing season under rain shortage resulting in decrease of shrub cover and C losses from the system on the long-term.