The aim of the present study was to investigate 50 years (1964–2013) of seasonal and annual rainfall trends and their fluctuation over time in northern Bangladesh. After testing the autocorrelation, ...non-parametric Mann–Kendall test along with Sen Slope estimator was used to examine rainfall trends and their magnitudes. The sequential Mann–Kendall test was used to identify any fluctuations in the trends over time and to detect the possible points of change in the rainfall series. We found that pre-monsoon and post-monsoon rainfall is increasing in most of the rainfall stations. The only decrement in pre-monsoon rainfall was found for Ishurdi (1.28mm/year). However, the sequential Mann–Kendall test detected decreasing pre-monsoon rainfall trend after early the 1990s. Monsoon rainfall showed a decreasing trend in the majority of the area studied. The maximum decrement in monsoon rainfall was found for Sylhet station (8.10mm/year) and minimum in Mymensingh (1.53mm/year). An upward monsoon rainfall trend was found for Rangpur (2.02mm/year). Annual rainfall followed the monsoon rainfall trend. However, all of the positive and negative trends were found statistically non-significant at 95% confidence limit with the only exception for monsoon and annual rainfall at Rajshahi station. Rajshahi station was the only region where the monsoon and annual rainfall has a significant negative trend at 95% confidence limit. The sequential Mann–Kendall test detected several non-significant points of change for seasonal and annual rainfall at most of the stations. Periodic fluctuations were also detected. We observed that there were decreasing seasonal rainfall trend after early the 1990s for the majority of the stations.
•We studied the seasonal and annual rainfall trend, their fluctuations, and possible change point for northern Bangladesh.•Monsoon rainfall is decreasing non-significantly in most of the northern Bangladesh.•Pre-monsoon and post-monsoon rainfall shows a non-significant (at 95% confidence limit) increasing trend in most of the study area.•The sequential Mann–Kendall test reveals periodic fluctuations for seasonal rainfall at majority of the stations.•Decreasing seasonal rainfall trend were found after the early 1990s in most of the stations
Runoff in Haihe River Catchment of China is steadily declining due to climate change and human activity. Determining abrupt changes in runoff could enhance identification of the main driving factors ...for the sudden changes. In this study, the sequential Mann–Kendall test analysis is used to determine abrupt changes in runoff in eight sub-catchments of Haihe River Catchment, while trend analysis via the traditional Mann–Kendall test for the period 1960–1999 is used to identify the basic trend of precipitation and runoff. The results suggest an insignificant change in precipitation and a significant decline in runoff in five of the eight sub-catchments. For most of the sub-catchments, abrupt changes in runoff occurred in 1978–1985. Through correlation comparisons for precipitation and runoff for the periods prior to and after abrupt runoff changes, human activity, rather than climatic change, is identified as the main driving factor of runoff decline. It is also noted that abrupt decline in runoff was actually at the beginning of China’s 1978–1985 land reform. Given that the land reform motivated farmers to productively manage their reallocated lands, agricultural water use therefore increased. Hence percent agricultural land is analyzed in relation to land use/cover pattern for the late 1970s and early 1980s. The analysis shows that when cultivated farmland exceeds 25% of a sub-catchment area, an abrupt decline in runoff occurs. It is therefore concluded that high percent agricultural land and related agricultural water use are the most probable driving factors of runoff decline in the catchment.
Trends in reference evapotranspiration (ETo) have been found highly diverse in different regions of the globe due to the contradictory changes in the meteorological variables that define ETo. Despite ...a significant impact of ETo in water resources and ecology, knowledge on the changes and the cause of the changes in ETo is very limited in tropical regions. The trends in ETo, the factors influencing the changes in ETo and the change point (year) that made the trend significant were evaluated in this study for tropical peninsular Malaysia. The modified version of Mann-Kendall (MK) test was used for the assessment of unidirectional changes in ETo and the driving meteorological variables. The innovative trend analysis (ITA) was conducted to understand the variations in change with time. Sobol's method was used to measure the sensitivity of ETo to different meteorological factors and the Sequential MK test was employed to identify the change point. The study revealed an increase in annual (0.009–0.026 mm/year) and seasonal (0.014–0.027 mm/year during southwest monsoon and 0.015–0.074 during northeast monsoon) ETo in peninsular Malaysia which contradicts to evapotranspiration paradox found in many regions. The minimum temperature (31.5–48.2%) was found as the most influencing factor followed by wind speed (15.1–32.8%.) in defining ETo in peninsular Malaysia. Analysis of ITA and sequential MK test results revealed that the rise in minimum temperature is the major cause of the increase in ETo in peninsular Malaysia. A faster rise in minimum temperature after 1981–1985 caused an increase in ETo after 1993–1996 in most of the locations. The minimum temperature in the region was noticed to rise much faster compared to the global average which indicates a large and continuous increase in ETo due to global warming and thus, reduction in atmospheric water balance in peninsular Malaysia.
•The factors influencing the changes in reference evapotranspiration (ETo) are evaluated for tropical peninsular Malaysia.•Modified MK test, innovative trend analysis, sequential MK test and Sobol's global sensitivity analysis are used .•Minimum temperature is the most influencing factor of ETo changes followed by wind speed in peninsular Malaysia•A faster rise in minimum temperature after 1981–1985 caused an increase in ETo after 1993–1996 in most of the stations.
Trend analysis of the mean (monsoon season, non-monsoon season and annual) and extreme annual daily rainfall and temperature at the spatial and temporal scales was carried out for all the 33 urban ...centers of the arid and semi-arid state of Rajasthan, India. Statistical trend analysis techniques, namely the Mann–Kendall test and Sen's slope estimator, were used to examine trends (1971–2005) at the 10% level of significance. Both positive and negative trends were observed in mean and extreme events of rainfall and temperature in the urban centers of Rajasthan State. The magnitude of the significant trend of monsoon rainfall varied from (−) 6.00mm/hydrologic year at Nagaur to (−) 8.56mm/hydrologic year at Tonk. However, the magnitude of the significant negative trends of non-monsoon rainfall varied from (−) 0.66mm/hydrologic year at Dungarpur to (−) 1.27mm/hydrologic year at Chittorgarh. The magnitude of positive trends of non-monsoon rainfall varied from 0.93mm/hydrologic year at Churu to 1.70mm/hydrologic year at Hanumangarh. The magnitude of the significant negative trends of annual rainfall varied from (−) 6.47mm/year at Nagaur to (−) 10.0mm/year at Tonk. The minimum, average and maximum temperature showed significant increasing warming trends on an annual and seasonal scale in most of the urban centers in Rajasthan State. The magnitude of statistically significant annual extreme daily rainfall varied from 2.00mm at Jhalawar to (−) 1.64mm at Tonk, while the magnitude of statistically significant extreme annual daily minimum and maximum temperature varied from 0.03°C at Ganganagar to 0.05°C at Jhalawar, respectively. The spatial variations of the trends in mean (monsoon season, non-monsoon season and annual) and extreme annual daily rainfall and temperature were also determined using the inverse-distance-weighted (IDW) interpolation technique. IDW results are helpful to identify trends and variability in mean and extreme rainfall and temperature in space and time for the study locations where the data is not available and the quality of data is not good. These spatial maps of temperature and rainfall can help local stakeholders and water managers to understand the risks and vulnerabilities related to climate change in terms of mean and extreme events in the region.
•Highlights the importance of climate change assessments to prepare suitable adaptation strategies in urban centres in India.•Identifies trends in mean and extreme annual daily rainfall and temperature in the 33 urban centres of Rajasthan State.•Found predominant changes in mean and extreme rainfall and temperature trends in most of the urban centres of Rajasthan.
ABSTRACT
Snowfall in mountainous areas provides indispensable water sources for arid and semi‐arid watersheds. Changes in snowfall and snowmelt regimes have significantly impacted the water ...resources' variations and management. This study addresses the ratio of snowfall to precipitation (S/P) in the Tian Shan Mountains, China. It aims to examine changes in S/P ratio and its synchronization with climate change. The results show that the average S/P ratio experienced a downward trend under the precipitation and snowfall increases as well as temperature increases during the cold seasons in 1961–2010. Spatially, the S/P ratios show an increasing trend in the Boertala Valley and the southern slope of the middle Tian Shan Mountains, but a decreasing trend in Yili Valley, and the western and the northern slope of the middle Tian Shan Mountains. The S/P ratios decreased significantly in middle altitudes with elevation ranging from 1500 to 2500 m, but either decreased or increased at elevations below 1500 m due to a temperature inversion in the elevation belt. In high altitudes with elevation over 3500 m, the magnitudes of the decreased S/P ratios were small because the temperature was always below freezing. The decreases in S/P ratio was mainly attributed to the relative changes in snowfall (dS/S) being less than that in precipitation (dP/P), whereas the increases in S/P ratio resulted from the relative changes in snowfall (dS/S) being more than that in precipitation (dP/P). Temperature increases have also lead to the decreases in S/P ratios to some extent.
•Interactions between lake, catchment and Yangtze River were investigated.•Yangtze River effect was the primary factor affecting recession period of Poyang Lake.•Modifications to the River from the ...Three Gorges Dam caused seasonal dryness of the Lake.•Hydraulic engineering between the Lake and River would resolve water supply crisis.
Changes in lake hydrological regimes and the associated impacts on water supplies and ecosystems are internationally recognized issues. During the past decade, the persistent dryness of Poyang Lake (the largest freshwater lake in China) has caused water supply and irrigation crises for the 12.4 million inhabitants of the region. There is conjecture as to whether this dryness is caused by climate variability and/or human activities. This study examines long-term datasets of catchment inflow and Lake outflow, and employs a physically-based hydrodynamic model to explore catchment and Yangtze River controls on the Lake’s hydrology. Lake water levels fell to their lowest during 2001–2010 relative to previous decades. The average Lake size and volume reduced by 154km2 and 11×108m3 during the same period, compared to those for the preceding period (1970–2000). Model simulations demonstrated that the drainage effect of the Yangtze River was the primary causal factor. Modeling also revealed that, compared to climate variability impacts on the Lake catchment, modifications to Yangtze River flows from the Three Gorges Dam have had a much greater impact on the seasonal (September–October) dryness of the Lake. Yangtze River effects are attenuated in the Lake with distance from the River, but nonetheless propagate some 100km to the Lake’s upstream limit. Proposals to build additional dams in the upper Yangtze River and its tributaries are expected to impose significant challenges for the management of Poyang Lake. Hydraulic engineering to modify the flow regime between the Lake and the Yangtze River would somewhat resolve the seasonal dryness of the Lake, but will likely introduce other issues in terms of water quality and aquatic ecosystem health, requiring considerable further research.
•The variations of water level in Dongting Lake from 1961 to 2010 were analyzed.•The aim was to evaluate the impacts of anthropogenic activity and climate change.•The time series was divided into ...1961–1980, 1981–2002, and 2003–2010.•Precipitation was the main driver during the period of 1961–1980 and 1981–2002.•Dam construction was the main driver during the period of 2003–2010.
Understanding the variation regularity of water level and the potential drivers can provide insights into lake conservation and management. In this study, inter- and inner-annual variations of water level in Dongting Lake during the period of 1961–2010 were analyzed to determine whether anthropogenic or climatic factor should be responsible for the variations. The results showed that water level decreased significantly during the period of 1961–1980, while increased significantly during the period of 1981–2002 at the 5% significance level. However, the variation trend of water level after 2002 did not reach a significant level. The variation in the dry season was more obviously than that in the wet season. The date when water level was firstly below 24m during the period of 2003–2010 appeared about 27days earlier than usual, and the date was even advanced to mid-September in 2006. As for the duration, water level was below 24m for about 185days in the period of 2003–2010 and 20–30days longer than the other two periods. In conclusion, water level might be influenced by a combination of anthropogenic and climatic factors, with rainfall probably as the main driver responsible for hydrological alteration during the period of 1961–1980 and 1981–2002 while dam construction as the main driver during the period of 2003–2010. Under the circumstance of uncontrollable climate change, effective measures for reservoir operation should be put forward to maintain the ecological integrity and ensure water release and storage capacity of aquatic ecosystems.
ABSTRACT
In recent times, trend analysis and change point detection in hydroclimatic variables receiving significant attention due to climate change and its socioeconomic consequences. In this study, ...long‐term trends of rainfall in four subdivisions of southern India namely Kerala, Tamil Nadu, North Interior (NI) Karnataka and Telangana regions are analysed using linear regression, nonparametric Mann–Kendall (MK) test and Sen's slope estimator methods. Trend analysis of annual rainfall time series shows an increasing trend in three subdivisions – Tamil Nadu, NI Karnataka and Telangana, and a decreasing trend in Kerala subdivision. Further the sequential change in trend of annual and seasonal rainfalls in the four subdivisions is conducted using sequential MK (SQMK) method. The SQMK analysis shows an early divergence of progressive and retrograde modes of post‐monsoon rainfall of Kerala and winter rainfall of Telangana subdivisions. Further it is observed that among different seasonal rainfalls, the post‐monsoon rainfall of Kerala subdivision shows a statistically significant increase in the recent past. Then the trend analysis based on discrete wavelet transform (DWT) in conjunction with SQMK method is performed on the post‐monsoon rainfall time series of Kerala subdivision, and the results show that there is a dominancy of short‐term periodicity of less than a decade in the subdivision.
Changes in rainfall play an important role in agricultural production, water supply and management, and social and economic development in arid and semi-arid regions. The objective of this study was ...to examine the trend of rainfall series from 18 meteorological stations for monthly, seasonal, and annual scales in Shanxi province over the period 1957–2019. The Mann–Kendall (MK) test, Spearman’s Rho (SR) test, and the Revised Mann–Kendall (RMK) test were used to identify the trends. Sen’s slope estimator (SSE) was used to estimate the magnitude of the rainfall trend. An autocorrelation function (ACF) plot was used to examine the autocorrelation coefficients at various lags in order to improve the trend analysis by the application of the RMK test. The results indicate remarkable differences with positive and negative trends (significant or non-significant) depending on stations. The largest number of stations showing decreasing trends occurred in March, with 10 out of 18 stations at the 10%, 5%, and 1% levels. Wutai Shan station has strong negative trends in January, March, April, November, and December at the level of 1%. In addition, Wutai Shan station also experienced a significant decreasing trend over four seasons at a significance level of 1% and 10%. On the annual scale, there was no significant trend detected by the three identification methods for most stations. MK and SR tests have similar power for detecting monotonic trends in rainfall time series data. Although similar results were obtained by the MK/SR and RMK tests in this study, in some cases, unreasonable trends may be provided by the RMK test. The findings of this study could benefit agricultural production activities, water supply and management, drought monitoring, and socioeconomic development in Shanxi province in the future.
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