Variability Of Rainfall In Time Research Articles

A New Physically Based Index to Quantify the Impact of Multiple Reservoirs on Flood Frequency at the Catchment Scale Based on the Concept of Equivalent Reservoir

AbstractThe presence of multiple reservoirs in a catchment generally induces the attenuation of flood events, thus mitigating their impact in downstream flood‐prone areas. Such mitigation results from the superposition of several contributions, coming from the regulated (i.e., altered by reservoir) or unregulated subcatchments as determined by the reservoirs spatial distribution within the catchment. Understanding how multiple reservoirs affect the probability distribution of flood peak at the catchment scale is a difficult task, requiring a detailed and generally costly hydrologic/hydraulic simulation. We provide here a simple yet physically based mathematical framework to account for the effect of multiple reservoirs, located in series along the main channel, on peak flood quantile at the catchment scale. The framework allows to disentangle the role of the main relevant parameters controlling the system behavior, such as the number of reservoirs, their relative location and relative storage coefficient within the catchment, and a climatic parameter governing rainfall variability in time. The combined effect of the reservoirs in reducing the peak flow is represented by a global index , which is bounded between zero and unit and it is independent of the return period. The index is based on the concept of equivalent reservoir and is easily calculated by the analytical formulas provided in this study, as function of the above parameters.

English

Rainfed agriculture is practiced in the central clay plains of Sudan and is affected by the high rainfall variability in time and space within and between seasons. This study focused on analyzing rainfall variability and trend using a 30-year record (1985-2014) of seven meteorological stations at the major agricultural production areas in Gedarif state in eastern Sudan. Yearly rainfall has relatively low variability compared to monthly variability. According to annual rainfall totals, it was possible to classify stations into two groups; one with annual rainfall more than 600 mm and the second with rainfall ranging between 500 to 600 mm. In both groups, the majority of rainfall (60%) occurred during July and August. Trends of rainfall were inconsistent and the cropping season extended from June to September. Farmers in areas having high rainfall and extended growing season (group I) could grow suitable crops and varieties and their appropriate management practices should be implemented. In areas of low rainfall and short growing seasons (group II), farmers could grow crops of short maturing varieties and water harvesting techniques. There is a need for research activities that examine rainfall trends and how agricultural practices might adapt accordingly. Key words: Rainfall, variability, trend, sorghum yield, Gedarif, Sudan.

A high space–time resolution dataset linking meteorological forcing and hydro-sedimentary response in a mesoscale Mediterranean catchment (Auzon) of the Ardèche region, France

Abstract. A comprehensive hydrometeorological dataset is presented spanning the period 1 January 2011–31 December 2014 to improve the understanding of the hydrological processes leading to flash floods and the relation between rainfall, runoff, erosion and sediment transport in a mesoscale catchment (Auzon, 116 km2) of the Mediterranean region. Badlands are present in the Auzon catchment and well connected to high-gradient channels of bedrock rivers which promotes the transfer of suspended solids downstream. The number of observed variables, the various sensors involved (both in situ and remote) and the space–time resolution ( ∼ km2, ∼ min) of this comprehensive dataset make it a unique contribution to research communities focused on hydrometeorology, surface hydrology and erosion. Given that rainfall is highly variable in space and time in this region, the observation system enables assessment of the hydrological response to rainfall fields. Indeed, (i) rainfall data are provided by rain gauges (both a research network of 21 rain gauges with a 5 min time step and an operational network of 10 rain gauges with a 5 min or 1 h time step), S-band Doppler dual-polarization radars (1 km2, 5 min resolution), disdrometers (16 sensors working at 30 s or 1 min time step) and Micro Rain Radars (5 sensors, 100 m height resolution). Additionally, during the special observation period (SOP-1) of the HyMeX (Hydrological Cycle in the Mediterranean Experiment) project, two X-band radars provided precipitation measurements at very fine spatial and temporal scales (1 ha, 5 min). (ii) Other meteorological data are taken from the operational surface weather observation stations of Météo-France (including 2 m air temperature, atmospheric pressure, 2 m relative humidity, 10 m wind speed and direction, global radiation) at the hourly time resolution (six stations in the region of interest). (iii) The monitoring of surface hydrology and suspended sediment is multi-scale and based on nested catchments. Three hydrometric stations estimate water discharge at a 2–10 min time resolution. Two of these stations also measure additional physico-chemical variables (turbidity, temperature, conductivity) and water samples are collected automatically during floods, allowing further geochemical characterization of water and suspended solids. Two experimental plots monitor overland flow and erosion at 1 min time resolution on a hillslope with vineyard. A network of 11 sensors installed in the intermittent hydrographic network continuously measures water level and water temperature in headwater subcatchments (from 0.17 to 116 km2) at a time resolution of 2–5 min. A network of soil moisture sensors enables the continuous measurement of soil volumetric water content at 20 min time resolution at 9 sites. Additionally, concomitant observations (soil moisture measurements and stream gauging) were performed during floods between 2012 and 2014. Finally, this dataset is considered appropriate for understanding the rainfall variability in time and space at fine scales, improving areal rainfall estimations and progressing in distributed hydrological and erosion modelling. DOI of the referenced dataset: doi:10.6096/MISTRALS-HyMeX.1438.

Analysis of Precipitation Concentration Index (PCI) for Awka Urban Area, Nigeria

Causes and consequences of climate variability of which the fluctuation in rainfall amounts over time represent an important indicator is indeed an area that has continued to generate research attention. The aim of this paper is, therefore, to evaluate the concentration and variability of rainfall in time using Precipitation Concentration Index (PCI) based on monthly precipitation over 38 years period (1976–2013) and its implications for flooding and water supply in the town. Data for the study were collected from NIMET, Lagos. The PCI was calculated for annual, seasonal and decadal scales using annual, wet and dry season equations. Result shows that annual PCI value range from the lowest of 12 in 1976 to the highest of 20 in 1987. About 78% of the years were characterized by PCI values that fall within moderate range, an indication of moderate precipitation distribution within the 38-year period, while 24% of the years recorded annual value within the irregular range indicating an irregular rainfall distribution and concentration across the months. Again, the fact that many months have PCI values within ≥ 16.7 is a further manifestation that almost 90% of the rainfall occurred in 6 months. These results show that the area is having reasonable floods with consequent pollution of the surface water bodies as well as soil erosion as observed in the town. It is, therefore, recommended that the Awka Capital Territory Development Authority (ACTDA) should take cognizance of this result in the planning of the new layouts and redevelopment of the old areas of the town.

Impact of suspended sediment load on the silting of SMBA reservoir (Algeria)

The phenomenon of suspended sediment load is very complex in Mina River basin because of its important soil heterogeneity, vegetation deficiency and rainfall variability in time and space. The methodological approach adopted in this paper consists of finding a regressive power model, which may explain better the suspended sediment discharge as a function of the flow discharge collected at Wadi El-Abtal and Sidi AEK Djilali hydrometric stations by studying this relation at various temporal scales: daily, annual, monthly and seasonal. The obtained monthly power relations, explaining the greatest part of the variance, lead to interpolate, extrapolate and analyse suspended and bed loads deposited on Sidi M’hamed Ben Aouda (SMBA) reservoir since being in service in 1977/1978. These allow authors to find relations between specific erosion and effective rainfall and propose some solutions for river basin managers and policy makers to reduce the silting of SMBA reservoir.

Venezuelan Rainfall Data Analysed by Using a Bayesian Space–time Model

SUMMARY We consider a set of data from 80 stations in the Venezuelan state of Guárico consisting of accumulated monthly rainfall in a time span of 16 years. The problem of modelling rainfall accumulated over fixed periods of time and recorded at meteorological stations at different sites is studied by using a model based on the assumption that the data follow a truncated and transformed multivariate normal distribution. The spatial correlation is modelled by using an exponentially decreasing correlation function and an interpolating surface for the means. Missing data and dry periods are handled within a Markov chain Monte Carlo framework using latent variables. We estimate the amount of rainfall as well as the probability of a dry period by using the predictive density of the data. We considered a model based on a full second-degree polynomial over the spatial co-ordinates as well as the first two Fourier harmonics to describe the variability during the year. Predictive inferences on the data show very realistic results, capturing the typical rainfall variability in time and space for that region. Important extensions of the model are also discussed.

TEMPORAL AND SPATIAL CHARACTERISTICS OF RAINFALL IN THE NAM RIVER DAM BASIN OF KOREA

An investigation into rainfall variability in time and space in the Nam River dam basin of Korea is made with the use of the coefficient of variation and the correlation coefficient. The Nam River dam basin is a small mountainous watershed where wind direction and orography are the dominant influences on the pattern and distribution of rainfall. Rainfall distribution was found to vary with elevation, position, wind direction and distance from a reference station. The results of this study can be used in the design of rain gauge network, hydrological forecasting and for other applications in the Nam River dam basin.

TEMPORAL AND SPATIAL CHARACTERISTICS OF RAINFALL IN THE NAM RIVER DAM BASIN OF KOREA

An investigation into rainfall variability in time and space in the Nam River dam basin of Korea is made with the use of the coefficient of variation and the correlation coefficient. The Nam River dam basin is a small mountainous watershed where wind direction and orography are the dominant influences on the pattern and distribution of rainfall. Rainfall distribution was found to vary with elevation, position, wind direction and distance from a reference station. The results of this study can be used in the design of rain gauge network, hydrological forecasting and for other applications in the Nam River dam basin.

On the Possible Use of Fractal Theory in Rainfall Applications

The rainfall data used in hydrological calculations are seldom sufficient to correctly represent the extreme variability of rainfall in time and space. Regarding for example traditional sewer design, the errors may be negligible, but the successively more complex hydrological models used today are much more sensitive to inadequate input data. The problem can be solved either by extensive data collection or by developing methods for extracting more information from limited data. An approach based on fractal theory has been shown to provide a suitable framework for this purpose. In this paper we apply two fractal-related analyzing methods to a two-year series of one-minute rainfall observations. The results indicate that the series exhibit statistical properties that are independent of scale, i.e., scale invariant. A possibility emerging from these findings is to make a shift from the scale defined by the available data to the scale needed for the hydrological problem in question, a prospect of tremendous practical importance.

Les variations spatiales et temporelles des précipitations au Sahel

The rainfall variability in time and space is very-high in the sahelian zone , it can occur in the whole West Africa. Dry or wet spells are recurrent phenomena. The drought of 1968 -1982, culminating in 1972 -1973 and once more in 1982 , exceeds all the previous drought episods in magnitude and spatial involvement. In order to understand climatic change and rainfall variability, the data required concern not merely the state of the atmosphere but the state of the whole climatic system consisting of atmosphere, ocean and land surface

Space-time quantification of rainfall inputs for hydrological transport models

Since the early dyas of the development of hydrology, hydrologists have attempted to represent the amount of rainfall on a catchment in both time and space through areal averaging techniques. Much work has been reported in the literature which deals with characterizing rainfall by depth, duration, intensity and frequency of occurrence. Defining rainfall variability in time and space has been the subject of many studies using dense experimental raingage networks and radar instrumentation. While the definition of variability and characterization of rainfall are important, the hydrologic response of the basin, due to rainfall time and space distribution for event and between events periods, is required in continuous simulation models. Raingage network densities, ranging from the experimental network density (23 km 2) to climatological density (400 km 2), are used to calculate the element rainfall amount. Using rainfall records from a dense network of recording raingages (168 gages on 2926 km 2), methods of distributing rainfall from point amounts to distributed runoff model area elements are tested. Density of the rainage network spacing vs. size of the distributive model elements are tested on basins ranging in size from 67 to 2926 km 2. Depending on the size of the distributed model element and the size of the basin, the computation and computer processing of rainfall input data can be prohibitive. The time scale increment and corresponding model responses for selected hydrological models are also tested. The computation requirements and relative cost of basin-scale rainfall inputs are presented.