Temporal Changes in Flow Regime along the River Vistula

The flow regime in the River Vistula is influenced by climatic and geographical factors and human intervention. In this study, we focus on an analysis of flow and precipitation variability over time and space following the course of the River Vistula. Multi-purpose statistical analyses of a number of runoff and precipitation characteristics were performed to present a general overview of the temporal and spatial changes. Since the important feature of the hydrological regime of Polish rivers is the seasonality of runoff associated with the occurrence of cold (winter) and warm (summer) seasons within a hydrological year, a seasonal approach is applied to describe specific seasonal features that can be masked when using annual data. In general, the results confirm popular impressions about changes in winter season runoff characteristics, i.e., significantly decreasing daily maxima, increasing daily minima and a decrease in concentration, and so a bigger uniformity of winter daily flows. An interesting behaviour of minimum flows in the summer has been revealed, which is contrary to social perceptions and the alarming changes taking place in the other parts of the world. Additionally, precipitation indexes related to the formation of droughts show no trends, e.g., the mean value of the maximum dry spell length.

Trends of Hydroclimatic Intensity in Colombia

Prediction of precipitation changes caused by global climate change is a practical and scientific problem of high complexity. To advance, we look at the record of all available rain gauges in Colombia and at the CHIRPS database to estimate trends in essential variables describing precipitation, including HY-INT, an index of the hydrologic cycle’s intensity. Most of the gauges and cells do not show significant trends. Moreover, the signs of the statistically significant trends are opposite between the two datasets. Satisfactory explanation for the discrepancy remains open. Among the CHIRPS database’s statistically significant trends, the western regions (Pacific and Andes) tend to a more intense hydrologic cycle, increasing both intensity and mean dry spell length, whereas for the northern and eastern regions (Caribbean, Orinoco, and Amazon), the tendencies are opposite. This dipole in trends suggests different mechanisms: ENSO affects western Colombia more directly, whereas rainfall in the eastern regions depends more on the Atlantic Ocean, Caribbean Sea, and Amazon basin dynamics. Nevertheless, there is countrywide accord among gauges and cells with significant increasing trends for annual precipitation. Overall, these observations constitute essential evidence of the need for developing a more satisfactory theory of climate change effects on tropical precipitation.

Variability and Trend Analysis of Temperature, Rainfall and Characteristics of Crop Growth Season in Eastern Zone of Tigray Region, Northern Ethiopia

To favour farmers and adjusting their farming practices, long term weather analyses is essential to determine future directions and making adjustments required to existing systems. The main purpose of this study was thus to analyze the variability and trends of climatic variables (temperature and rainfall) and characteristics of crop growth season in Eastern zone of Tigray region for the period of 1980–2009. Detail investigations were carried out using parametric (Linear regression) and non-parametric tests (Mankendall and Sen’s slope estimator). Moreover, homogeneity test was applied using a method developed by Van Belle and Hughes for the general trend analysis. Furthermore, the trend of rainfall end to characterize crop growth season using R-Instat and XLSTAT software. It was found that the general trend of monthly rainfall experienced an overall significant increasing trend. The seasonal rainfall experienced significantly increasing trend during the summer rainy season (June–September) whilst a significant decreasing trend occurred in the short rainy season (February–May). Likewise, the seasonal maximum temperature trends exhibited a significant increasing trend in all seasons whereas the minimum temperature showed inhomogeneous trend across seasons as well as stations. Despite significant increase of rainfall in summer season, the trend of growing season characteristics (onset, cessation, length of growing period and dry spell length) did not change significantly over the study period. However, the variability of rainfall and dry spell length was found to be very large. Hence, crop production in the study area demands appropriate adaptation strategies that considers the erratic nature of the rainfall, the long dry spell length in the season and increasing trends of temperature.

Future Changes in Wet and Dry Season Characteristics in CMIP5 and CMIP6 simulations

Climate change will result in more dry days and longer dry spells, however, the resulting impacts on crop growth depend on the timing of these longer dry spells in the annual cycle. Using an ensemble of Coupled Model Intercomparison Project Phase 5 and Phase 6 (CMIP5 and CMIP6) simulations, and a range of emission scenarios, here we examine changes in wet and dry spell characteristics under future climate change across the extended tropics in wet and dry seasons separately. Delays in the wet seasons by up to two weeks are projected by 2070-2099 across South America, Southern Africa, West Africa and the Sahel. An increase in both mean and maximum dry spell length during the dry season is found across Central and South America, Southern Africa and Australia, with a reduction in dry season rainfall also found in these regions. Mean dry season dry spell lengths increase by 5-10 days over north-east South America and south-west Africa. However, changes in dry spell length during the wet season are much smaller across the tropics with limited model consensus. Mean dry season maximum temperature increases are found to be up to 3°C higher than mean wet season maximum temperature increases over South America, Southern Africa and parts of Asia. Longer dry spells, fewer wet days, and higher temperatures during the dry season may lead to increasing dry season aridity, and have detrimental consequences for perennial crops.

Variability and Trend Analysis of Temperature, Rainfall and Characteristics of Crop Growth Season in Eastern Zone of Tigray Region, Northern Ethiopia

Background: Long term weather analyses are very useful indicators in determining future directions and in making adjustments required to existing systems. And, in order to favor farmers to adjust their farming practices, seasonal climate outlooks are needed. The main purpose of this manuscript was thus to analyze the variability and trends of maximum and minimum temperature, monthly and seasonal rainfall series and characteristics of crop growth season in Eastern zone of Tigray region over the period of 1980–2009.Methods: Detail investigations were carried out using parametric (Linear regression) and nonparametric tests (Mankendall (Mk) and Sen’s slope estimator). Moreover, homogeneity test using a method developed by Van Belle and Hughes was used for general trend analysis. In addition, daily rainfall data to characterize crop growth season were analysed using R-Instat and XLSTAT software for trend analysis.Results: It was found that the general trend of monthly rainfall experienced an overall significant increasing trend. The seasonal rainfall experienced significantly increasing in summer main rainy season, June–September (Kiremt) while significantly decreasing in short rainy season, February– May (Belg). Likewise, the seasonal maximum temperature trends exhibited significant increase in each season while minimum temperature trend had inhomogeneous trend across seasons as well as stations. The trend of growing season characteristics (onset, cessation, LGP and dry spell length did not change significantly over the study period (1980–2009) in all stations. However, the coefficient of variability of LGP was (CV, >15%) and dry spell length was (CV, >25%) inassociation with short nature of LGP (68–85 days had a negative impact on the agricultural activities of the study area during the study period.Conclusions: Despite significant increase of rainfall in summer season, the variability of rainfall and dry spell length was very large. Hence, the study recommends crop production in the study area demands appropriate adaptation strategies that considers the erratic nature of the rainfall, the long dry spell length in the season and increasing trends of temperature.

Droughts and the Impacts of Dry Spells in North of Iraq

Different sets of dry spell length such as complete series, monthly maximum, seasonal maximum, and annual maximum are applied and modeled with different probability distribution functions (such as Gumbel Max, generalized extreme value, Log-Logistic, generalized logistic, inverse Gaussian, Log-Pearson 3, generalized Pareto) to recognize in which duration, dry spells cause drought. The drought situation and temporal analysis in the North of Iraq region were done using the SPI index and by software of DrinC at a time scale of 3.6 and 12 months. Because of applicability, availability of data and the aim of the study, SPI is selected to analyze the dry spells in this study. Based on the maximum length of the available statistical period, the statistics for the years 1980 to 2019 were used from nine meteorological stations for analysis. The results of the study showed the severity of drought during the study period which related to dry spells. The results of this research confirm the variation of drought occurrence with varying degrees in different time and different dry spells condition in Iraq.

The Climatic Analysis of Summer Monsoon Extreme Precipitation Events over West Africa in CMIP6 Simulations

We evaluate the capability of 21 models from the new state-of-the-art Coupled Model Intercomparison Project, Phase 6 (CMIP6) in the representation of present-day precipitation characteristics and extremes along with their statistics in simulating daily precipitation during the West African Monsoon (WAM) period (June–September). The study uses a set of standard extreme precipitation indices as defined by the Expert Team on Climate Change Detection and Indices constructed using CMIP6 models and observational datasets for comparison. Three observations; Global Precipitation Climatology Project (GPCP), Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS), and Tropical Applications of Meteorology using SATellite and ground-based observation (TAMSAT) datasets are used for the validation of the model simulations. The results show that observed datasets present nearly the same spatial pattern but discrepancies in the magnitude of rainfall characteristics. The models show substantial discrepancies in comparison with the observations and among themselves. A number of the models depict the pattern of rainfall intensity as observed but some models overestimate the pattern over the coastal parts (FGOALS-f3-L and GFDL-ESM4) and western part (FGOALS-f3-L) of West Africa. All model simulations explicitly show the pattern of wet days but with large discrepancies in their frequencies. On extreme rainfall, half of the models express more intense extremes in the 95th percentiles while the other half simulate less intense extremes. All the models overestimate the mean maximum wet spell length except FGOALS-f3-L. The spatial patterns of the mean maximum dry spell length show a good general agreement across the different models, and the observations except for four models that show an overestimation in the Sahara subregion. INM-CM4-8 and INM-CM5-0 display smaller discrepancies from their long-term average rainfall characteristics, in terms of extreme rainfall estimates than the other CMIP6 datasets. For the frequency of heavy rainfall, TaiESM1 and IPSL-CMGA-LR perform better when compared with observational datasets. MIROC6 and GFDL-ESM4 displayed the largest error in representing the frequency of heavy rainfall and 95th percentile extremes, and therefore, cannot be reliable. The study has assessed how rainfall extremes are captured in both observation and the models. Though there are some discrepancies, it gives room for improvement of the models in the next version of CMIP.

Multi-type global drought projection using multi-model hydrological simulations

<p>A multi-drought study that covers several draught types is required to better understand future drought. It is anticipated that drought will be exacerbated under climate change due to altered precipitation patterns and/or increased evapotranspiration. However, IPCC AR5 and SREX report stated with barely medium confidence that drought is expected to intensify over several regions in the world by the end of the 21st century, while elsewhere there is overall low confidence.</p><p>One of the reasons for these confidence levels stems from a definitional issue. As drought is a complex phenomenon and involves several processes, there are multiple hydrological variables and relevant-indicators used to quantify drought. Nonetheless, very few studies have comprehensively discussed future drought considering several drought types within a single study, hence leaving a gap on the holistic picture of future drought. Besides, most studies referred to in AR5 and SREX are based on coarse general circulation model (GCM) or regional climate model projections which have inherent model biases. Also, scenario uncertainties need to be examined more on drought projections, using the latest greenhouse gas emission scenarios.</p><p>This study presents a comprehensive multi-drought-type assessment on a global-scale until 2099. Using a set of multiple state-of-art global hydrological model (GHM) simulations forced by four bias-corrected GCM projections, meteorological (precipitation), agricultural (soil moisture) and hydrological (runoff) droughts are investigated by using the Standardized method at monthly-scale and another hydrological drought (discharge) by using a variable threshold method. The multi-model data set, which was developed in the Inter-Sectoral Impact Model Inter-comparison Project phase2b under a consistent simulation protocol, provides finer and detailed hydrological simulations at 0.5°x0.5° resolution. To explore potential pathways of drought changes, this study examined the Representative Concentration Pathways (RCP) 2.6, 6.0 and 8.5 scenarios. For each case, four drought features; drought intensity, spatial extent, the number of events, dry spell length, were studied, compared to those of the period before the 1960s.</p><p>The results highlight the hotspots of future droughts and show the development of each drought type for each RCP scenarios. As well as consistencies, differences among drought types were found in change trends and drought features. For instance, meteorological drought will decrease in some parts of middle-latitude in the northern hemisphere but the other two drought types will increase due to an increase in evapotranspiration over the regions. Or, dry spell length tends to be longer in runoff > soil moisture > precipitation drought in this order. These differences indicate that it is crucial to clearly define drought in discussing the phenomenon and it is critical to properly select drought types and index for one’s interest. Also, differences among RCP scenarios pose a question for mitigation discussions from the viewpoint of drought. Two types of uncertainties in this projection concerning model (GHMs and GCMs) uncertainty and parameter uncertainty in the drought analysis methods are also presented along with the drought projections.</p>

Observed Changes in Temperature and Precipitation Extremes Over the Yarlung Tsangpo River Basin during 1970–2017

Twenty-five climate indices based on daily maximum and minimum temperature and precipitation at 15 meteorological stations were examined to investigate changes in temperature and precipitation extremes over the Yarlung Tsangpo River Basin (1970–2017). The trend-free prewhitening (TFPW) Mann–Kendall test and Pettitt’s test were used to identify trends and abrupt changes in the time series, respectively. The results showed widespread significant changes in extreme temperature indices associated with warming, most of which experienced abrupt changes in the 1990s. Increases in daily minimum and maximum temperature were detected, and the magnitude of daily minimum temperature change was greater than that of the daily maximum temperature, revealing an obvious decrease in the diurnal temperature range. Warm days and nights became more frequent, whereas fewer cold days and nights occurred. The frequency of frost and icing days decreased, while summer days and growing season length increased. Moreover, cold spell length shortened, whereas warm spell length increased. Additionally, changes in the precipitation extreme indices exhibited much less spatial coherence than the temperature indices. Spatially, mixed patterns of stations with positive and negative trends were found, and few trends in the precipitation extreme indices at individual stations were statistically significant. Generally, precipitation extreme indices showed a tendency toward wetter conditions, and the contribution of extreme precipitation to total precipitation has increased. However, no significant regional trends and abrupt changes were detected in total precipitation or in the frequency and duration of precipitation extremes.