The Frequency of Precipitation Days in the Yangtze River Basin
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Based on the daily,monthly and annual precipitation data from the meteorological stations of Tianshui,Baoji,Wugong and Xi'an during the period of 1951- 2011,the change trend of precipitation was analyzed using the linear regression,Mann-Kendall test and five-year moving average method. It could be concluded that the seasonal distribution of precipitation was quite different in the Wei River Basin,precipitation occurred mainly in summer and autumn,it was in an obvious decrease trend in spring and autumn,and its variation trend was not obvious in summer and winter. However,the seasonal distribution of precipitation was quite different from different years.There was a sharp change of seasonal precipitation except in summer,and also of annual precipitation. There was a28- 32-year fluctuation period of precipitation. The precipitation days in the drainage basin were in a reduction trend,but the intensity of light rain was in an increase trend. During the period of 1976- 1990,the rainstorm days were the most at the meteorological stations of Tianshui,Baoji and Wugong except at Xi'an Meteorological Station,and they were quite different from different years
Trend analysis
Seasonality
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The temporal and spatial variations of extreme precipitation were studied by using extreme precipitation indices Based on the daily precipitation data of 23 meteorological stations over the Red River Basin in China from 1960 to 2007.The intensity and frequency of extreme precipitation gradually reduces from southeast to northwest,high value area was distributed along the south of the Jiangcheng-Luchun-Jinping-Hekou while low value area existed along the north of Weishan-Nanjian-Midu and the middle reaches of the Yuanjiang River Valley.The peak of extremely heavy precipitation occurred in July,while extreme precipitation occurrence frequency in flood season accounted for 91.48% of the annual total.In the past 48 years,an increasing trend was observed in indices such as the total wet day precipitation(PRCPTOT),the maximum 1-day precipitation amount(Rx1d),the maximum 5-day precipitation amount(Rx5d),the number of heavy precipitation days(R95N),the percent of total precipitation from heavy precipitation(R95T) and the simple daily intensity index(SDII),among which R95T and SDII increased significantly at the rates of 0.68%·(10 a)-1 and 0.17 mm·d-1·(10 a)-1 respectively.The trends of these indices exhibited spatial differences except for SDII,regions with the increasing trends are distributed mostly in the upstreams of the Lixianjiang and Yuanjiang rivers,as well as Tengtiao River Basin,while those of the decreasing trends were found in the downstreams of the Lixianjiang and Yuanjiang rivers,and Panlong River Basin as well.
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Influenced by intensifying human activities and constant changes of global climate, hydrological factors of the Pearl River Basin have been changed significantly. By using the Mann-Kendall trend test method and the sequential cluster analysis, this paper systematically analyzed the trends and mutation of the 16 precipitation indices in the Pearl River Basin. Results indicate that: 1) In the Pearl River Basin, the annual total precipitation, the total precipitations in spring and winter increase faintly, while the precipitation amount in autumn shows a stable decreasing trend. Besides, the precipitation amount in summer does not indicate a significant changing tendency. Meanwhile, the trends of precipitation amount in summer have the greatest impacts on the spatial distribution of the annual total precipitation variations. Additionally, the precipitation amount trend in spring takes the second place, which followed by the precipitation amount trends in autumn and winter. 2) There is a slight increase in the proportions of spring and winter precipitation to annual precipitation, while decreasing trends are found in the proportions of summer and autumn precipitation to annual precipitation in the Pearl River Basin. 3) In the Pearl River Basin, the total numbers of annual rainy days and light rainy days are decreasing significantly, however, the downfall days, torrential rainy days and the 95% percentile of the rainy days distributions tend to increase. Moreover, the significant tendency of light rainy days, torrential rainy days and the 95% percentile of the rainy days distributions have mutations in the early 1990s. 4) In the Pearl River Basin, the mean daily precipitation intensity is increasing stably, and precipitation period is centralizing, which can easily trigger occurrence of floods and droughts.
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Based on the daily precipitation datasets recorded by 9 meteorological stations in the Nujiang River Basin in Tibet during the period from 1971 to 2010,the thresholds of extreme precipitation at these stations were defined using 99 percentile values,and the spatiotemporal change of extreme precipitation events in the basin was analyzed.The results show that the thresholds of extreme precipitation varied in a range of 20.6-33.8 mm in the Nujiang River Basin,and the spatial distribution of extreme precipitation thresholds was consistent with annual precipitation.In recent 30 years(1981-2010),the maximum daily precipitation was in an increase trend in most areas including Nagqu,Baxoi and Zayu,especially in Zayu where the increase rate was 5.2 mm·(10a)-1.The frequency of extreme precipitation tended to an increase in Dengqen,Baxoi and Zayu,but it decreased at other stations with rates of-0.06——0.41 d·(10a)-1.The annual extreme precipitation increased significantly with a rate of 29.4 mm·(10a)-1 at Zayua,changed slightly at Zogang,and was in a decrease trend with rates of-1.0——27.2 mm·(10a)-1 at other stations.In addition,the extreme precipitation intensity increased with rates of 0.6-6.2 mm·d-1·(10a)-1 at Nagqu,Sog,Zogang and Zayu,it was in a decrease trend at Amdo and Dengqen,but there was no change at Biru,Lhorong and Baxoi.It was also found that there was a significant 4-5-year period of frequency of extreme precipitation,maximum daily precipitation,extreme precipitation and extreme precipitation intensity before the 1990s,and a significant 2-4-year period after 2000.
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Using measured daily precipitation data of 5 meteorological stations in the Yellow River delta from 1961 to 2007 and the general extreme precipitation indices suitable for the actual local conditions,the temporal characteristics of extreme precipitation events change was analyzed.The results show that the annual maximum daily precipitation,heavy precipitation days and intensity,rainstorm days and intensity present a decreasing trend in the area.From the 1960s,the extreme precipitation experiences about 3 change processes which falls first and rises later.The extreme precipitation has a good correlation with the total annual precipitation,and the correlation coefficient is more than 0.75.The annual maximum daily precipitation,heavy precipitation days and rainstorm intensity change abruptly at the end of 1970s and the beginning of 1980s,which is similar to the change characteristic in central north China.
Intensity
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This paper studied the temporal and spatial variations of extreme precipitation in Huaihe river basin from 1960 to 2008 with the geo-spatial statistics,time series analysis and trends in diagnosis and other methods.The results showed that the extreme precipitation,precipitation intensity and the days of heavy rainfall had not significantly changed in annual and seasonal trend.The incidents without precipitation lasting over 15 d were increasing from south to north about 2-5 times per year,it happened the most in winter and the least in summer.The events with precipitation lasting over 5 d were increasing from northeast to southwest about 1-8 times per year.In the northwest of Huangchuan-Zhengyang-Zhengzhou this kind of events were the most in the fall,in other areas the events were the least in summer.In the more than 40% of sites the chance with no persistant precipitation had increased significantly.The climate trend rate was 0.22-0.60 times/a.It occurred the most in the 1970s during abruptly climatic change.The events of precipitation reduced obviously over 30% of the sites.The climate trend rate was-0.24 to-0.70 times/a,no abruptly climatic change was found.There were no obvious relationship between persistant(no) precipitation events and the annual precipitation.
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Based on the data of precipitation from 1951 to 2014 in Xi’an,Shanxi Province,China(hereinafter referred to as Xi’an),after analyzing the characteristics of changes in precipitation over the last 64 years,the multiple scale analysis of annual precipitation and flood season precipitation are analyzed by wavelet analysis in Xi'an.The conclusions are as follows:(1)Flood season precipitation accounts for 79.32% of annual precipitation, and the correlation coefficient is 0.94,so,the contribution of precipitation in flood season to annual precipitation is very great.(2) Annual precipitation and flood season precipitation shows a decreasing trend, with a decreasing rate of -11.3mm/10a and -3.17mm /10a, respectively,which shows 72% of the annual precipitation decreases in the non flood season.(3)Annual precipitation sequence and flood season precipitation sequence have the similar main cycle in Xi'an.Their first main periods are 29 years and 31 years, respectively, and the second, the third,the forth main cycles all are 13 years, 6 years and 3 years.(4)Over any time scale,the annual precipitation and the precipitation in flood season will be into a less period after 2014.Since the oscillation intensities of the first and the second main cycles of annual precipitation is basically same,the possibilities of the less period of annual precipitation continuing to 2020 or 2017 are the same.However,the oscillation intensity of the first main cycle of precipitation in flood season is much greater than that of the second main cycle,so the probability of the less period of precipitation in the flood season is more likely to continue to 2020 than 2017.
Wet season
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Using the daily precipitation data and daily temperature data at 90weather stations in Northeast China from 1961to 2010,the climate characteristics,trends and variation field characteristics of different grades in precipitation events have been studied.The results show that the trend of annual mean temperature increases in the past 50years,with higher increasing rate than other areas of China.An abrupt change temperature occurred in 1987,followed by a sharp increase in temperature.Annual precipitation and precipitation days decrease,with the decrease of annual precipitation days being more obvious,which is mostly due to the decline in light rainfall(days).Annual rainfall(days)shows the increasing trend,with annual snowfall(days)showing a decreasing trend.Annual precipitation decreases from southeast to northwest,and the annual number of precipitation days is larger in the east and north.The proportion of extreme heavy precipitation and different grade snowfall increase after temperature has abrupt change.The fluctuations of annual precipitation(days)also become larger after 1987,which may be the main reason of increase of extreme drought and flood events in Northeast China.The seasonal distribution of precipitation also shows significant changes.Rainfall(days)in summer and autumn are reducing,and rainfall(days)in spring and snowfall(days)in winter are increasing,which will be beneficial to maintain the soil moisture in winter and snow in spring.
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Long-term trends in summer precipitation totals, the number of rainy days, and precipitation intensity were investigated with daily rainfall datasets for China from 1961 to 2000. Total precipitation significantly increased in summer in the Yangtze River basin and northwestern China, while total precipitation decreased in other regions. The number of rainy days increased in the Yangtze River basin, and over northwestern China. In contrast, the number of rainy days decreased over Tibet and over northern and northeastern China. Seasonal mean precipitation intensity became large at most of the stations in China.To investigate trends in heavy rainfall, daily rainfall totals during the whole investigation period were grouped into ten classes, with class width equal to l0% of total number of rainy days from 1961 to 2000, and the number of rainy days, and summer mean rainfall amount for each class, were obtained for each summer. A simple linear fit was made to determine the linear trends in the 10-class precipitation time series. The upper 20 percentile of daily precipitation totals showed a statistically significant increase over the Yangtze River basin, and over northwestern China during the study period. Class average precipitation decreased in almost 10 classes over Tibet and north and northeastern China.
Yangtze river
Wet season
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