Standardized Precipitation Evapotranspiration Index Series Research Articles

Spatiotemporal heterogeneity in meteorological and hydrological drought patterns and propagations influenced by climatic variability, LULC change, and human regulations

This study aims to quantify meteorological–hydrological drought propagations and examine the potential impacts by climatic variability, LULC change (LULC), and human regulations. An integrated observation-modeling framework quantifies drought propagation intervals and assesses mechanisms influencing hydrological droughts. Meteorological droughts are characterized using the Standardized Precipitation Evapotranspiration Index (SPEI), and hydrological droughts are assessed through the Standardized Streamflow Index (SSI) across diverse climatic zones. Cross-correlation analysis between SPEI and SSI time series identifies the lag time associated with the highest correlation as the drought propagation interval. Mechanisms are investigated via a coupled empirical-process modeling framework incorporating the Soil and Water Assessment Tool (SWAT). Discrepancies between simulated and observed SSI time series help quantify the extent of human regulation impacts on hydrological drought characteristics and propagation. The Yellow River Basin (YRB), divided into six subzones based on climate characteristics, is selected as the case study. Key findings include: (1) Meteorological droughts were extremely severe across most YRB during the 1990s, while the 2000s showed some mitigation primarily due to precipitation increases. (2) Hydrological droughts and propagation times from meteorology to hydrology demonstrated substantial spatiotemporal variability. In general, summer propagation times were shorter than other seasons. (3) Propagation times were shorter in arid regions with cropland or built-up land cover versus grassland and woodland, while the reverse held for humid regions. (4) Human regulations prolonged propagation times, likely due to reservoir regulations designed to overcome water deficits. While the YRB is the focus of this paper, the methodologies and findings are applicable to other regions worldwide to enhance drought forecasting and water resource management. In various hydrological and climatic contexts worldwide.

Drought Characterization in Croatia Using E-OBS Gridded Data

Droughts are among the major natural hazards that are spreading to many parts of the world, with huge multi-dimensional impacts. An extensive analysis of drought phenomenon is presented for continental Croatia based on a meteorological E-OBS gridded dataset (0.25° × 0.25°), within the period of 1950–2022. The drought events were characterized by the Standardized Precipitation Evapotranspiration Index (SPEI), applied to different time-scales (6 and 12 months), in order to describe the subannual and annual variability of drought. The spatiotemporal patterns of drought are obtained through principal component analysis (PCA) and K-means clustering (KMC) applied to the SPEI field. An areal drought evolution analysis and the changes in the frequency of occurrence of the periods under drought conditions were achieved using a kernel occurrence rate estimator (KORE). The modified Mann–Kendall (MMK) test, coupled with the Sen’s slope estimator test, are applied to the SPEI series in order to quantify the drought trends throughout the country. According to the history drought events and considering the different morphoclimatic characteristics of the study area, the results showed that Croatia could be divided into three different and spatially well-defined regions with specific temporal and spatial characteristics of droughts (central northern, eastern and southern regions). A manifest increase is shown in the percentage of area affected by drought, as well as in the yearly drought occurrences rates, in both central northern and eastern regions, and an evident decrease is shown in the southern region for both 6- and 12-month SPEI time-scales. In the observation of the drought’s temporal characteristics, it was found that downward trends expressing increasing drought severities were strongly significant in northern and eastern regions, while a few significant upward trends were seen in the southern region. From this study, it is possible to obtain a broader view of the historical behaviour of droughts in Croatia, with the results providing useful support for drought risk assessment and decision-making processes.

Multifractal characteristics of multiscale drought in the Yellow River Basin, China

As a climate-sensitive area in China, the Yellow River Basin (YRB) is increasingly affected by droughts resulting from climate change and frequent human activities. Droughts in the YRB are non-stationary, yet the temporal scaling properties of their complex fluctuations are still unclear. To address this deficiency, we investigated the spatiotemporal characteristics of multifractal nature and their sources through the multifractal detrended fluctuation analysis (MFDFA). Our research was conducted utilizing the Standardized Precipitation Evapotranspiration Index (SPEI) derived for 80 meteorological stations located within the YRB between 1961 and 2017 on various timescales. The main findings were as follows: (1) YRB droughts exhibited persistence since the Hurst exponent exceeded 0.5, indicating that future drought trends were expected to be consistent with the present. Drought persistence increased with increasing timescales, with the strongest persistence recorded in the mid-temperate semi-arid zone. (2) Multifractality was observed in all the SPEI series, as evidenced by the Generalized Hurst Exponent [h(q)] varied nonlinearly with q. Multifractal intensity strengthen with increasing drought duration, as measured by the width in the multifractal spectrum presented in detail at different timescales. (3) Small fluctuations dominated the multifractal behaviors of droughts as most of the multifractal spectrums were right-skewed. (4) In contrast to the original series, the shuffle and surrogate procedures significantly narrowed the width of the multifractal spectrum (while spectrum width remained greater than 0.05), revealing that the multifractality of the drought originated from the long-range correlations (LRC) and the broad probability density distribution (PDF). Moreover, the spectrum width of the shuffled sequences was more variable, demonstrating that LRC was a dominant source of multifractality. This study contributes to a better understanding of the complex fluctuations of drought, as well as a new perspective for improving the accuracy of drought prediction.

Hydrological drought evolution and its response to meteorological drought in the middle-lower reach of the Yangtze River triggered by Three Gorges Reservoir

利用1960—2016年长江流域183个气象站逐月气温和降水数据以及干流3个水文站逐月径流资料, 采用标准化降水蒸散指数(SPEI)和标准化径流指数(SRI)分析三峡水库蓄水运行前后长江中下游宜昌、汉口和大通站水文干旱的多时间尺度演变以及对气象干旱的响应特征. 结果表明: (1) 三峡水库运行后下游各站冬春季旱情明显趋缓, 而秋季干旱状况略有加重; 水库蓄水后各站中旱和重旱发生频率均呈减少趋势, 其中中旱减幅明显, 而特旱发生频率则总体表现为增加趋势; (2) 三峡水库蓄水后3个站平均干旱历时的变幅相对较小, 而干旱烈度和烈度峰值的均值增幅较大; 同时, 各站短时间尺度(1和3个月) 干旱特征变量的变幅总体呈现沿程递增趋势, 而长时间尺度(6和12个月) 干旱特征变量的变幅整体表现为沿程递减趋势; (3) 水库蓄水后各站短时间尺度SRI与SPEI的相关性减小, 但相关性随时间尺度增加而迅速增强, 12个月时间尺度的相关系数达到最大并略高于蓄水前; 在年内相关性上, 蓄水后各站短时间尺度SRI与SPEI的相关系数明显减小, 冬季表现尤为突出, 而长时间尺度的相关系数则略有增加; (4) 水库影响下不同时间尺度宜昌站水文干旱滞后于气象干旱平均天数有增有减, 而汉口和大通站的滞后天数均呈显著增加趋势, 且总体表现为沿程递增趋势. 研究结果可为变化环境下水文干旱预警与水库优化调度提供参考.;Based on monthly temperature and precipitation data from 183 meteorological stations in the Yangtze River (YZR) Basin and monthly runoff data from three hydrological stations in the mainstream of the YZR during 1960—2016, this study, using the standardized precipitation evapotranspiration index (SPEI) and standardized runoff index (SRI), analyzed the multiple timescales evolution of hydrological drought and its response to meteorological drought at Yichang, Hankou, and Datong stations in the middle-lower reach of the YZR before and after the operation of the Three Gorges Reservoir (TGR). The results showed that with the operation of TGR, drought situations at the downstream hydrological stations in winter and spring were significantly reduced, while drought conditions were slightly aggravated in autumn. The occurrence frequency of moderate and severe drought at each hydrological station showed a declining trend in the post-TGR period, particularly for the moderate drought. The frequency of extreme drought demonstrated an increasing trend in general. With the TGR was impounded, the average drought duration of three hydrological stations showed relatively mild variations, whereas the average drought severity and peak exhibited a significant upward trend. Furthermore, the variation amplitude of drought characteristic variables at the short-term timescale (1 and 3 months) from three hydrological stations displayed an increasing trend along the middle-lower reach of the YZR, while decreasing trend happened at the long-term timescale (6 and 12 months). The correlation between SRI and SPEI series from three hydrological stations at short-term timescale decreased significantly, but the correlation increased rapidly with timescale increase. Correlation coefficient between the SRI and SPEI series reached the maximum at 12-month timescale and were slightly higher than that in the pre-TGR period. Meanwhile, the intra-annual correlation coefficients between the SRI and SPEI series at the short-term timescale declined evidently in the post-TGR period, particularly in winter. However, the correlation coefficients of the SRI and SPEI series at long-term timescale increased slightly. With the impacts of the TGR, average day number that hydrological drought lagged behind meteorological drought at Yichang station at different timescales increased or decreased, while the number of lagged days at Hankou and Datong stations increased significantly, and had an increasing trend along the middle-lower reach of the YZR. These results can provide reference for hydrological drought warning and optimal operation of reservoir under the changing environment.

Assessment of Drought Characteristics under Changing Climatic Conditions using SPI and SPEI Indices in Semi-Arid Environment of Southeastern Niger

In the sahelian zone where 80% of the populations rely on rainfed-agricultural activities for their livelihood, drought episodes had significant socio-economics and ecological impacts. In recent decades, there has been an increase in the intensity, frequency and severity of drought occurrence mainly attributed to climate change. Thus, the main objectives of this study were: i) to understand drought multi-scale patterns and trend; ii) to assess drought duration, frequency and temporal extent over Mainé-soroa and Diffa located in the lake chad basin. To achieve these objectives standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI) at 1, 3, 6, 9, 12 and 24-month timescales were employed for the stations of Mainé-soroa and Diffa. Stations’ monthly rainfall, air minimum and maximum temperature spanning 1950-2009 and 1988-2017 respectively for Mainé-soroa and Diffa were used for the analysis. The Mann–kendall trend test was performed and revealed negative SPEI and SPI trends in the station of Mainé-soroa. Results indicate a significant negative SPI-12 and SPI-24 trend (p-value < 0.05), while no trend was detected in the rest of the time-scale series. The absolute value of declining trend was gradually increasing when SPEI was calculated with more lagged months. Meanwhile, at the Diffa station both SPEI and SPI showed positive trends. The pettitt's t-test on the SPEI series indicated particularly 1968 as the change point detected for three time scales including spei-9, SPEI-12, and SPEI-24. Drought frequency generally increased in Mainé-soroa over the period of 1950–2009. SPEI appear to be the most powerful tool of monitoring drought in semi-arid environment in the context of climate change. To build resilience to drought and cope with its effects in the area we stress the importance of the establishment of early risk identification and advices framework at local level such study should be extended to lake chad basin.

Drought and Wetness Variability and the Respective Contribution of Temperature and Precipitation in the Qinghai-Tibetan Plateau

Quantifying drought and wetness fluctuations is of great significance to the regional ecological environment and water resource security, especially in the fragile Qinghai-Tibetan Plateau (QTP). In this paper, the standardized precipitation evapotranspiration index (SPEI) was calculated based on the observed data and China Meteorological Forcing Dataset (CMFD) in the QTP for the period of 1979–2015, and the drought and wetness evolution based on the SPEI series and respective contribution of temperature and precipitation were also analyzed. Results indicated that meteorological stations are mainly concentrated in the eastern part of the plateau, which cannot reflect the drought and wetness trend of the whole QTP. The linear trend and Mann–Kendall test revealed that SPEI series calculated based on CMFD data at 1-, 3-, 6-, 9-, 12-, and 24-month time scales all showed significant upward trend p < 0.01 , indicating that the QTP as a whole tended to be wetter. Spatially, the regions with significant drying p < 0.1 and increased drought probability were mainly concentrated in the Qaidam Basin and the southern part of the QTP, and the mean contribution rates of temperature and precipitation variability to SPEI trend in these regions were 60% and −11%, respectively. The regions with significant wetting p < 0.1 and decreased drought probability were mainly concentrated in the northeast, central, and western parts of the plateau, and the mean contribution rates of temperature and precipitation variability to SPEI trend were −9% and 61% in these regions. From the statistics in different climatic regions, most of the arid and humid regions in the QTP tended to be drier, while the semiarid regions tended to be wetter.

Wavelet Packet-Genetic Programming: A New Model for Meteorological Drought Hindcasting

This study presents developing procedures and verification of a new hybrid model, namely wavelet packet-genetic programming (WPGP) for short-term meteorological drought forecast. To this end, the multi-temporal standardized precipitation evapotranspiration index (SPEI) has been used as the drought quantifying parameter at two meteorological stations at Ankara province, Turkey. The new WPGP model comprises two main steps. In the first step, the wavelet packet, which is a generalization of the well-known wavelet transform, is used to decompose the SPEI series into deterministic and stochastic sub-signals. Then, classic genetic programming (GP) is applied to formulate the deterministic sub-signal considering its effective lags. To characterize the stochastic component, different theoretical probability distribution functions were assessed, and the best one was selected to integrate with the GP-evolved function. The efficiency of the new model was cross-validated with the first order autoregressive (AR1), GP, and random forest (RF) models developed as the benchmarks in the present study. The results showed that the WPGP is a robust model, superior to AR1 and RF, and significantly increases the predictive accuracy of the standalone GP model.

A Novel Fuzzy Random Forest Model for Meteorological Drought Classification and Prediction in Ungauged Catchments

This paper presents a new tree-based model, namely Fuzzy Random Forest (FRF), for one month ahead Standardized Precipitation Evapotranspiration Index (SPEI) classification and prediction with a noteworthy application in ungauged catchments. The proposed FRF model uses global SPEI dataset as the meteorological drought quantifier and applies a fuzzy inference system to extract fuzzified and crisp SPEI values for an ungauged catchment. The evolved crisp series is then transformed into the polynomial label vector of extremely wet, wet, near normal, dry, and extremely dry categories. In the end, the state-of-the-art random forest algorithm was used to classify and predict the label vector using the lagged SPEI series of the selected global grid points. To demonstrate the development and verification process of the FRF model, the global SPEI-6 values for the period of 1961–2015 were retrieved from four global grid points around the Central Antalya Basin, Turkey. The new model was trained and validated using 70% and 30% of the data sets, respectively. The performance of the new model was examined in terms of total accuracy, misclassification, and Kappa statistics and cross-validated with the fuzzy decision tree model developed as the benchmark in this study. The results showed the promising performance of the FRF for drought classification and prediction with outstanding efficiency for extremely wet and dry events classification. According to the Kappa statistic, the proposed FRF model is 25% more accurate than the benchmark FDT model.

Drought modeling using classic time series and hybrid wavelet-gene expression programming models

The standardized precipitation evapotranspiration index (SPEI) at three different time scales (i.e., SPEI-3, SPEI-6, and SPEI-12) from six meteorology stations located in Turkey are modeled in this study. To this end, two types of classic time series models, namely linear autoregressive (AR) and non-linear bi-linear (BL) are used. Furthermore, the hybrid models are proposed by coupling the wavelet (W) and gene expression programming (GEP). Five various mother wavelets (i.e., Haar, db4, Symlet, Meyer, and Coifflet), for the first time, are employed and compared for implementing the hybrid W-GEP approach in drought modeling. The modeling results of SPEI droughts via the time series models illustrated that the non-linear BL performs slightly better than the linear AR. Moreover, all the hybrid W-GEP models developed in the study region provide superior performances compared to the standalone GEP. In general, db4 in SPEI-3 modeling and Symlet for modeling the SPEI-6 and SPEI-12 of the studied locations are the optimal wavelets to develop the W-GEP. Finally, the SPEI series at each target station is modeled through applying the SPEI data of the five neighboring stations. It is found that the SPEI data of neighboring stations are appropriate for modeling the SPEI series of the target station when the SPEI data of the target station is not at hand. For this case, the performance of standalone GEP for modeling the SPEI-3 and SPEI-6 of the stations is generally better than the case of utilizing the original SPEI data at each target station.

Spatiotemporal variability of standardized precipitation evapotranspiration index in mainland China over 1961–2016

AbstractA good understanding of spatiotemporal characteristics of historical droughts is important for managing and tackling drought hazards. This paper aims to investigate spatiotemporal variations of drought in mainland China over 1961–2016 based on the standardized precipitation evapotranspiration index (SPEI) at the 1‐ and 12‐month timescales. The trend significance and magnitude of 12‐month SPEI were analysed using the modified Mann–Kendall (MMK) method and Sen's slope values. The empirical orthogonal function (EOF) and ensemble empirical mode decomposition (EEMD) were applied to analyse the spatial patterns and periods in 1‐month SPEI series. The results showed that the temporal fluctuations of SPEI were weaker as the timescale increased. The droughts in 1960s were the severest than in other decades. The sites with SPEI < −5 (denoting the extreme droughts) mainly located in the northwestern and southwestern China. An overall wetter trend was detected in most regions of China, except the belt from northeastern to middle and southwestern China. The 1‐month SPEI had shorter than 1.20‐year of mean periods for the seven sub‐regions and China, while had 0.22–2.95 years of mean periods at different sites. Residuals (decomposed from 1‐month SPEI) showed increasing trends in most of the sub‐regions except sub‐region 3. In northern and northwestern China, longer periods of SPEI were detected. The highest values of EOF1 and EOF2 illustrated the eastern and northeastern China were drought‐sensitive areas. From the correlations between 1‐month SPEI and the atmospheric circulations, the AO was the main physical factor that influenced drought occurrence in mainland China. The identified spatiotemporal characteristics of SPEI could offer references for drought prevention and management in China.

Spatiotemporal Characteristics of Drought in the Heihe River Basin Based on the Extreme-Point Symmetric Mode Decomposition Method

Assessment of spatiotemporal characteristics of drought under climate change is significant for drought mitigation. In this study, the standardized precipitation evapotranspiration index (SPEI) calculated at different timescales was adopted to describe the drought conditions in the Heihe River Basin (HRB) from 1961 to 2014. The period characteristics and spatiotemporal distribution of drought were analyzed by using the extreme-point symmetric mode decomposition (ESMD) and inverse distance weight interpolation methods. Four main results were obtained. (1) The SPEI series of the upper reaches of the HRB at different timescales showed an upward trend (not significant) during 1961–2014. In the middle and lower reaches, the SPEI series exhibited significant downward trends. (2) The annual SPEI series of the lower reaches was decomposed through the ESMD method and exhibited a fluctuating downward trend as a whole. The oscillation showed quasi-3.4-year and quasi-4.5-year periods in the interannual variation, while a quasi-13.5-year period occurred in the interdecadal variation. The interannual period plays a leading role in drought variation across the HRB. (3) The entire research period was divided into three subperiods by the Bernaola–Galvan segmentation algorithm: 1961–1966, 1967–1996, and 1997–2014. The spring drought frequency and autumn drought intensity arrived at their maxima in the lower reaches during 1997–2014, with values of 72.22% and 1.56, respectively. The high frequency and intensity areas of spring, summer, and autumn drought moved from the middle-upper reaches to the middle-lower reaches of the HRB during 1961–2014. (4) Compared to the wavelet transform, the ESMD method has self-adaptability for signal decomposition and is more accurate for drought period analysis. Extreme-point symmetric mode decomposition is a more efficient decomposition method for nonlinear and nonstationary time series and has important significance for revealing the complicated change features of climate systems.

Drought characteristics and its impact on changes in surface vegetation from 1981 to 2015 in the Yangtze River Basin, China

AbstractLocated in the subtropical monsoon climate area of China, the Yangtze River Basin (YRB) had been frequently affected by drought events recently. Based on standardized precipitation evapotranspiration Index (SPEI) and enhanced vegetation index (EVI), the characteristics of drought in the YRB and the influence of drought on vegetation in different seasons were investigated by maximum correlation (Rmax) between 1981 and 2015. The results showed that: (a) Droughts occurred frequently but had no obvious regularity in the YRB from 1981 to 2015. The drought intensity in autumn and winter were slightly higher than that in spring and summer. The autumn droughts in Sichuan Basin were the most serious, followed by the spring droughts in the middle and lower reaches of the YRB; (b) Annual SPEI series at 102 stations exhibited a downward trend and 72 stations exhibited an upward trend. There was no station showing a significant downward trend and two stations showing a significant upward trend. There was a drying trend in autumn and winter, and a wetting trend in spring and summer. The eastern area of the YRB shows a more arid trend in spring and autumn, as well as in the southwest in winter; (c) 90% of YRB showed a significant positive correlation (p < .05) between SPEI and EVI during the growing season (March–October), and the vegetation was more sensitive to short‐term and medium‐term drought. However, there were spatial and seasonal differences in the impact of drought on vegetation. Different types of land cover had different responses to droughts. Grasslands and croplands were more sensitive to droughts, grasslands at low elevation was dominated by short‐term droughts, while at high elevation was influenced by medium‐long‐ and long‐term droughts. Croplands and woodlands were more vulnerable to short‐term droughts.

Analysis of Dry/Wet Variations in the Poyang Lake Basin Using Standardized Precipitation Evapotranspiration Index Based on Two Potential Evapotranspiration Algorithms

Global warming has resulted in unevenly distributed changes in precipitation and evapotranspiration, which has some influence on dry/wet conditions, thus exerting a tremendous impact on national life and the social economy, especially agricultural production. In order to characterize the dry/wet variations in the Poyang Lake basin during 1958–2013, based on the potential evapotranspiration (PET) estimated by the Thornthwaite (TH) and Penman–Monteith (PM) formulas, two types of Standardized Precipitation Evapotranspiration Index (SPEI), namely SPEI_th and SPEI_pm, were calculated in this study. The characteristic of dry/wet variations in the Poyang Lake basin was analyzed and a comparative analysis of two SPEIs was conducted. The results indicate that both SPEI series showed a wet trend in the Poyang Lake basin on an annual scale as well as seasonal scales during 1958–2013, except for spring and autumn. A drying trend was observed in spring, while in autumn, the dry and wet conditions in two SPEIs had opposite trends. However, all trends from two SPEIs were not significant, except for summer SPEI_pm. Meanwhile, significant positive correlations were detected between precipitation and two SPEIs, with the correlation coefficients above 0.95, whereas negative correlations were detected between PET and two SPEIs, with the correlation coefficients ranging from −0.17 to −0.85. This indicates that precipitation was the main climatic factor to determine change in dry/wet conditions in the Poyang Lake basin. Although there were obvious differences between the accumulated values of the Penman–Monteith-based PET (ET_pm) and Thornthwaite-based PET (ET_th), trends in the SPEI_pm series were generally consistent with those in the SPEI_th series, revealing that the method for PET calculation was not critical to the change in dry/wet conditions. Moreover, the results of the conditional probability of SPEI_pm and SPEI_th show that both SPEI_pm and SPEI_th could detect wet or dry events that were identified by SPEI_pm or SPEI_th.

Assessment of candidate distributions for SPI/SPEI and sensitivity of drought to climatic variables in China

Drought occurs more frequently in the context of climate change and threatens water security worldwide. An appropriate fitting distribution is crucial for accurately identifying drought using drought indices. Here, seven two‐parameter distributions (Gamma, Gumbel, Logistic, Log–Logistic, Log‐normal, Normal, and Weibull) and four distributions (general logistic, generalized extreme value, Normal, and Pearson Type III) are applied to calculate the standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI), respectively, to choose the most appropriate fit for the 541 stations in China. The results show that, in most cases, Gamma and general logistic were the best distributions for SPI and SPEI, respectively. Nevertheless, we noted that Pearson Type III and generalized extreme value also fit the SPEI series well, indicating the importance of distribution fitting assessment for various regions. The annual and seasonal drought evolutions across China were clarified, with drought decreasing significantly in western Northwest China (annually and each season), northern North China (spring and winter), the Tibetan Plateau (spring), and the lower reaches of the Yangtze River (winter) and increasing mainly in South China (spring) and western South China (summer and autumn). The intensity, duration and severity of light, moderate, and severe drought were also detected; the results suggest that drought in China is mainly concentrated around the changes in light and moderate drought. Additionally, we assessed the sensitivity of drought evolution to various meteorological variables. The most sensitive variable in South China, North China and the Tibetan Plateau is precipitation, followed by wind speed, temperature, relative humidity, and sunshine duration; in Northwest China, the order is wind speed, precipitation, temperature, relative humidity, and sunshine duration. Furthermore, the correlation results also indicate that the drought evolution is affected by the multivariate ENSO index, while the influencing characteristics are different in various seasons and regions. The results in this paper contribute to drought mitigation and the effective utilization of water resources in China.

Effect of Air Temperature on Historical Trend of Long-Term Droughts in Different Climates of Iran

Effective monitoring of drought plays an important role in water resources planning and management, especially under global warming effect. The aim of this paper is to study the effect of air temperature on historical long-term droughts in regions with diverse climates in Iran. To this end, monthly air temperature (T) and precipitation (P) data were gathered from 15 longest record meteorological stations in Iran covering the period 1951–2014. Long-term meteorological droughts behavior was quantified using two different drought indices, i.e. the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). Linear and non-linear trends in T, P, SPI and SPEI were evaluated using non-parametric and parametric statistical approaches such as non-modified and modified Mann-Kendall Test, Theil-Sen approach, and simple regression. The results indicated that the significant trends for temperature are approximately all increasing (0.2 °C to 0.5 °C per decade), and for precipitation are mostly decreasing (−7.2 mm to −14.8 mm per decade). It was also indicated that long-term drought intensities monitored by the SPI and SPEI have had significant downward trend (drought intensification with time) at most stations of interest. The observed trends in the SPI series can be worsen if air temperature (in addition to precipitation) participates in drought monitoring as SPEI. In arid and extra arid climates, it was observed that temperature has strong effects on historical drought characteristics when comparing the SPI and SPEI series. Due to the determinative role of temperature in mostly dry regions like Iran, the study suggests using the SPEI rather than SPI for more effective monitoring of droughts.

Drought reconstruction based on grape harvest dates for the Czech Lands, 1499-2012

The timing of maturity of grapes depends on the weather conditions during the growing season. This study relies on the dependence of harvest dates on the air temperature and dry/wet conditions. Recorded observations show that increases in air temperature and dryness are associated with earlier grape harvests. Documentary data of grape harvests from the Bohemian wine-growing region (mainly northwest of Prague) were combined with mean Standardized Precipitation Evapotranspiration Index (SPEI) series starting in 1841 and ordinary least square regression with subsequent scaling to reconstruct the mean SPEI values for this area for April to August from 1499 to 2012. The reconstructed SPEI series explains 75% of the drought variability since 1841. All dry years that were detected by the reconstructed April-August SPEI values correlate well with years of excellent and good red wine of vintage quality for 1499-1840. A comparison of the reconstructed series with other SPEI reconstructions from the Czech Lands (the recent Czech Republic) based on documentary and instrumental data shows good agreement. The results demonstrate that grape harvest series may be used as a proxy for drought reconstruction in the central European region.

Long-term drought variability and trends in Barcelona (1787-2014)

Long-term drought variability and trends were assessed in Barcelona at annual and seasonal scale for the period 1787-2014 and sub-periods 1851-2014, 1901-2014 and 1951-2014 to identify changes in drought patterns across time. High quality and adjusted monthly temperature and precipitation series were required for this purpose. The Standardized Precipitation Index (SPI), based on precipitation, and the Standardized Precipitation Evapotranspiration Index (SPEI), based on the difference between precipitation and reference evapotranspiration (ET0), were calculated to describe temporal drought fluctuations. Therefore, major droughts and wet events were identified and an accurate analysis of drought severity, magnitude and duration were also carried out.Both drought indices provided similar results related to drought variability and trends in Barcelona across time, although the SPEI showed larger drought severity than SPI especially during the second half of the 20th century. Trends analysis revealed a significant drying trend at annual scale according to the SPEI since mid-19th century while the SPI did not show changes in drought patterns. At seasonal scale, both the SPI and SPEI found a clear drying trend only for summer (JJA) during the current period (1951-2014), although the SPEI was indicating the trend towards drier conditions for the whole period (1787-2014). Drought severity in SPEI series increased 13% during the second half of the 20th century compared with the whole period under study while drought magnitude and duration did not present significant changes in both the SPI and SPEI series. The increasing atmospheric evaporative demand associated to the large temperature rising experienced in Barcelona during the last decades could have played a substantial role in explaining the increase of drought severity and trends found in the SPEI series.

Transitional properties of droughts and related impacts of climate indices in the Pearl River basin, China

Summary Drought is the natural hazard poorly understood so far due to various mechanisms behind. Moreover, disastrous effects of drought on human society necessitate accurate forecasting of drought behaviors. In this case, to improve forecasting of drought in the Pearl River basin, a trivariate copula model has been developed and used to include the El Nino Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Indian Ocean Dipole (IOD) and Pacific Decadal Oscillation (PDO) into model structure of Markov chain. The Standardized Precipitation Evapotranspiration Index (SPEI) has been used to monitor the drought in this study. Comparison with the preliminary correlation analysis between each month climate index and SPEI series indicated that the trivariate copula performs satisfactorily well in evaluation of influences of climate indices on the transition probabilities of drought. It is considered that the region with the negative vertical velocity is dominated by more precipitation and vice versa. Moreover, field patterns of 500 hPa vertical velocity anomalies related to each climate index have further corroborated the influences of climate indices on the drought behaviors. Besides, the mean extreme drought durations under different conditions of each climate index have also been investigated in this study. Results indicated that the mean extreme drought duration tends to be longer in the western part of the Pearl River basin during positive phase of ENSO while tends to be shorter during the positive phase of NAO and vice versa; in the central part of the Pearl River basin, the mean extreme drought duration tends to be shorter during the positive phase of ENSO, NAO and PDO while tends to be longer during the positive episode of IOD, and vice versa; in the eastern part of the Pearl River basin, the mean extreme drought duration tends to be shorter during the positive episode of ENSO and PDO, and vice versa. This study sheds new light on transitional behaviors of droughts as a result of influences from climate indices.

Similarity, difference and correlation of meteorological and hydrological drought indices in a humid climate region – the Poyang Lake catchment in China

Based on the estimation of standardized precipitation–evapotranspiration index (SPEI) and standardized runoff index (SRI), this study investigated the variability and correlation of hydrological drought and meteorological drought in a humid climate region – the Poyang Lake catchment in China. Results indicate that the occurrences of hydrological droughts in the catchment are different from those of meteorological drought on both a seasonal and annual basis. However, annual variability of both indices showed the same periodic variation characteristics during the study period. With comparison of the performance of SPEI and SRI time series at different timescales, our observation reveals that the two drought indices show a higher degree of similarity and correlation as timescales increased. In addition, SRI is found to be less variable than SPEI at shorter timescales and it shows an obvious hydrologic delay of about 1–2 months in response to SPEI at timescales >12 months. Due to hydrologic detention of subsurface soil moisture, shallow groundwater and perhaps reservoir storage, a 2-month timescale of SPEI is found to be more appropriate for river discharge monitoring, especially for those rivers with similar drainage area, climate and geographical conditions as in this study region.

A Standardized Precipitation Evapotranspiration Index Reconstruction in the Taihe Mountains Using Tree-Ring Widths for the Last 283 Years.

Tree-ring samples from Chinese Pine (Pinus tabulaeformis Carr.) that were collected in the Taihe Mountains on the western Loess Plateau, China, were used to analyze the effects of climate and drought on radial growth and to reconstruct the mean April-June Standardized Precipitation Evapotranspiration Index (SPEI) during the period 1730–2012 AD. Precipitation positively affected tree growth primarily during wet seasons, while temperature negatively affected tree growth during dry seasons. Tree growth responded positively to SPEI at long time scales most likely because the trees were able to withstand water deficits but lacked a rapid response to drought. The 10-month scale SPEI was chosen for further drought reconstruction. A calibration model for the period 1951–2011 explained 51% of the variance in the modeled SPEI data. Our SPEI reconstruction revealed long-term patterns of drought variability and captured some significant drought events, including the severe drought of 1928–1930 and the clear drying trend since the 1950s which were widespread across northern China. The reconstruction was also consistent with two other reconstructions on the western Loess Plateau at both interannual and decadal scales. The reconstructed SPEI series showed synchronous variations with the drought/wetness indices and spatial correlation analyses indicated that this reconstruction could be representative of large-scale SPEI variability in northern China. Period analysis discovered 128-year, 25-year, 2.62-year, 2.36-year, and 2.04-year cycles in this reconstruction. The time-dependency of the growth response to drought should be considered in further studies of the community dynamics. The SPEI reconstruction improves the sparse network of long-term climate records for an enhanced understanding of climatic variability on the western Loess Plateau, China.