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A consideration of rainfall, runoff and losses at Plynlimon in the context of long term hydrological variability in the UK and maritime Western Europe
S. Green,T. J. Marsh
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 1997,
Abstract: Important questions concerning the resilience of current water management strategies have been raised by the recent volatility of climatic conditions across large parts of western Europe. The last decade, overall, has been exceptionally warm and there have been very large spatial and temporal variations in rainfall, river flows and aquifer recharge rates. Examination of historical rainfall and runoff records for parts of maritime western Europe confirms that there is no close modern parallel to the conditions experienced recently. Some-but far from complete-consistency with a number of favoured climate change scenarios may be recognised. Analyses of recent trends in lengthy rainfall and runoff series for the UK demonstrate significant regional differences and provide conflicting signals especially in relation to trends in catchment losses. Difficulties in reconciling the results from different areas may reflect both real hydroclimatological differences between catchments and variations in the precision of hydrometric time series-uncertainties in the assessment of areal precipitation in upland areas in particular. The dense monitoring networks at Plynlimon together with a rigorous data quality control programme underpins the value of the hydrometric datasets as important benchmarks against which to assess the significance of the very unusual patterns of rainfall and runoff which have characterised the recent past. This paper places the rainfall, runoff and losses data for Plynlimon in the perspective provided by a number of long hydrometric records for maritime western Europe. The representativeness of the Plynlimon base period is considered with particular reference to both the historical stability which typifies the great majority of European hydrometric time series and the recent extension in the recorded range of accumulated rainfall and runoff totals which has been identified in some regions (e.g. western Scotland and Norway). Particular attention is directed to changes in seasonal rainfall and runoff patterns and the recent increases in evaporative demands. Some of the implications for the overall water balance and for water resource management are considered.
Variability of rainfall in Peninsular Malaysia
C. L. Wong,R. Venneker,S. Uhlenbrook,A. B. M. Jamil
Hydrology and Earth System Sciences Discussions , 2009,
Abstract: This study analyzed and quantified the spatial patterns and time-variability of rainfall in Peninsular Malaysia on monthly, yearly and monsoon temporal scales. We first obtained an overview of rainfall patterns through the analysis of 16 point data sources. The results led to choosing three distinct regions, i.e.~the east coast, inland and west coast regions. For detailed analysis, Shepard's interpolation scheme was applied to the station data to produce daily rainfall fields on a 0.05 degree resolution grids for the period 1971–2006. The rainfall characteristics in time and space derived from a frequency analysis were found to be distinctly different in these three regions. In the east coast region, monthly rainfall shows a significant periodicity dominated by an annual cycle, followed by a half-year cycle. The inland and west coast regions show that the dominant periodic fluctuations in the monthly rainfall are dominated by a half-year cycle, followed by an annual cycle. The long-term rainfall variability analysis shows that the dry and wet conditions in Peninsular Malaysia are not primarily governed by the ENSO events. The results from the individual regions suggest that although the relative variability is influenced by ENSO, local and regional conditions have an effect on the interannual rainfall variability, which is superimposed on the large-scale weather conditions. A significant increasing trends in annual rainfall (9.3 mm/year) and northeast monsoon rainfall (6.2 mm/monsoon) were only detected in the west coast region. No trend was found in the monthly rainfall, except for November in the west coast region. The spatial variation analysis shows that the east coast region, which received substantially higher amounts of rainfall during the northeast monsoon, has lower spatial rainfall variability and a more uniform rainfall distribution than other regions. A larger range for the monthly spatial variation was observed in the west coast region.
Daily rainfall variability at a local scale (1,000 ha), in Piracicaba, SP, Brazil, and its implications on soil water recharge
Reichardt, K.;Angelocci, L.R.;Bacchi, O.O.S.;Pilotto, J.E.;
Scientia Agricola , 1995, DOI: 10.1590/S0103-90161995000100008
Abstract: daily rainfall variability at a local scale (1,000 ha) was studied at piracicaba, sp, brazil, for the period of one year (1993-1994), in order to better understand the process of soil water recharge. coefficients of variation of daily data for ten observation points varied from 2.2 to 169.3% and the variability was independent of rain type, i.e. whether convective, frontal or of other origin. data were not related to separation distances between observation points and it is concluded that one observation point does not represent areas as far as 1,000 to 2,500 m apart, for daily, monthly or even quarterly averages. yearly totals for the ten observation points presented a coefficient of variation as low as 3.06%, indicating that all points can replace each other in annual terms.
Mapping an index of extreme rainfall across the UK  [PDF]
D. S Faulkner,C. Prudhomme
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 1998,
Abstract: Distance from the sea, proximity of mountains, continentality and elevation are all useful covariates to assist the mapping of extreme rainfalls. Regression models linking these and other variables calculated from a digital terrain model have been built for estimating the median annual maximum rainfall, RMED. This statistic, for rainfall durations between 1 hour and 8 days, is the index variable in the rainfall frequency analysis for the new UK Flood Estimation Handbook. The interpolation of RMED between raingauge sites is most challenging in mountainous regions, which combine the greatest variation in rainfall with the sparsest network of gauges. Sophisticated variables have been developed to account for the influence of topography on extreme rainfall, the geographical orientation of the variables reflecting the prevailing direction of rain-bearing weather systems. The different processes of short and long-duration extreme rainfall are accounted for by separate regression models. The technique of georegression combines estimates from regression models with a map of correction factors interpolated between raingauge locations using the geostatistical method of kriging, to produce final maps of RMED across the UK.
A Possible Linkage in the Interdecadal Variability of Rainfall over North China and the Sahel
REN Baohu,LU Riyu,XIAO Ziniu,
REN Baohu
,LU Riyu,XIAO Ziniu

大气科学进展 , 2004,
Abstract: The instrumental records of precipitation, including some historical documentary evidence, show that the rainfall in North China during the rainy season (July and August) exhibits an interdecadal variability similar to the Sahelian rainfall. Both these areas exhibited a weak interdecadal rainfall variability prior to the 1950s, and experienced a long-lasting drought since the 1960s, with two rainfall decreasing transitions,one around the year 1965 and another in the late 1970s. NCEP/NCAR reanalysis data are used to analyze the associated changes in atmospheric circulation during the second decrease transition. The changes of local atmospheric circulation at the end of the 1970s, at both lower and upper levels, contribute to the less precipitation in North China and the Sahel.
Downscaling summer rainfall in the UK from North Atlantic ocean temperatures  [PDF]
R. L. Wilby
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2001,
Abstract: Annual series of three stochastic rainfall model parameters — the seasonal wet day amount (or intensity), the conditional dry–day probability (or dry–spell persistence), and the conditional wet-day probability (or wet-spell persistence) — were examined using daily rainfall records for ten UK stations for the period 1901–1995. The purpose was first, to determine the extent to which these indices of summer (June–August) rainfall were correlated with empirical orthogonal functions (EOFs) of summer North Atlantic sea surface temperature (SST) anomalies: second, to evaluate the skill of EOFs of preceding winter (December–February) SSTs for summer rainfall forecasting and downscaling.Correlation analyses suggest that observed increases in summer dry-spell persistence since the 1970s coincided with positive SST anomalies in the North Atlantic. In contrast, wet-spell persistence and intensities were relatively weakly correlated with the same patterns, implying that the use of SSTs is justifiable for conditioning occurrence but not intensity parameters. Furthermore, the correlation strengths were greater for EOFs of SSTs than those reported for area-average SST anomalies, indicating that the pattern of SST anomalies conveys important information about seasonal rainfall anomalies across the UK. When EOFs of winter SSTs were used to forecast summer rainfall in Cambridge, the skill was once again greater for dry-spells than either wet-spells or intensities. However, even for dry–spells, the correlation with observations — whilst statistically significant — was still rather modest (r<0.4). Nonetheless, the results are comparable to previous investigations of summer rainfall across Europe, and suggest that forecasting skill (across the UK) originates from the predictability of the rainfall occurrence process. Keywords: North Atlantic, ocean temperatures, downscaling, rainfall, forecasting, UK
Variability and Trends of Summer Monsoon Rainfall over Bangladesh  [PDF]
MN Ahasan,Md AM Chowdhary,DA Quadir
Journal of Hydrology and Meteorology , 2010, DOI: 10.3126/jhm.v7i1.5612
Abstract: In this paper, the updated rainfall data of 50 years (1961-2010) for 30 selected rain gauge stations of Bangladesh have been used. The data were analyzed to investigate the variability and trends of summer monsoon (June- September) rainfall over Bangladesh. The possible teleconnection of monsoon rainfall variability with ENSO has also been investigated. Annual profile of the station mean monthly rainfall of Bangladesh shows a unimodal pattern with high rainfall between June-September (monsoon season) with highest in July and low rainfall between December – February with lowest in January. All Bangladesh mean summer monsoon rainfall is 1769.14 mm, standard deviation 209.16 mm (coefficient of variance 11.82 %) and annual country average rainfall is 2456.38 mm. Summer monsoon rainfall widely varies over the geographical areas with lowest in central-western part and highest in southeastern part with next highest in northeastern part of the country. The trend analysis shows that the trend of the country average monsoon rainfall is decreasing (-0.53 mm/year). The spatial distribution of the trend values indicates that the summer monsoon rainfall exhibits increasing trends at the rate of 5-6 mm/year in the NW region and 3-4 mm/ year in the south-central and extreme SE region. The eastern region exhibits decreasing trends of about -2 to -7 mm/year with highest (-6 to -7 mm/year) in the east-central part. The time series plot of country average summer monsoon rainfall shows the inter-annual variability in the timescales of 2-3 years and 4-6 years. The time series of 5 year moving average reveals existence of low frequency variability of timescales of 9-14 years. The time series of Bangladesh monsoon rainfall shows that there were 11 strong monsoon years and 8 weak monsoon years within the periods of 1961-2010 (50 years). The analysis of the decadal mean rainfall shows that the decades 1961-1970 and 1981-1990 were wet and the decades 1971-1980, 1991-2000 and 2001-2010 were dry. Floods in Bangladesh result from the excess rainfall occurring both inside and outside the country. Summer monsoon rainfall is characterized by active and weak (break) spells, which are associated with the fluctuation of monsoon rainfall in the time scales of 20-25 and 40-50 days. Such fluctuations are caused due to north-south movement of the monsoon trough. The fluctuations in the time scales of 4-7 and 10-14 days are associated with the formation of low pressure systems over the head Bay. The possible atmospheric teleconnections of summer monsoon rainfall with ENSO have also been
Rainfall Variability and Droughts in the Drylands of Baringo County, Kenya
Richard Ochieng, Charles Recha, Bockline Omedo Bebe, George Morara Ogendi
Open Access Library Journal (OALib Journal) , 2017, DOI: 10.4236/oalib.1103827
Abstract:
The study was undertaken to determine rainfall variability and droughts in arid and semi-arid lands (ASALs) of Baringo County. The study used rainfall data for Perkerra Agricultural Research Station (LM5) and Nginyang rainfall station (IL6) for the period 1970-2008 and 1974-2013 respectively. Standardized anomalies were calculated and plotted to establish trends of drought events across the study period. Plots of rainfall data from the two stations displayed an oscillating trend with major rainfall peaks being observed in 1977 and 2007 for Perkerra, 1982 and 1997 for Nginyang. Analysis of rainfall data from the two stations indicates four catastrophic drought periods recognized with standard anomalies of less than-0.9 (SA (t) <-0.9) as a function of the time scales. The study findings reveal that MAM seasonal drought index plots for both Perkerra and Nginyang rainfall stations had a declining long-term MAM seasonal rainfall trend. Perkerra OND Seasonal Drought Index plot indicates a constant trend in the long-term OND seasonal rainfall while Nginyang long-term OND seasonal drought trend shows a gentle upward trend, an indication that the conditions are improving. The study recommends the strengthening of uptake of seasonal climate forecast to inform appropriate decision making regarding response to rainfall variability and drought events.
Universal inverse power-law distribution for temperature and rainfall in the UK region  [PDF]
A. M. Selvam
Physics , 2013,
Abstract: Meteorological parameters, such as temperature, rainfall, pressure etc., exhibit selfsimilar space-time fractal fluctuations generic to dynamical systems in nature such as fluid flows, spread of forest fires, earthquakes, etc. The power spectra of fractal fluctuations display inverse power-law form signifying long-range correlations. The author has developed a general systems theory which predicts universal inverse power-law form incorporating the golden mean for the fractal fluctuations of all size scales, i.e., small, large and extreme values. The model predicted distribution is in close agreement with observed fractal fluctuations in the historic month-wise temperature (maximum and minimum) and rainfall in the UK region. The present study suggests that fractal fluctuations result from the superimposition of an eddy continuum fluctuations. The observed extreme values result from superimposition of maxima (or minima) of dominant eddies (waves) in the eddy continuum.
Bayesian objective classification of extreme UK daily rainfall for flood risk applications
M. A. Little,H. J. E. Rodda,P. E. McSharry
Hydrology and Earth System Sciences Discussions , 2008,
Abstract: In this study we describe an objective classification scheme for extreme UK daily precipitation to be used in flood risk analysis applications. We create a simplified representation of the spatial layout of extreme events based on a new digital archive of UK rainfall. This simplification allows a Bayesian clustering algorithm to compress these representations down to eight prototypical patterns of extreme falls. These patterns are then verified against a five-class, manual, subjective typing scheme, produced independently using known meteorological mechanisms, isohyetal maps and additional descriptive text from the archive. Compared against the manual scheme, the new objective scheme can reproduce the known meteorological conditions, both in terms of spatial layout and seasonal timing, and is shown to be of hydrological relevance when matched to several notable flooding events in the past century. Furthermore, it is computationally simple and straightforward to apply in classifying future extreme rainfall events. We discuss the practical use of this new typing scheme in flood simulations and climate change applications.
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