Hydrological Drought Duration Research Articles

Impact of hydrological drought occurrence, duration, and severity on Murray-Darling basin water quality

The severity and frequency of droughts are projected to increase globally due to climate change, but the effects of this on water quality are uncertain. The Murray-Darling Basin (MDB) is the largest river system in Australia and has been impacted by droughts of varying severity within recent decades. In this study, we assessed the influence of hydrological droughts and their characteristics (severity and duration) on water quality, utilising a long-term (1980–2017) dataset from two monitoring sites. The main drought periods, and their duration and severity, were identified using the calculated Standardised Drought Index values (SDI) from averaged monthly streamflow data. While several hydrological drought periods were identified, the longest duration and greatest severity were during the Millennium Drought (1998–2010). Nutrient loads and concentrations of Total Nitrogen and Total Phosphorus of drought and post-drought periods were significantly different. The drought period showed the lowest median and interquartile range of nutrient (total nitrogen, TN; oxidised nitrogen, NOX; total phosphorus, TP; and soluble reactive phosphorus, SRP) concentrations and loads for both sites, whereas the highest nutrient loads and concentrations were reported during the post-drought period (approx. 1 × 103 to 1 × 105 kg day−1 increase in nutrient loads). Our analysis found significant relationships between nutrient loads and SDI during droughts. The load of N and P in the initial flush post-drought increased with drought at both sites. This suggests that nutrients were retained in the landscape during the drought and released in higher loads post-drought when the catchment became wetter, the hydrology was activated, and nutrients were mobilised. Hydrology is a key driver controlling the water quality within the inter-drought period and the peak nutrient loads post-drought. The duration and the severity of droughts had a significant (p = 0.01) influence on peak TN and TP monthly loads but not cumulative loads over a 12-month period. Hydrological droughts are important factors in controlling the water quality of the MDB. Therefore, management efforts should be focused on reducing the occurrence and duration of these events, along with the implementation of catchment nutrient control measures.

Evaluation of climate-change impacts on the temporal and spatial behaviour of drought in South-Central Chile

ABSTRACT Chile is particularly vulnerable to climate change due to its geomorphology, ecological wealth and economic dependence on water resources, and has been facing a severe drought since 2010. The objective of this work is to propose a methodology able to project and evaluate changes in the magnitude, duration and frequency of meteorological and hydrological droughts, as a decision-making tool to support adaptation to climate change. Four basins were assessed using different downscaled General Circulation Models (GCM) and the Soil & Water Assessment Tool (SWAT) model. Results were evaluated using Standardized Precipitation Index (SPI) and Standardized Streamflow Index (SSFI) comparing drought events over two periods of time. An increase in magnitude and duration of meteorological droughts is projected, especially in the Andes Mountain range. Regarding hydrological drought, greater increases in magnitude and duration are projected for the Itata River Basin. In both types of droughts, an increase in frequency is expected.

Assessment and Prediction of Hydrometeorological Drought in Corong River Basin, Indonesia

Hydrological drought analysis in a river basin is crucial because it impacts water resource management, agriculture, drinking water supply, industry, ecology, and disaster risk mitigation. It plays a key role in water usage planning, safeguarding agricultural yields, and ensuring a stable drinking water supply. In the context of this research, the Corong River basin is used as a case study. This study aims to determine the level of hydrological drought (deficit) using the Threshold Level Method (TLM) and predict hydrological drought using the Seasonal Autoregressive Integrated Moving Average (SARIMA) model. This model understands the characteristics of drought and predicts drought in the Corong River basin for early mitigation and anticipation of drought in the region. The results of this research indicate a strong relationship between the Hydrological Drought Index (HDI) and streamflow and Precipitation (PRCP), which can be used to forecast future droughts. This model is highly robust and accurate in observing the level of hydrological drought (deficit, duration, and sharpness) and predicting drought events in the Corong River basin. These findings have significant practical implications for water management and disaster risk mitigation in this river basin while also advancing the scientific understanding of hydrological drought.

Future changes in hydrological drought across the Yangtze River Basin: identification, spatial–temporal characteristics, and concurrent probability

Hydrological droughts have caused greater damage due to climate change and rapid socio-economic development. Exploring future hydrological drought variations is imperative for drought defense and infrastructure construction. This study aims to investigate future changes in hydrological droughts across the Yangtze River Basin during 2025 ∼ 2100. The Distributed Time-Variant Gain Model coupled with the Standard Operation Policy module (DTVGM-SOP) was employed to project future runoff driven by the bias-corrected Coupled Model Intercomparison Project Phase 6 (CMIP6) data. Subsequently, future spatial–temporal changes in the characteristics of hydrological droughts were analyzed based on the Standardized Runoff Index (SRI). Lastly, future changes in the probability of concurrent hydrological droughts were evaluated using the bivariate copula functions. Results show that: (1) The DTVGM-SOP model can satisfactorily simulate monthly streamflow with a high Nash-Sutcliff Efficiency (NSE) of 0.75 ∼ 0.92. Future annual maximum discharge and annual average discharge in the mainstream and the northern tributaries are expected to increase by 3.34%∼58.19% and 3.11%∼32.66% respectively, while annual maximum discharge in the southern tributaries decreases by 0.54%∼20.09%. (2) The future annual and seasonal hydrological drought frequency is expected to decrease in most regions, while the frequency in spring increases in the Qinghai Tibet Plateau. The duration and severity of future hydrological droughts are expected to decrease by 9.52%∼78.69% and 5.13%∼81.36% in the whole basin except the source area and the Yalong River Basin. (3) Future concurrent hydrological droughts will be mitigated with decreasing frequency. The probability of concurrent hydrological droughts is expected to decrease by 46.15%∼96.47%, while future changes in the conditional probability of the mainstream are diverse in different concurrent scenarios with a range of −93.41%∼40.09%. The findings are helpful to improve the understanding of future hydrological drought variations in the Yangtze River Basin and provide essential information for damage reduction.

Multidecadal trend analysis of hydrological drought along River Niger using the Streamflow Drought Index

Droughts affect human well‐being and the economy of countries across the world. Understanding the long‐term evolution of droughts within a particular region will help in drought mitigation and adaptation plans, thereby reducing drought impact on the environment. This study examined the multidecadal trends in hydrological droughts at two stations along River Niger using 3‐month, 6‐month, and annual time scales. Hydrological drought events were identified using the streamflow drought index (SDI) between 1915 and 1990 based on the Theil‐Sen slope and Mann‐Kendall approaches. Across the timescales, extreme and severe droughts occurred in 1982/84 and 1983/84 with ‐2 ≤ Sd < ‐1.5. On an annual scale, the results from the annual SDI further showed that the 7th and 8th decades (1971−1990) recorded more drought events of varying degrees ranging from mild to extreme drought in both stations than in other decades. The last two decades (7th and 8th) further revealed the most extended hydrological drought duration from 1974/75 to 1988/89 for Baro and from 1979/80 to 1988/89 for Lokoja. The highest severity recorded at Baro was ‐15.56 and ‐14.26 at Lokoja. The prolonged duration of drought events across the stations and their associated yearly intensities suggests that more proactive measures are needed to ameliorate the hydrological drought impact in the study area.

The role of human activities in the weakening of the propagation relationship between meteorological and hydrological droughts in the Heihe River Basin

AbstractMeteorological droughts could propagate into hydrological droughts and cause agricultural and societal damage. However, the quantitative impact of human activities on the drought propagation has not been well understood. Based on the precipitation and streamflow observational data and hydrological model simulations during 1961–2013, this study analysed the drought propagation characteristics and quantified the impact of human activities on the propagation in the upper and middle reaches of Heihe River Basin. The matched meteorological and hydrological drought events indicated that the drought propagation was characterized by a lagged onset, weakened drought severity and reduced development and recovery speeds. In the upstream area, the correlations between meteorological and hydrological droughts were significant, and human activities had a negligible impact on the drought propagation. In contrast, the impact from human activities was significant in the midstream area, and the effect was two‐fold. Specifically, human activities (e.g., reservoir regulation) reduced the propagation probability of one or several meteorological droughts propagating into a hydrological drought by 16%. Meanwhile, human activities had remarkable effects on long‐lasting and extreme hydrological droughts through reducing their duration and severity by 18% and 37% respectively. On the other hand, the occurrence probability of mismatched drought events, that is, meteorological droughts without causing hydrological droughts and hydrological droughts without preceding meteorological droughts, increased by 34% due to human interventions. Human activities also accelerated the development speed of slowly developing hydrological droughts. In addition, human activities increased the mean duration and severity of moderate hydrological droughts by 32% and 18%. The results are expected to provide scientific bases for understanding the formation and development process of hydrological drought and improving drought forecasting under global change.

Identifying driving processes of drought recovery in the southern Andes natural catchments

Study regionThe natural river basins of Chile. Study focusDrought effects on terrestrial ecosystems produce hydroclimatic stress with variable extensions. Particularly, hydrological drought duration can provide a better understanding of recovery together with catchment characteristics and climatology. This study focuses on the impacts of the multi-year drought experienced in Chile for more than a decade.The recovery of relevant catchment variables to quantify the drought termination (DT) and drought termination duration (DTD) after the hydrological drought is presented. A composite analysis of natural catchments using the CAMELS-CL data set discharge (1988–2020), k-NDVI (2000–2020), and soil moisture (1991–2020) provides the average response of the recovery after severe droughts. New hydrological insights for the regionThis study demonstrates that local catchment properties can explain the recovery of studied variables after a hydrological drought.Explanatory variables from CAMELS-CL to derive the DT using random forest regression (RFR) were used with a strong correlation of 0.92, 0.84, and 0.89 for discharge, vegetation productivity, and soil moisture, respectively.The discharge patterns show longer recovery over environments dominated by shrublands with less precipitation and higher temperatures, in central Chile, while higher latitudes with higher vegetation cover, increasing precipitation, and lower temperatures present shorter recovery times. The vegetation productivity shows longer recovery over highly vegetated mountains in central Chile. The soil moisture recovery spatial distribution presented patterns that connect them with the discharge recovery. This work enables the identification of drought vulnerability, which is valuable for managing water resources and ecosystems and is helping to predict drought recovery periods in regions with a lack of observations.

Comparative Analysis of Drought Indices in Hydrological Monitoring in Ceará’s Semi-Arid Basins, Brazil

The purpose of this work is to evaluate the applicability of five drought indices (Surface Water Supply Index—SWSI, Reclamation Drought Index—RDI, Streamflow Drought Index—SDI, Standardized Precipitation Index—SPI, and Evaporative Demand Drought Index—EDDI) as tools for monitoring, in identifying the duration, intensity, and frequency of hydrological droughts in the basins of the Banabuiú, Castanhão, and Orós reservoirs, located in the state of Ceará, Brazil. The analysis focused on determining the performance of the indexes for capturing the droughts characteristics from 2002 to 2020. Thus, the comparison of the values of the indexes with the Target Levels of the reservoir’s operation were used, as well as the analysis of six decision criteria: robustness, tractability, transparency, sophistication, extensibility, and dimensionality, to compare the behavior of drought indices. The results of the evaluation criteria showed that the SPI was superior to the other indices, being able to significantly represent the drought episodes, capturing a greater number of events. Thus, SPI received the highest total weighted score (118), followed by SWSI (97), EDDI (95), SDI (95), and RDI (88). In this context, it was found that the SPI and SWSI are the most suitable indices to monitor drought in the region and that the use of longer time scales can be recommended to manage hydrological droughts, in order to improve planning and management of water resources.

Possible Future Climate Change Impacts on the Meteorological and Hydrological Drought Characteristics in the Jinghe River Basin, China

Revealing the impact of future climate change on the characteristics and evolutionary patterns of meteorological and hydrological droughts and exploring the joint distribution characteristics of their drought characteristics are essential for drought early warning in the basin. In this study, we considered the Jinghe River Basin in the Loess Plateau as the research object. The standardized precipitation index (SPI) and standardized runoff index (SRI) series were used to represent meteorological drought and hydrological drought with monthly runoff generated by the SWAT model. In addition, the evolution laws of the JRB in the future based on Copula functions are discussed. The results showed that: (1) the meteorological drought and hydrological drought of the JRB displayed complex periodic change trends of drought and flood succession, and the evolution laws of meteorological drought and hydrological drought under different spatiotemporal scales and different scenario differ significantly. (2) In terms of the spatial range, the JRB meteorological and hydrological drought duration and severity gradually increased along with the increase in the time scale. (3) Based on the joint distribution model of the Copula function, the future meteorological drought situation in the JRB will be alleviated when compared with the historical period on the seasonal scale, but the hydrological drought situation is more serious. The findings can help policy-makers explore the correlation between meteorological drought and hydrological drought in the background of future climate change, as well as the early warning of hydrological drought.

Anthropogenic drought in the Yellow River basin: Multifaceted and weakening connections between meteorological and hydrological droughts

Connections between meteorological and hydrological droughts (CMHDs) have changed due to increasingly intensified human activities. Understanding such changes from an anthropogenic perspective is, therefore, of critical importance for drought monitoring, early warning, and adaptation. As the second-longest river system in China, the Yellow River basin (YRB) has been seriously impacted by humans and has a high risk of drought due to its dry climate, dense population and massive water demand. However, the effects of human activities on CMHDs in the YRB are still unclear. We thus used the standardized precipitation evapotranspiration index (SPEI) and the standardized streamflow index (SSI) to indicate meteorological drought (MD) and hydrological drought (HD), respectively, and analysed CMHDs with multiple indicators including trends, correlation, difference, and drought propagation characteristics. Our results highlighted the heterogeneity of CMHDs in terms of subregions and indicators and showed that this heterogeneity was impacted by humans by quantifying the effects of human activities (mainly referring to water abstraction and regulation) on CMHDs in each subregion. We found that 1) human activities aggravated the disconnections between MD and HD by sharpening the downward trend in the SSI and weakening the correlation (amplifying the difference) between the SPEI and SSI, and 2) human activities reduced the drought propagation rate from MD to HD, manifested by a decreased number of HD but a lengthening of the HD duration. In addition to the abovementioned effects that were consistent for most of the YRB, there were also some effects, for example, on HD intensity and drought propagation time, that differed among subregions, and these kinds of diverse responses of CMHDs might be related to the various human activities in terms of intensities, types, and regions. By analysing human effects on CMHDs, this study may be helpful in understanding drought and drought propagation in the Anthropocene, which is beneficial to watershed management.

Future hydrological drought changes over the upper Yellow River basin: The role of climate change, land cover change and reservoir operation

The upper Yellow River Basin (UYRB) provides more than 50 % of freshwater for the Yellow River. Recent research reveals continuous climatic wetting and vegetation greening over the UYRB in a warming future, but future hydrological drought changes remain unclear due to complex interactions between climate change, land cover change and reservoir operations. Here we project hydrological drought changes at different global warming levels and quantify contributions from each driving factor. The Conjunctive Surface-Subsurface Process eco-hydrological model is coupled with a reservoir module, and the new CSSPv2 + Reservoir model is used to perform future projections driven by bias-corrected meteorological forcings from 11 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The CSSPv2 + Reservoir model simulates monthly streamflow over the UYRB reasonably well with a Kling-Gupta efficiency of 0.75. The increasing precipitation during dry seasons reduces hydrological drought duration by 9 %/12 %/19 % at the 1.5℃/2.0℃/4.0℃ warming level, while its impact on drought severity is limited. The dramatic increase of leaf area index at the 4.0℃ warming level robustly exacerbates hydrological drought severity to 28 % by intensifying evapotranspiration. The reservoir operation, designed to reduce seasonality in monthly releases to ensure stable hydropower generation, further decreases drought duration, but increases drought frequency and severity to 63 %/48 %/108 % and 9 %/11 %/32 % at the 1.5℃/2.0℃/4.0℃ warming levels respectively. A mitigated operation rule which stores less water during wet seasons, however, can reduce hydrological drought severity and duration significantly. Thus, future reservoir operations should pay more attention to the trade-off between hydropower generation and hydrological drought mitigation.

Wetland mitigation functions on hydrological droughts: From drought characteristics to propagation of meteorological droughts to hydrological droughts

Wetlands have been singled out as a potential nature-based solution for improving the resilience and reducing the risks of hydrometeorological extremes. However, whether and to what extent wetlands can affect hydrological droughts is not well understood. To fill this gap, we proposed a general framework to discern the effect of wetlands on: (i) the characteristics (duration, severity, development and recovery processes) of hydrological droughts, and (ii) the propagation of meteorological droughts to hydrological droughts. First, hydrological modeling was conducted with a spatially-explicit model integrated with wetland modules. Then, the run theory and pooling method were selected to recognize hydrological drought events and identify their characteristics. Further, the Pearson correlation coefficient, temporal shift method and cross wavelet transform were used to explore the propagation processes. Finally, the characteristics and propagation processes were compared to quantify wetland mitigation services on hydrological droughts. To validate the proposed framework, two river basins from China and Canada (the Gan River Bain and the Nelson River Bain), with distinct land cover, were chosen to perform hydrological modeling and quantify wetland effects. The results indicate that wetlands mainly contribute to alleviating hydrological droughts by decelerating the development process, accelerating the recovery, shortening the duration, and reducing the severity of the hydrological drought events. However, the effects are variable as they may have weak impacts and even worsen drought conditions. Wetlands can extend drought propagation time and weaken the transition of meteorological to hydrological droughts. The likelihood of hydrological drought formation due to meteorological decreased by 19% and 18% respectively, for the Gan River Bain and Nelson River Bain, thanks to the mitigation services of wetlands. These findings highlight the drought-mitigation roles of wetlands and the proposed modelling framework has the potential to be useful in beneficial in assisting basin management on drought risks in the context of climate change mitigation.

Analysis of drought conditions and their impacts in a headwater stream in the Central European lower mountain ranges

Headwaters represent a significant fraction of the global stream length and are important for streamflow quality and quantity. Since climate change is predicted to affect runoff generation processes fundamentally, it is essential to understand potential consequences for the water availability in headwater catchments. The Lehstenbach catchment, located in the Fichtel Mountains (Germany), represents many headwater catchments in the lower mountain ranges in Central Europe. This study’s primary objective is to predict and analyze potential shifts in the catchment’s water balance, estimate periods of hydrological drought conditions, and their characteristics. For this purpose, we used an integrated process-based hydrological model to represent surface/groundwater interactions and runoff generation mechanisms for the Lehstenbach catchment until 2100, using a Regional Climate Model Ensemble. The simulations indicate decreased water availability in summer and autumn, mainly due to increased evapotranspiration rates. The Minimum Environmental Flow (MEF), a quantitative measure of aquatic species’ exposition to abnormally low streamflow conditions, implies an increase of low flow conditions towards 2100. A first estimate indicates a possible increase of hydrological drought duration and intensity in the future. These findings suggest severe impacts on ecosystem health and services, such as decreasing water availability, leading to consequences like forest and wetland degradation and declining biodiversity. These findings can be used to implement suitable mitigation strategies to reduce climate change effects on the headwater ecosystems, such as water shortage for irrigation and drinking water supply and loss of flora and fauna.

Analysis of the Potential Impact of Climate Change on Climatic Droughts, Snow Dynamics, and the Correlation between Them

Climate change is expected to increase the occurrence of droughts, with the hydrology in alpine systems being largely determined by snow dynamics. In this paper, we propose a methodology to assess the impact of climate change on both meteorological and hydrological droughts, taking into account the dynamics of the snow cover area (SCA). We also analyze the correlation between these types of droughts. We generated ensembles of local climate scenarios based on regional climate models (RCMs) representative of potential future conditions. We considered several sources of uncertainty: different historical climate databases, simulations obtained with several RCMs, and some statistical downscaling techniques. We then used a stochastic weather generator (SWG) to generate multiple climatic series preserving the characteristics of the ensemble scenario. These were simulated within a cellular automata (CA) model to generate multiple SCA future series. They were used to calculate multiple series of meteorological drought indices, the Standardized Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), and a novel hydrological drought index (Standardized Snow Cover Index (SSCI)). Linear correlation analysis was applied to both types of drought to analyze how they propagate and the time delay between them. We applied the proposed methodology to the Sierra Nevada (southern Spain), where we estimated a general increase in meteorological and hydrological drought magnitude and duration for the horizon 2071–2100 under the RCP 8.5 emission scenario. The SCA droughts also revealed a significant increase in drought intensity. The meteorological drought propagation to SCA droughts was reflected in an immediate or short time (1 month), obtaining significant correlations in lower accumulation periods of drought indices (3 and 6 months). This allowed us to obtain information about meteorological drought from SCA deficits and vice versa.

Analysis of trends, recurrences, severity and frequency of droughts using standardised precipitation index: Case of OR Tambo District Municipality, Eastern Cape, South Africa.

South Africa is susceptible to droughts. However, little documentation exists on drought occurrence in South Africa at national, provincial and municipal administrative boundaries. This study profiles hydrological drought in OR Tambo District Municipality from 1998 to 2018, computing frequency, severity and intensity in order to show areas of high vulnerability. Data used were obtained from South African Weather Services. Standardised precipitation index (SPI) was calculated using the Meteorological Drought Monitor (MDM) software. Results showed a wide variation in monthly precipitation throughout the year. Coastal areas receive higher rainfall than inland municipalities. The study revealed that Nyandeni experienced the highest drought frequency of 62%, Mhlontlo (58%), King Sabatha Dalindyebo Municipality (57%), Ngquza Hill (55%) and Port St Johns Municipality showing the least at 52%. Hydrological drought severity frequency and duration varied between seven days and nine weeks. Drought intensity class exposed the annual average intensity for the five local municipalities represented as follows: KSDM (–0.71), PSJM (–0.99), Ngquza Hill (–0.81), Nyandeni (–0.71) and Mhlontlo (–0.62). The longest drought duration across OR Tambo was experienced in 2014 with durations varying from 3 to 11 weeks across the municipalities. OR Tambo District Municipality is susceptible to hydrological droughts and the extent varies across local municipalities. Results could be used for both adaptation planning and mitigating the impacts of future droughts. In addition, they could assist in guiding allocation of drought relief resources in ways that prioritise drought prone and vulnerable municipality.

Assessment of Hydrological Drought Using the Standardized Streamflow Index (SSFI): A Case Study of the Tien Yen River Basin of Quang Ninh Province, Vietnam

Probabilistic assessment of drought plays an important role in providing valuable information for evaluating water resources systems under drought conditions, and bivariate copulas are effective and efficient for the probabilistic assessment of drought based on joint distributions and/or joint return periods of drought characteristics. In this study, hydrological drought events and their characteristics (including duration and severity) in the Tien Yen River Basin of Quang Ninh province are detected using the Standardized Streamflow Index (SSFI). The BB8Copula is selected as the best-fit copula for hydrological drought duration and severity. Joint probabilities and joint return periods of drought duration and severity in the cases “and” and “or” are calculated based on the BB8Copula, which are employed for drought assessment. The results show that the drought events with 1-season or cross-quarter duration were more popular than others; joint probabilities and joint return periods of the detected drought events from 1962 to 2009, ranged from 0.2% to 92.2% and from 0.782 years to 315.414 years, respectively, in the case “and”, and ranged from 3.8% to 99.6% and from 0.724 years to 18.785 years, respectively, in the case “or”.

Hydrological drought variability and its teleconnections with climate indices

In this study, the spatial–temporal variability of hydrological drought in Meriç Basin, Turkey, has been examined, and its relationship with global climate oscillations has been explored. Prior to drought analysis, streamflow data gaps were filled, and series homogeneity was examined. The standardized streamflow index (SSFI) was used to detect drought periods from monthly streamflow data of nine stations well spread over the study area. The hydrological drought intensity, magnitude, and duration were investigated using the SSFI time series. The drought variability was assessed by performing trend analysis and wavelet transforms. Trend analysis showed an increase in drought events in the Northern region and a decrease in the Southern region of the basin. Wavelet analysis reveals that the 4–10 year scales explicate the drought series pattern. The identification of climate patterns that are most associated with drought variability is performed via correlation analysis. The investigation showed that distinct climate patterns simultaneously influence drought events. Significantly, the influence of El Nino Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Atlantic Multidecadal Oscillation (AMO) is notable. The analysis demonstrated a remarkable correlation between ENSO and SSFI series at the interannual scale (2–8 years). The obtained phase difference suggests that the maximum dry (wet) conditions coincide with La Nina (El Nino) events. In addition, positive NAO phases and negative AMO phases lead to drought occurrence.

Unravelling the role of vegetation on the different trends between climatic and hydrologic drought in headwater catchments of Spain

The availability of water resources is a major challenge of the Mediterranean region. Intense land use transformation has impacted the headwaters of river basins that generate most of the water resources, reducing streamflow This study analyses the evolution of hydrological and climatic drought in headwater catchments of Spain and it explores the extent to which vegetation can reinforce trends in hydrological drought severity in comparison to the evolution of cFlimatic drought severity. We have used the Standardized Precipitation Evapotranspiration Index and Standardized Streamflow Index to examine hydrological and climatic drought, respectively, and examined the changes in the frequency, magnitude, and duration of climatic and hydrologic droughts over non-perturbed headwater catchments of Spain from 1961 to 2013. We quantified vegetation changes over time in the analysed catchments and we compared the changes in the climatic and hydrological droughts with changes in vegetation coverage using Pearson’s r correlations and linear regression model. The results show that the trends toward higher frequency, duration and severity of hydrological droughts are more marked than the observed trends in climatic droughts, which can be associated to the dominant increase in vegetation coverage and activity in the study domain. Finally, it is concluded that the spatial differences observed between the trends of climatic and hydrological droughts show some relationship with the patterns of forest succession observed in recent decades. Our results stress the relevance of land transformation processes on trends in water resources availability, particularly during periods of climatic droughts in which competence between vegetation water consumption and streamflow production is much more relevant.

Drought propagation modification after the construction of the Three Gorges Dam in the Yangtze River Basin

The construction of Three Gorges Dam (TGD) has exerted substantial effects on local hydrological regime and eco-system since its operation in 2003. Due to this significant anthropogenic activity, drought propagation processes from Meteorological drought to Hydrological (M-H) and Soil moisture (M-S) drought are inevitably modified. However, current studies are still insufficient to answer whether the TGD operation mitigated or exacerbated the drought propagation process due to its complex impacts. To explore this key issue, this study proposed an integrated framework to quantify the impact of TGD construction on drought propagation process in the Yangtze River Basin (YRB). The drought characteristics propagation models and propagation time were calculated to identify the modification of drought propagation process between different TGD operation scenarios. Trend and attribution analyses were also utilized to explore potential influence factors. Results demonstrated that TGD construction did significantly aggravate the drought characteristics propagation. Specifically, the slopes of hydrological drought duration and severity in linear propagation models increased 23.98% and 29.23% in average. While for M-S propagation, the slopes of duration and severity models increased 28.72% and 21.91%, respectively. This research also found that, after TGD operation, the M-H propagation speed had been slowed down but the M-S lag time became shorter. Additionally, the variations and trend patterns of six potential influence factors at four separate periods were full analyzed to provide implications for the drought propagation modifications. And TGD operation was confirmed to become a major factor inducing the variations of annual runoff, which accounted for 56.75% for the entire YRB. The above quantitative findings are expected to reveal the potential changes of drought propagation process caused by the construction of TGD in YRB. The proposed integrated framework is also beneficial to the drought early warning system and understanding of drought development not only in YRB, but also in other basins at the global scale.

Discussion “A probability distribution for hydrological drought duration”

Discussion “A probability distribution for hydrological drought duration”