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Flood trends and variability in the Mekong river
J. M. Delgado, H. Apel,B. Merz
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2010,
Abstract: Annual maximum discharge is analyzed in the Mekong river in Southeast Asia with regard to trends in average flood and trends in variability during the 20th century. Data from four gauging stations downstream of Vientiane, Laos, were used, covering two distinct hydrological regions within the Mekong basin. These time series span through over 70 years and are the longest daily discharge time series available in the region. The methods used, Mann Kendal test (MK), ordinary least squares with resampling (OLS) and non-stationary generalized extreme value function (NSGEV), are first tested in a Monte Carlo experiment, in order to evaluate their detection power in presence of changing variance in the time series. The time series are generated using the generalized extreme value function with varying scale and location parameter. NSGEV outperforms MK and OLS, both because it resulted in less type II errors, but also because it allows for a more complete description of the trends, allowing to separate trends in average and in variability. Results from MK, OLS and NSGEV agreed on trends in average flood behaviour. However, the introduction of a time-varying scale parameter in the NSGEV allowed to isolate flood variability from the trend in average flood and to have a more complete view of the changes. Overall, results showed an increasing likelihood of extreme floods during the last half of the century, although the probability of an average flood decreased during the same period. A period of enhanced variance in the last quarter of the 20th century, estimated with the wavelet power spectrum as a function of time, was identified, which confirmed the results of the NSGEV. We conclude that the absence of detected positive trends in the hydrological time series was a methodological misconception due to over-simplistic models.
Flood trends and variability in the Mekong river  [PDF]
J. M. Delgado,H. Apel,B. Merz
Hydrology and Earth System Sciences Discussions , 2009,
Abstract: Annual maximum discharge is analyzed in the Mekong river in Southeast Asia with regard to average flood trends and variability during the 20th century. Data from four gauging stations downstream of Vientiane, Laos, were used, covering two distinct hydrological regions within the Mekong basin. These time series span through over 70 years and are the longest daily discharge time series available in the region. The methods used, Mann Kendal test (MK), ordinary least squares with resampling (OLS) and non-stationary general extreme value function (NSGEV), are first tested in a Monte Carlo experiment, in order to evaluate their detection power in presence of changing variance in the time series. The time series are generated using the general extreme value function with linearly varying scale and location parameter. NSGEV outperforms MK and OLS, both because it incurred in less type II errors, but also because it allows for a more complete description of the trends, allowing to separate trends in average and in variability Results from MK, OLS and NSGEV agreed on trends in average flood behaviour. However, the introduction of a time-varying scale parameter in the NSGEV allowed to isolate flood variability from the average flood trend and to have a more complete view of the changes. Overall, results showed an increasing likelihood of extreme floods during the last half of the century, although the probability of an average flood decreased during the same period. The variance, estimated with the wavelet power spectrum as a function of time, allowed to identify a period of enhanced variance in the last quarter of the 20th century, which confirmed the results of the NSGEV. We conclude that the initial absence of detected positive trends in the hydrological time series was a methodological misconception due to over-simplistic models.
RIVER LATERAL DYNAMICS AND FLOODS IN RELATION TO HUMAN COMMUNITIES. CASE STUDY: MOARA DOMNEASC VILLAGE (ON LOWER TELEAJEN RIVER)
L. ZAHARIA,G. IOANA-TOROIMAC,S. CATAN?,G. MINEA
Aerul ?i Apa : Componente ale Mediului , 2012,
Abstract: River lateral dynamics and floods in relation to human communities. Case study: Moara Domneasc Village (on Lower Teleajen River). One of the challenges set at the beginning of the 21st century is living with floods and natural dynamics of rivers. In this general framework, the main goal of our paper is to elaborate a diagnosis regarding lateral migrations and floods of Teleajen River in the proximity of Moara Domneasc Village (Rafov Commune, County of Prahova). Our analysis includes two approaches: a hydrological and a geomorphological one. The hydrological approach aims at analysing maximum discharges variability of Teleajen River at Moara Domneasc gauging station and at showing its floods’ frequency. The geomorphological approach leads to outlining the wandering area of Teleajen River based on a diachronic cartography, which takes into account documents covering the last 150 years; the boundaries correspond to the maximum extension of its channel and abandoned channels. The area overflowed in September 2005 is assessed based on drawings from the urban plans of Rafov Commune. The outcome shows that the area appropriated by Teleajen River was partially exploited by society. Therefore the village of Moara Domneasc is exposed to these hazards.
Variability and Trends of Daily Heavy Rainfall Events over Niger River Basin Development Authority Area in Nigeria  [PDF]
Joseph Sunday Babatolu, Rufus Temidayo Akinnubi, Akintade Taiwo Folagimi, Omosuyi Oluwayemisi Bukola
American Journal of Climate Change (AJCC) , 2014, DOI: 10.4236/ajcc.2014.31001
Abstract: Using 70 years of daily rainfall records in eight stations, an analysis of variability and trends of daily heavy rainfall events over Niger River Basin Development Authority Area was carried out by using Standardized Anomaly Index and Spearman Rank Correlation Coefficient. Significant temporal variability on interannual and decadal time-scales was observed in the frequency of heavy rainfall events and annual heavy rainfall amount. Both the annual heavy rainfall amount and frequency of heavy rainfall events demonstrated no pronounced temporal decreasing or increasing trend. However, more recent data records from 1981 onwards revealed an increasing trend. Thus, evidence of a temporal change is apparent in heavy rainfall events in the last three decades in sympathy with global warming.
The influence of decadal-scale variability on trends in long European streamflow records  [PDF]
J. Hannaford,G. Buys,K. Stahl,L. M. Tallaksen
Hydrology and Earth System Sciences Discussions , 2013, DOI: 10.5194/hessd-10-1859-2013
Abstract: This study seeks to provide a long-term context for the growing number of trend analyses which have been applied to river flows in Europe. Most studies apply trend tests to fixed periods, in relatively short (generally 1960s–present) records. This study adopts an alternative "multi-temporal" approach, whereby trends are fitted to every possible combination of start and end years in a record. The method is applied to 132 catchments with long (1932–2004) hydrometric records from northern and central Europe, which were chosen as they are minimally anthropogenically influenced and have good quality data. The catchments are first clustered into five regions, which are broadly homogenous in terms of interdecadal variability of annual mean flow. The multi-temporal trend approach was then applied to regional time series of different hydrological indicators (annual, monthly and high and low flows). The results reveal that the magnitude and even direction of short-term trends are heavily influenced by interdecadal variability. Some short-term trends revealed in previous studies are shown to be unrepresentative of long-term change. For example, previous studies have identified post-1960 river flow decreases in southern and eastern Europe: in parts of eastern Europe, these trends are resilient to study period, extending back to the 1930s; in southern France, longer records show evidence of positive trends which reverse from the 1960s. Recent (post-1960) positive trends in northern Europe are also not present in longer records, due to decadal variations influenced by the North Atlantic Oscillation. The results provide a long-term reference for comparison with published and future studies. The multi-temporal approach advocated here is recommended for use in future trend assessments, to help contextualise short-term trends. Future work should also attempt to explain the decadal-scale variations that drive short-term trends, and thus develop more sophisticated methods for trend detection that take account of interdecadal variability and its drivers.
What drives flood trends along the Rhine River: climate or river training?
S. Vorogushyn,B. Merz
Hydrology and Earth System Sciences Discussions , 2012, DOI: 10.5194/hessd-9-13537-2012
Abstract: The Rhine River catchment was heavily trained over the past decades and faced the construction of the Rhine weir cascade, flood protection dikes and detention basins. For the same time period, several studies detected positive trends in flood flows and faced the challenge of flood trend attribution, i.e. identifying the drivers of observed change. The presented study addresses the question about the responsible drivers for changes in annual maximum daily flows at Rhine gauges starting from Maxau down to Lobith. In particular, the role of river training measures including the Rhine weir cascade and a series of detention basins in enhancing Rhine floods was investigated. By applying homogenisation relationships to the original flow records in the period from 1952 till 2009, the annual maximum series were computed that would have been recorded had river training measures not been in place. Using multiple trend analysis, the relative changes in the homogenised time series were found to be smaller up to about 20% points compared to the original records. This effect is attributable to the river training measures and primarily to the construction of the Rhine weir cascade. The increase in Rhine flood discharges was partly caused by the unfavourable superposition of the Rhine and Neckar flood waves. It resulted from the acceleration of the Rhine waves due to construction of the weir cascade. However, at the same time, the tributary flows across the entire Upper and Lower Rhine, which enhance annual Rhine peaks, showed very strong positive trends. This suggests the dominance of a large-scale driver such as climate variability/change which acted along with river training. In particular, the analysis suggests that the river training measures fell in a period with increasing flood trends driven by factors other than river training of the Rhine main channel.
Trends in timing and magnitude of flow in the Upper Indus Basin
M. Sharif,D. R. Archer,H. J. Fowler,N. Forsythe
Hydrology and Earth System Sciences Discussions , 2012, DOI: 10.5194/hessd-9-9931-2012
Abstract: River flow is a reflection of the input of moisture and its transformation in storage and transmission over the catchment. In the Upper Indus Basin (UIB), since high altitude climate measurement and observations of glacier mass balance are weak or absent, analysis of trends in magnitude and timing in river flow provides a window on trends and fluctuations in climate and glacier outflow. Trend analysis is carried out using a Mann-Kendall nonparametric trend test on records extending from 1960 to 1998. High level glacial catchments show a falling trend in runoff magnitude and a declining proportion of glacial contribution to the main stem of the Indus. Elsewhere annual flow has predominantly increased with several stations exhibiting statistically significant positive trends. Analysis of timing using spring onset date (SOT) and centre of volume date (CoV) indicated no clear trends – in direct contrast to what has been observed in Western North America. There is, however, a consistent relationship between CoV and annual runoff volume. A consistently positive correlation was also found between SOT and CoV for all the stations implying that initial snowpack conditions before the onset of runoff influence timing throughout the season. The results of the analysis presented here indicate that the magnitude and timing of streamflow hydrograph is influenced both by the initial snowpack and by seasonally varied trends in temperature. The study contributes to the understanding of the links between climate trends and variability and river runoff and glacier mass balance and runoff. The Upper Indus Basin is predominantly influenced by winter precipitation; similar trend analysis applied to summer monsoon dominated catchments of the Central Himalaya is recommended.
FLOW CONTROL FACTORS AND RUNOFF CHARACTERISTICS IN THE VALEA CERBULUI RIVER BASIN  [PDF]
RUTH PERJU
Aerul ?i Apa : Componente ale Mediului , 2012,
Abstract: CHARACTERISTICS OF THE AVERAGE LIQUID FLOW IN THE VALEA CERBULUI RIVER BASIN. This article aims to emphasize the characteristics of the average liquid flow in the Valea Cerbului Basin (a tributary of the upper course of Prahova River). Although it is a small hydrographic basin (26 km2), it has distinct hydrological features, which are imposed by the morphological, morphometrical and climatic conditions of this area. In order to achieve this purpose, the hydrological data series (recorded on Valea Cerbului) on the average flow have been statistically analyzed. Analyses revealed as main hydrological features of this basin: a relatively reduced interannual variability of the average flow (Cv = 0.33); a hydrologic regime with high flow in July and reduced flow in January and February; a significant share of the annual flow (52%) from April to July and a reduced one during the winter (13% of the annual flow). According to Mann-Kendall test, at both monthly and seasonal level, average discharge shows (generally) ascending trends, but particularly significant are the trends between August and December and the trend calculated for the season autumn.
Scale effects on the hydrological impact of upland afforestation and drainage using indices of flow variability: the River Irthing, England  [PDF]
D. Archer
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2003,
Abstract: Frequent assertions by river users that rivers in northern England now rise and fall more quickly than in the past, have never been validated by analysis on catchments of more than 10 km2. The method using indices of flow variability provides a basis for making direct measurements of the annual number and duration of pulses, i.e. rises above a given flow, and for comparing catchments of different sizes. A comparison is made between the small afforested headwater Coalburn catchment (1.5 km2) and the larger River Irthing catchment (335 km2) on which the afforested area comprises 19%. A simple but effective means is provided for decoupling the effect of climatic variability from the effects of land use. The analysis shows that major changes have occurred on the small catchment, first with rising pulse numbers after pre-afforestation drainage, then with a much greater progressive decrease in pulse number accompanied with increasing pulse duration. In contrast, the larger catchment shows little change in variability indices from the beginning of its record in 1968 until the late 1980s after which the pattern of change mirrors that at Coalburn but the proportional change is much smaller. The direction of change is the opposite of that asserted by river users. Keywords: hydrology, flow variability, land-use impacts, forests, scale effects
Change trends and driving factors of base flow in Kuye River Catchment
窟野河流域河川基流量变化趋势及其驱动因素

LEI Yongnan,ZHANG Xiaoping,ZHANG Jianjun,LIU Erjia,
雷泳南
,张晓萍,张建军,刘二佳

生态学报 , 2013,
Abstract: The variation of base flow is a comprehensive reflection of the change of regional climate and human activities, and it is an important element to maintain sustainable development of the ecosystem. In this study, the Kuye River catchment, a tributary of the Yellow River, with a drainage area of 8706 km2 on the Loess Plateau, was chosen to investigate the change trends and evolution characteristics of base flow. Based on Chapman-Maxwell base flow separation technique, the total runoff from Wenjiachuan hydrological station, which is the maximum control station in the Kuye River catchment, was separated into surface runoff and base flow. The nonparametric Mann-Kendall test and the Pettitt test were used to identify the trends and change points in the base flow during 1959 to 2005. The results indicated that: (1) In the past 50 years, a significant negative trend was detected in the annual base flow in all 7 time periods. For total year period, the average change rate is 0.628mm/a. The change points significantly occurred in 1980 and 1996. (2) Analysis of flow duration curves showed that, compared to the contrast period from 1959 to 1979, the relative changes of annual base flow for 5%, 50% and 95% exceeding time were lowered by 30%, 38% and 54%, respectively, in the treat period from 1980 to 1995, and the relative changes of annual base flow for 5%, 50% and 95% exceeding time were lowered by 57%, 68% and 100%, respectively in the coal mining period from 1996 to 2005. (3) The significant negative trend in base flow is a comprehensive effect of the change of regional climate and human activities, and the main factor is that excessive exploitation of a large area of coal resources and utilization of groundwater.
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