The hydrological cycle is expected to intensify with global warming, which likely increases the intensity of extreme precipitation events and the risk of flooding. The changes, however, often differ ...from the theorized expectation of increases in water-holding capacity of the atmosphere in the warmer conditions, especially when water availability is limited. Here, the relationships of changes in extreme precipitation and flood intensities for the end of the twenty-first century with spatial and seasonal water availability are quantified. Results show an intensification of extreme precipitation and flood events over all climate regions which increases as water availability increases from wet to dry regions. Similarly, there is an increase in the intensification of extreme precipitation and flood with the seasonal cycle of water availability. The connection between extreme precipitation and flood intensity changes and spatial and seasonal water availability becomes stronger as events become less extreme.
Analysis of hydrological extremes is challenging due to their rarity and small sample size and the interconnections between different types of extremes and gets further complicated by an ...untrustworthy representation of meso-scale processes involved in extreme events by coarse spatial and temporal scale models as well as biased or missing observations due to technical difficulties during extreme conditions. The special issue “Statistical Analysis and Stochastic Modelling of Hydrological Extremes”—motivated by the need to apply and develop innovative stochastic and statistical approaches to analyze hydrological extremes under current and future climate conditions —encompass 13 research papers. Case studies presented in the papers exploit a wide range of innovative techniques for hydrological extremes analyses. The papers focus on six topics: Historical changes in hydrological extremes, projected changes in hydrological extremes, downscaling of hydrological extremes, early warning and forecasting systems for drought and flood, interconnections of hydrological extremes and applicability of satellite data for hydrological studies. This Editorial provides an overview of the covered topics and reviews the case studies relevant for each topic.
► A decreasing trend was found in annual precipitation at about 60% of the stations. ► The significant negative trends occurred mostly in the northwest of Iran. ► The strongest negative trend of the ...seasonal series was detected in winter.
Precipitation is a principal element of the hydrological cycle and its temporal variability is important from both the scientific and practical point of view. The annual and seasonal precipitation trends of 41 stations in Iran for the period 1966–2005 have been analyzed using the Mann–Kendall test, the Sen’s slope estimator and the linear regression. The effective sample size method was applied to eliminate the effect of serial correlation on the Mann–Kendall test. The results indicated a decreasing trend in annual precipitation at about 60% of the stations. The decreasing trends were significant at seven stations at the 95% and 99% confidence levels. The magnitude of the significant negative trends in annual precipitation varied from (−)1.999
mm/year at Zanjan station to (−)4.261
mm/year at Sanandaj station. The spatial distribution of the annual precipitation trends showed that the significant negative trends occurred mostly in the northwest of Iran. On the seasonal scale, the trends in the spring and winter precipitations time series were mostly negative. The highest numbers of stations with significant trends occurred in winter while no significant positive or negative trends were detected by the trend tests in autumn precipitation. The significant negative trends ranged between (−)0.283
mm/year at Zahedan station and (−)0.807
mm/year at Sanandaj station in winter season. In addition, the highest and lowest significant increases of precipitation values were obtained over Semnan and Mashhad in summer at the rates of (+)0.110
mm/year and (+)0.036
mm/year, respectively.
Climate change can put an intense pressure on already scarce water resources in the Middle East, potentially catalyzing the risk of serious water conflicts. Albeit most efforts to quantify the ...potential impact of climate change in the Middle East use global climate model outputs, here we employ two regional climate model outputs which are expected to provide more trustworthy projections for the region with a complex terrain and variable land surfaces. We find that not ubiquitous does future climate change lead to a decline in annual precipitation total in the region; nevertheless, the projected decline stems from an increase in number of dry days (NDD) rather than a decrease in precipitation intensity on rainy days. The increase in annual precipitation total in the southern part is driven by changes in both NDD and precipitation intensity. The drought periods will be longer (up to 90%) in about 80% of the Middle East area. The prolonged droughts in the future will increase the already high level of water stress in the region and force people to migrate or live in dreadful conditions of water poverty.
•The large-scale patterns of changes are congruous between the BM and POT methods.•There is discrepancy between the BM and POT results at the local scale.•The discrepancies increase with event ...extremity and vary with season and space.•The differences are statistically significant for longer return periods.•The differences get larger seasonally for summer and spatially for arid regions.
A reliable estimation of hydrological extremes with potentially severe socio-economic impacts is of crucial importance for efficient planning and design of hydraulic structures. Extreme value theory provides a firm theoretical foundation for the statistical modelling of extreme hydrological events. The dilemma in the modelling is on whether to use block maxima (BM) or peak-over-threshold (POT) method, each with its own cons and pros. It remains unexplored to what extent future projected changes in extreme hydrological events are influenced by the method choice, especially when some simplifications are made to lessen the computational burden for large-scale studies. This study addresses this research question by a comparative analysis between the BM and POT methods on future climate change impact on global flood and extreme precipitation. The extreme precipitation analysis is performed using 24 CMIP5 general circulation models (GCMs), while the flood analysis is based on a multi-model ensemble of 20 members including five global impact models forced by four CMIP5 GCMs. The results reveal that the BM and POT methods agree on the sign of changes in flood and extreme precipitation intensities, but disagree on the magnitude. The discrepancy between the BM and POT results increases with event extremity. The difference also varies with season, where the difference in the global land area with increasing signals peaks in winter at the rates of 11% for extreme precipitation and in summer at the rates of 3.5% for flood.
Projected precipitation from climate models is used in a wide range of fields for climate change impact assessment. However, the spatial pattern of uncertainty across latitudes and the global ...uncertainty hotspots are not well understood despite their importance for regional adaptation planning. In this study, we describe uncertainties in projected extreme precipitation changes per K global warming across latitudes, and decompose the overall uncertainty into climate model and internal variability uncertainties. We then identify global uncertainty hotspots and discuss the broader implications. Our results show that both uncertainty sources are highly heterogeneous across latitudes, while climate model uncertainty exceeds internal variability uncertainty for all seasons and precipitation intensities. The largest difference between model and internal variability uncertainties is found in tropical regions where model uncertainty is thrice as large as internal variability uncertainty in June-July-August season and twice as large as that in the other seasons. Tropical and subtropical regions are identified as the global uncertainty hotspots, with the Sahara desert and the southern part of the Middle East being the local hotspots. The large uncertainty in the tropics and subtropics is primarily due to the convective nature of rainstorms which cannot be adequately represented by coarse-scale climate models, and also to sparse observation networks based on which climate models can be tuned and improved. The results highlight areas where future model development and improvement efforts should focus to reduce the overall uncertainties in projected precipitation extremes.
Evaluation of simple reference evapotranspiration (ET
o
) methods has received considerable attention in developing countries where the weather data needed to estimate ET
o
by the Penman–Monteith FAO ...56 (PMF-56) model are often incomplete and/or not available. In this study, eight pan evaporation-based, seven temperature-based, four radiation-based and ten mass transfer-based methods were evaluated against the PMF-56 model in the humid climate of Iran, and the best and worst methods were selected from each group. In addition, two radiation-based methods for estimating ET
o
were derived using air temperature and solar radiation data based on the PMF-56 model as a reference. Among pan evaporation-based and temperature-based methods, the Snyder and Blaney–Criddle methods yielded the best ET
o
estimates. The ET
o
values obtained from the radiation-based equations developed here were better than those estimated by existing radiation-based methods. The Romanenko equation was the best model in estimating ET
o
among the mass transfer-based methods. Cross-comparison of the 31 tested methods showed that the five best methods as compared with the PMF-56 model were: the two radiation-based equations developed here, the temperature-based Blaney–Criddle and Hargreves-M4 equations and the Snyder pan evaporation-based equation.
Abstract
Compound hot-dry events have the potential to cause significant damages and propel socioeconomic systems towards tipping points by overwhelming the ability of natural and human systems to ...cope with the combined stressors. As climate change continues to alter hazard patterns, the impacts of these events will be further compounded by changes in exposure and vulnerability. However, the future risk of these events and the role of these components remain poorly understood. Using a multimodel ensemble, we find that by the end of the 21st century, an additional 0.7–1.7 billion people globally will be exposed to amplified compound events, depending on the scenarios. Additionally, the cropland exposure to these events is projected to increase by 2–5.7 million km². Our findings also suggest that countries with weak governance will experience a twice larger increase in the risk of compound events than those with good governance. This underscores the importance of effective governance in mitigating and managing the escalating risks of compound events.
The analyses of the spatial and temporal trends of precipitation are pertinent for the future development and sustainable management of water resources of a given region. Annual and seasonal ...precipitation data from 28 synoptic stations of Iran (1967–2006) were analyzed to determine the spatial and temporal trends and approximate year of the beginning of the significant trends by using the Mann–Kendall and Mann–Kendall rank statistic tests, respectively. The trend free pre-whitening (TFPW) method was applied to eliminate the influence of serial correlation on the Mann–Kendall test, and the magnitude of the precipitation trends was obtained from the Theil–Sen's slope estimator. Over the 40-year period, negative trend in annual precipitation occurred at 22 sites (79%), while just three sites had statistically significant (α=0.05) negative trend in precipitation. The magnitude of the significant negative trends of annual precipitation at the 95% confidence level varied from (−)2.53±0.69mm/year at Tabriz station to (−)3.43±0.81mm/year at Khoy station. The change points of the annual precipitation at Khoy, Oroomieh and Tabriz stations were 1982, 1994 and 1981, respectively. In the seasonal series, the negative trends in spring and winter precipitation were larger compared with those in the other seasonal series, so that, five significant negative trends were detected in the winter time series. A noticeable decrease in the winter precipitation series was observed mostly in northern Iran, as well as along the coasts of the Caspian Sea. In summer precipitation, two significant positive trends were found at Mashhad and Torbateheydarieh stations, whereas no significant positive or negative trends were detected by the trend tests in autumn precipitation.
► The significant trends were found in the annual precipitation time series. ► The significant annual trends started in 1982, 1994 and 1981. ► The significant trends were more obvious in winter than the other season.
PDF
