Drought Length Research Articles

Comparison of three reanalysis‐driven regional climate models over New Zealand: Climatology and extreme events

AbstractThree ~12‐km reanalysis‐driven regional climate models (RCMs) are evaluated in terms of capturing climatologies and extremes of precipitation, temperature and surface wind over Aotearoa/New Zealand (NZ). NZ provides an excellent case study for evaluating high‐resolution RCMs due to its coastal and complex terrain and isolated geographical position in the midlatitudes, exposed to both tropical and polar influences. Overall, we find that the RCMs faithfully reproduce the observed climate, with precipitation and temperature climatologies particularly well‐captured. However, the RCMs display significant differences from observations in capturing the surface wind climatology, highlighting a remaining key challenge. The excess “drizzle problem” is apparent to varying degrees, leading to a weaker representation in the length of meteorological drought in some regions. The RCMs also diverge in reproducing diurnal temperature range which appears partly related to cloud cover. Finally, we discuss the important role of observational uncertainty in the context of model evaluation.

Freshwater wetlands for flood control: How manipulating the hydroperiod affects plant and invertebrate communities.

Thoughtfully managed hydroperiods in natural and artificial wetlands could potentially provide a combination of desirable flood control services and high ecological functions. To explore how managed freshwater wetlands typical of the Houston, Texas area would respond to different hydrological regimes that might occur if wetlands were drained in anticipation of a heavy rain that did not materialize, we conducted a mesocosm experiment with six flooding depths and seven drought durations, followed by seven months of recovery. We found that the speed in which mesocosms dried out was a function of initial water depth, with mesocosms initially set with greater water depths (30 cm) taking ~ 38 days to dry out versus zero days for wetlands that were completely drained. Individual plant species (14 species planted; 8 species common at the end of the recovery period) were affected by drought length, flooding depth, or their interaction, although details of these responses varied among the species. The composition of the plant community at the end of the drought period was strongly affected by drought length, and the effect of the drought length treatment persisted through seven months of post-drought recovery, with the 80- and 160-day drought treatments diverging most strongly from shorter drought treatments. Above- and below-ground biomass of plants was not affected by the treatments, but above-ground dead biomass (litter) decreased with increasing drought length. Densities of mosquito larvae, snails and tadpoles were temporally variable, and were affected more during the treatment period and early in recovery than after a disturbance event late in recovery. Our results indicate that managed wetlands in southeast Texas would be quite resilient to dry periods of up to 40 days in duration, especially if water was not completely drained at the beginning of the drought. In addition, many species would persist in managed wetlands even with droughts of up to 160 days. This indicates considerable potential for managing the hydroperiods of artificial detention ponds by retaining water longer to increase ecological function, with little to no loss of flood control services, and for managing the hydroperiods of natural wetlands by draining them in advance of anticipated rains to increase flood control services, with little to no loss of ecological function.

Global groundwater droughts are more severe than they appear in hydrological models: An investigation through a Bayesian merging of GRACE and GRACE-FO data with a water balance model

Realistic representation of hydrological drought events is increasingly important in world facing decreased freshwater availability. Index-based drought monitoring systems are often adopted to represent the evolution and distribution of hydrological droughts, which mainly rely on hydrological model simulations to compute these indices. Recent studies, however, indicate that model derived water storage estimates might have difficulties in adequately representing reality. Here, a novel Markov Chain Monte Carlo - Data Assimilation (MCMC-DA) approach is implemented to merge global Terrestrial Water Storage (TWS) changes from the Gravity Recovery And Climate Experiment (GRACE) and its Follow On mission (GRACE-FO) with the water storage estimations derived from the W3RA water balance model. The modified MCMC-DA derived summation of deep-rooted soil and groundwater storage estimates is then used to compute 0.5∘ standardized groundwater drought indices globally to show the impact of GRACE/GRACE-FO DA on a global index-based hydrological drought monitoring system. Our numerical assessment covers the period of 2003–2021, and shows that integrating GRACE/GRACE-FO data modifies the seasonality and inter-annual trends of water storage estimations. Considerable increases in the length and severity of extreme droughts are found in basins that exhibited multi-year water storage fluctuations and those affected by climate teleconnections.

Changes in climate extremes in Zambia during green and dry Sahara periods and their potential impacts on hominid dispersal

Northern Africa experienced humid periods known as African humid periods or Green Sahara periods during the late Pleistocene and Holocene. The waxing and waning of the African Monsoon over the last several million years raises the question of how the climatic variability in the African Saharan region could have influenced the evolution and dispersion of hominins in Africa. Little is yet known about the changes in climate extremes in central southern Africa associated with these cycles and their potential impacts on human migration. In this study, we use a regional climate model to simulate archetypal Green and Desert Sahara periods under high and low boreal summer insolation and investigate the resulting changes in climate variability and extremes in South Tropical Africa, with a focus on Zambia. Our results indicate drier and warmer conditions under Green Sahara conditions relative to the Dry Sahara periods. In particular, an increase in the length of droughts and higher temperature extremes have been simulated over the Zambian region in the Green Sahara experiment. Our results suggest that during the Dry Sahara periods, Zambia may have offered better environmental conditions for hominin populations than the Central African Plateau (CAP). In contrast, the Green Sahara periods offered opposite conditions, potentially encouraging hominins to disperse through the large river valleys into the CAP and northward into the Sahel and Sahara.

Mortality thresholds of juvenile trees to drought and heatwaves: implications for forest regeneration across a landscape gradient

Tree loss is increasing rapidly due to drought- and heat-related mortality and intensifying fire activity. Consequently, the fate of many forests depends on the ability of juvenile trees to withstand heightened climate and disturbance anomalies. Extreme climatic events, such as droughts and heatwaves, are increasing in frequency and severity, and trees in mountainous regions must contend with these landscape-level climate episodes. Recent research focuses on how mortality of individual tree species may be driven by drought and heatwaves, but how juvenile mortality under these conditions would vary among species spanning an elevational gradient—given concurrent variation in climate, ecohydrology, and physiology–remains unclear. We address this knowledge gap by implementing a growth chamber study, imposing extreme drought with and without a compounding heatwave, for juveniles of five species that span a forested life zones in the Southwestern United States. Overall, the length of a progressive drought required to trigger mortality differed by up to 20 weeks among species. Inclusion of a heatwave hastened mean time to mortality for all species by about 1 week. Lower-elevation species that grow in warmer ambient conditions died earlier (Pinus ponderosa in 10 weeks, Pinus edulis in 14 weeks) than did higher-elevation species from cooler ambient conditions (Picea engelmannii and Pseudotsuga menziesii in 19 weeks, and Pinus flexilis in 30 weeks). When exposed to a heatwave in conjunction with drought, mortality advanced significantly only for species from cooler ambient conditions (Pinus flexilis: 2.7 weeks earlier; Pseudotsuga menziesii: 2.0 weeks earlier). Cooler ambient temperatures may have buffered against moisture loss during drought, resulting in longer survival of higher-elevation species despite expected drought tolerance of lower-elevation species due to tree physiology. Our study suggests that droughts will play a leading role in juvenile tree mortality and will most directly impact species at warmer climate thresholds, with heatwaves in tandem with drought potentially exacerbating mortality especially of high elevation species. These responses are relevant for assessing the potential success of both natural and managed reforestation, as differential juvenile survival following episodic extreme events will determine future landscape-scale vegetation trajectories under changing climate.

Examining past and projecting future: an 800-year streamflow reconstruction of the Australian Murray river

Managing water security and sustaining ecosystem functions under future warming poses substantial challenges for semi-arid regions. The Murray–Darling Basin (MDB) is particularly vulnerable given the considerable demand for water that underpins Australia’s agricultural production and contribution to the national economy. Understanding future drought risk requires a robust assessment of natural variability in drought length, frequency, and magnitude. In the absence of long instrumental records, past drought characteristics can be inferred from paleo-records. We reconstruct over 800 years of Murray River streamflow using a suite of tree-ring chronologies from regions with strong climate teleconnections to the MDB. The reconstruction (1190–2000 CE) captures a broad spectrum of natural climate variability, not fully represented in instrumental records, contributing to an improved understanding of the occurrence rate of multi-year droughts. We found that the Millennium Drought, which occurred in the 2000s, was the most severe (joint duration, magnitude, and peak) during the 800-year reconstruction. The return period of this event is estimated to be ∼2500 years. However, droughts in the early-1200s were of a longer duration and similar magnitude to the Millennium Drought. We used climate models to assess how the occurrence probability of severe droughts might change in the future. Compared to the 800-year baseline, climate models project an increase in future drought severity. While the increase in drought occurrence is within the uncertainty range for most future projections, the driest forecast shows a significant increase in the likelihood of severe droughts compared to natural variability. Our results highlight the need for water management strategies not to rely solely on instrumental data as it may not fully represent current and future risks. Ensuring a resilient MDB under future warming will require a robust water security policy that captures a broader range of natural and anthropogenic variability than currently recognised.

Compensatory response of ecosystem carbon-water cycling following severe drought in Southwestern China

Climate change has increased the frequency and length of droughts, but many uncertainties remain regarding the impacts of this aridification on terrestrial ecosystem function. Vegetation water use efficiency and carbon sequestration capacity are crucial determinants that both respond to and mediate the effects of drought. However, it is important to note that the consequences of drought on these processes can persist for years. A deeper exploration of these “drought legacy effects” will help improve our understanding of how climate change alter ecosystem carbon-water cycling. Here, we investigate the spatial patterns of drought legacy effects on remotely-sensed vegetation greenness (NDVI), net primary productivity (NPP) and water use efficiency (WUE) in southwestern China, a biodiversity hotspot that was impacted by an extreme drought in 2009–2010, with a particular focus on the tradeoff between WUE and NPP. Despite widespread negative drought legacy effects in NDVI (impacting 61.26 % of the study region), there was a general increase in NPP (58.68 %) and a decrease in WUE (67.53 %) in the year following drought (2011). This drought legacy effect was most evident in forests, while drought legacies in grasslands were less common. Drought legacies were also most apparent in relatively warm and humid areas. During the study period (2002 to 2018), we found that drought impacts on WUE also lagged behind changes in NPP by 1–2 years in forests, which highlights how drought legacies may manifest differently across ecosystem processes. Our study demonstrated that severe drought conditions may significantly affect the carbon sequestration capacity and water use efficiency of vegetation in southwestern China, and that forests may compensate for the detrimental effects of water stress by increasing water use and biomass growth after drought episodes.

The Potential of Deep Learning for Satellite Rainfall Detection over Data-Scarce Regions, the West African Savanna

Food and economic security in West Africa rely heavily on rainfed agriculture and are threatened by climate change and demographic growth. Accurate rainfall information is therefore crucial to tackling these challenges. Particularly, information about the occurrence and length of droughts as well as the onset date of the rainy season is essential for agricultural planning. However, existing rainfall models fail to accurately represent the highly variable and sparsely monitored West African rainfall patterns. In this paper, we show the potential of deep learning (DL) to model rainfall in the region and propose a methodology to develop DL models in data-scarce areas. We built two DL models for satellite rainfall (rain/no-rain) detection over northern Ghana from Meteosat TIR data based on standard DL architectures: Convolutional neural networks (CNNs) and convolutional long short-term memory neural networks (ConvLSTM). The Integrated Multi-satellitE Retrievals for the Global Precipitation Measurement (GPM) mission (IMERG) and Precipitation Estimation from Remotely Sensed Imagery Using an Artificial Neural Network Cloud Classification System (PERSIANN-CCS) products are used as benchmarks. We use rain gauge data from the Trans-African Hydro-Meteorological Observatory (TAHMO) for model development and performance evaluation. We show that our models compare well against existing products despite being considerably simpler, developed with a small training dataset—i.e., 8 stations covering 2.5 years with 20.4% of the data missing—and using TIR data alone. Concretely, our models consistently outperform PERSIANN-CCS for rain/no-rain detection at a sub-daily timescale. While IMERG is the overall best performer, the DL models perform better in the second half of the rainy season despite their simplicity (i.e., up to 120 k parameters). Our results suggest that DL-based regional models are a promising alternative to state-of-the-art global products for providing regional rainfall information, especially in meteorologically complex regions such as the (sub)tropics, which are poorly covered by ground-based rainfall observations.

Reservoir Capacity Estimation by the Gould Probability Matrix, Drought Magnitude, and Behavior Analysis Methods: A Comparative Study Using Canadian Rivers

Among the various methods for estimating reservoir volumes, the Gould probability matrix (GPM) method has been touted as a powerful method for estimating reservoir volumes. The other methods in vogue are the Behavior analysis (BA) with the latest induction of the Drought magnitude (DM) method. A comparison of the above methods in terms of ease, efficiency, and relative merits from each other is currently lacking in the literature. This paper compares the above three methods with a detailed analysis of the GPM method using the monthly flows from 16 Canadian rivers at the draft ratios of 75 and 50% with the probability of failure of 2.5, 5 and 10%. The results reported in this paper indicate that fifteen zones are sufficient in the GPM method to yield the reservoir capacity for the Canadian rivers while requiring no standardization of the data, similar to the BA method. In the DM method, standardized monthly flow sequences in combination with a scaling parameter Φ yielded effective drought length, which, when multiplied by drought intensity and the average of 12 monthly standard deviations, resulted in the appropriate values of reservoir capacity. The results of this paper affirm that the GPM method offers little special merit in obtaining reservoir capacity in view of the rigor of computational efforts and uncertainty in the correction factors for significantly autocorrelated (dependent) annual flows. The DM method was found to be comparable to the BA method, though it requires standardization of the monthly flow data. The study suggests that all three methods result in comparable estimates of reservoir capacity for nearly independent annual flows with a slight edge to the Behavior analysis (BA) method.

Intensity and spatiotemporal variations of drought in Tumakuru district, India using drought indices

Depletion of water resources and soil moisture leading to drought is a global concern and the effective assessment and monitoring using a drought model has become essential. A detailed account of frequency, run length and temporal trend of the drought events are presented in the study from 1981 to 2019 at the 1-, 3-, 6-, 9-, 12- and 24-month timescale using the Standardized Precipitation Index and Standardized Precipitation Evapotranspiration Index. SPI recorded more drought months in the extreme category with 14 months at Gubbi station for example, while SPEI showed only 5 months. SPEI showed longer drought length in moderate and severe categories for agricultural and hydrological drought. At a 1-month timescale, both the indices reported extreme drought events where Tiptur station in May 2016 (–4.75) and Chikkanayakanahalli station in March 1992 (–2.76) were the worst-case scenarios. The study aims at providing practical results to facilitate decision-makers for drought risk management.

Featured Collection Introduction: Severe Sustained Drought Revisited: Managing the Colorado River System in Times of Water Shortage 25 Years Later — Part I

Featured Collection Introduction: Severe Sustained Drought Revisited: Managing the Colorado River System in Times of Water Shortage 25 Years Later — Part I

Quantification of meteorological drought risks between 1.5 °C and 4 °C of global warming in six countries

We quantify the projected impacts of alternative levels of global warming upon the probability and length of severe drought in six countries (China, Brazil, Egypt, Ethiopia, Ghana and India). This includes an examination of different land cover classes, and a calculation of the proportion of population in 2100 (SSP2) at exposed to severe drought lasting longer than one year. Current pledges for climate change mitigation, which are projected to still result in global warming levels of 3 °C or more, would impact all of the countries in this study. For example, with 3 °C warming, more than 50% of the agricultural area in each country is projected to be exposed to severe droughts of longer than one year in a 30-year period. Using standard population projections, it is estimated that 80%-100% of the population in Brazil, China, Egypt, Ethiopia and Ghana (and nearly 50% of the population of India) are projected to be exposed to a severe drought lasting one year or longer in a 30-year period. In contrast, we find that meeting the long-term temperature goal of the Paris Agreement, that is limiting warming to 1.5 °C above pre-industrial levels, is projected to greatly benefit all of the countries in this study, greatly reducing exposure to severe drought for large percentages of the population and in all major land cover classes, with Egypt potentially benefiting the most.

What drives forest carbon storage? The ramifications of source-sink decoupling.

This article is a Commentary on Martínez‐Sancho et al. (2022), 236: 58–70.

Influence of drought duration and severity on drought recovery period for different land cover types: evaluation using MODIS-based indices

Drought is a slow-onset phenomenon driven by the lack of precipitation, affecting the performance of plants and functionality of terrestrial ecosystems. In addition to the length and severity of drought, the period it takes for the plants to return to normal conditions is critical. Remote sensing data with appropriate spatial and temporal coverage facilitates monitoring drought and its consequences on local and global scales. This study investigated the influence of drought duration and severity on the drought recovery period (DRP) for different land use and land cover (LULC) types in Iran. The moderate resolution imaging spectroradiometer (MODIS)-based vegetation health index (VHI) was used to monitor drought in the period 2000–2020. The results identified 2000, 2001, and 2008 as drought years. DRP was estimated using gross primary productivity (GPP). The findings revealed that shrubland and cropland experienced more prolonged droughts than forests, which experienced the shortest drought duration. Similarly, shrublands and croplands had the most prolonged recovery, and forests had the shortest recovery time. A direct relationship was observed between drought severity and DRP in all LULC types, however the local correlation between drought duration and recovery time better revealed the heterogeneity of relationships. This study provides valuable information on the drought resilience of different LULC types for use in achieving better management and a deeper understanding of drought.

A Procedure for Estimating Drought Duration and Magnitude at the Uniform Cutoff Level of Streamflow: A Case of the Weekly Flows of Canadian Rivers

At times, hydrological drought is defined using Q90 or Q95 (90% or 95% flows equaling or exceeding) or even at higher levels, such as Q75 as the cutoff level regardless of their seasonal variation (i.e., truncation at the uniform flow level). In the past, the estimation of drought length and magnitude at the aforesaid uniform cutoff levels of flow has been a challenging issue. A procedure is presented to first estimate the drought magnitude (M), which then forms the basis for estimating the drought duration or length (L). The drought magnitude (M) and the length of the critical period (Lcr) are estimated using the concept of behavior analysis prevalent in the hydrologic design of reservoirs. This information is used for estimating the drought length (LT-e′, the estimated value of drought length for the return period of T weeks) involving a Markov chain model on the standardized weekly flow sequences. A weighted average of Lcr and LT-e′ (=0.60 Lcr + 0.40 LT-e′) results in the estimate of drought length, which is compatible to the observed counterpart. The performance of the procedure to estimate drought length was found to be satisfactory up to the truncation level of Q75, whereas the estimation of drought magnitude was rated as good.

History and Projection of Hydrological Droughts in the Benin Basin of the Niger River (Benin)

In the context of a changing climate, the Beninese Niger River basin has been the focus of several research studies for the quantification, planning, and modeling of water and related resources for sustainable use. This research aims to characterize the historical (1976-2019) and projected (2021- 2050) hydrological drought of the Beninese Niger River basin. The study used daily observations of rainfall, maximum and minimum temperatures, runoff rates and simulations of HIRHAM and REMO RCMs from fifteen (15) rainfall stations installed around the basin. It uses standardized streamflow indices (SDI) at 12-month and 36-month time steps. The results show that the calculated SDI indices show, on average, for all the model scenarios used, chronological trends of increase. These increases are not significant (are of the order of 0.00001 per year). The analysis of the SDI indices shows that, on average, the hydrological droughts in the Beninese basin of the Niger River will increase at 36 months and decrease at 12 months of the SDI. In fact, these small variations of hydrological droughts will be accompanied by the increase of their duration and the decrease of their magnitudes. The droughts detected in the Benin basin of the Niger River during the historical period will continue until 2050 in the same range but with more extended drought lengths. It should be noted that most of the changes observed in the calculated and analyzed indices are not significant.

Range-wide persistence of the endangered arroyo toad (Anaxyrus californicus) for 20+ years following a prolonged drought.

Prolonged drought due to climate change has negatively impacted amphibians in southern California, U.S.A. Due to the severity and length of the current drought, agencies and researchers had growing concern for the persistence of the arroyo toad (Anaxyrus californicus), an endangered endemic amphibian in this region. Range‐wide surveys for this species had not been conducted for at least 20 years. In 2017–2020, we conducted collaborative surveys for arroyo toads at historical locations. We surveyed 88 of the 115 total sites having historical records and confirmed that the arroyo toad is currently extant in at least 61 of 88 sites and 20 of 25 historically occupied watersheds. We did not detect toads at almost a third of the surveyed sites but did detect toads at 18 of 19 specific sites delineated in the 1999 Recovery Plan to meet one of four downlisting criteria. Arroyo toads are estimated to live 7–8 years, making populations susceptible to prolonged drought. Drought is estimated to increase in frequency and duration with climate change. Mitigation strategies for drought impacts, invasive aquatic species, altered flow regimes, and other anthropogenic effects could be the most beneficial strategies for toad conservation and may also provide simultaneous benefits to several other native species that share the same habitat.

The impact of drought length and intensity on N cycling gene abundance, transcription and the size of an N2O hot moment from a temperate grassland soil

This study aimed to investigate the relationship between drought length, drought intensity and the size of the N2O hot moment. It selected two treatments to deduce the main nitrogen cycling process producing N2O (increasing WFPS from 40% to 90%, and from 70% to 90%), by destructively sampling soil cores to analyse gene abundance, transcription, and changes in soil chemistry (TON, NH4+, DOC). Five other drought and rewetting treatments on packed soil cores were selected to create the drought curves described in Barrat et al. (2020): these included increases of WFPS from 40% to 90%, 50%–90%, 60%–90%, 70%–90%, and 30%–60%. For each treatment, drought lengths were imposed from 0 to 30 days. A quadratic linear regression was fitted to the cumulative emissions data. This model explained a significant proportion of the total variation in the data (R2 = 0.72, p ≤ 0.001). All treatments had an increase in daily N2O emissions post wetting typical of a hot moment apart from the 30%–60% WFPS treatment. In terms of drought intensity, the 40%–90% WFPS was significantly larger than rest, probably due to a relatively larger change in water potential compared to the other treatments. The response to drought length followed a quadratic curve with a downward linear trend, with the largest emissions observed between 10 and 15 days of drought, and the smallest at 0 and 30 days. We suggest a 2-stage dormancy strategy to explain this, where microbes under dry conditions store osmolytes which are catabolised upon rewetting, however at prolonged negative water potentials this strategy is no longer effective, and so they enter a deeper state of dormancy where they can no longer rapidly respond to the changing water potential. Given the delayed response after rewetting, and the inverted U shaped curve in terms of drought length, it seems likely that the majority of emissions are of biological origin. The soil's chemistry data suggested that NH4+ was a key factor controlling the emission flux, but the transcriptional and genomic data were inconclusive. This study therefore suggests that future experiments should focus changes in osmolyte accumulation and catabolism as the key explanation for N2O hot moments, rather than changes in genomic and transcriptomic data or soil substrates, which do not always correlate with emissions.

An approach to identify the best climate models for the assessment of climate change impacts on meteorological and hydrological droughts

Abstract. This paper describes the benefits of using more reliable local climate scenarios to analyse hydrological responses. It assumes that Regional Climate Model (RCM) simulations are more reliable when they provide better approximations to the historical basic and drought statistics after applying a bias correction to them. We have investigated whether the best solutions in terms of their approximation to the local meteorology may also provide the best hydrological assessments. We have carried out a classification of the corrected RCM simulations used for both approximations. This has been applied in the Cenajo basin (south-eastern Spain), where we show that the best approximations of the historical meteorological statistics also provide the best approximations for the hydrological statistics. The selected RCMs were used to generate future (2071–2100) local scenarios under the RCP8.5 emission scenario. The two selected RCMs predict significant changes in mean precipitation (−31.6 % and −44.0 %) and mean temperature (+26.0 % and +32.2 %). They also predict higher frequency (from 5 events in the historical period to 20 and 22 in the future), length (4.8 to 7.4 and 10.5 months), magnitude (2.53 to 6.56 and 9.62 SPI) and intensity (0.48 to 1.00 and 0.94 SPI) of extreme meteorological droughts. These two RCMs also predict higher changes in mean streamflow (−43.5 % and −57.2 %) and hydrological droughts. The two RCMs also predict worrying changes in streamflow (−43.5 % and −57.2 %) and hydrologically extreme droughts: frequency (from 3 to 11 for the first model and 8 events for the second model), length (8.3 to 15.4 and 29.6 months), magnitude (from 3.98 to 11.84 and 31.72 SSI), and intensity (0.63 to 0.90 and 1.52 SSI).

Trends in drought occurrence and severity at mid-latitude European stations (1951\u20132015) estimated using standardized precipitation (SPI) and precipitation and evapotranspiration (SPEI) indices

One of the negative consequences of climate change is the also increase in the severity, frequency and length of droughts appearing in Europe. The effects of meteorological drought are often substantial, not only for the natural environment but also for humans. Hence, the main purpose of this research was to determine the trends in the severity and occurrence of droughts in Europe during the period 1951–2015 using the standardized precipitation index and the standardized precipitation and evapotranspiration index (SPEI). For six European sites located in mid latitudes, the number of dry months was determined and the trend of their occurrence was examined. Moreover, for the summer months in which the indicators fell below 0, the trend related to the severity of the drought was determined for each site. Despite the absence of a statistically significant trend of an increase in the occurrence of dry months in general, an increase in the severity of droughts occurring in summer was observed for the SPEI over a 6-month scale for all the investigated stations. The highest number of dry months since the 1970s appeared during the last 5 years of analysis.