摘要:

The use of the bootstrap technique to estimate the reference level of137Cs in an uneroded site is tested. The analysis is developed using137Cs measurements made in a small experimental Sicilian basin. In the reference area the137Cs activity is normally distributed with a known sample mean value,mequal to 94.4 mBq cm−2. The influence of137Cs reference site sampling was determined generating samples having a fixed size,Nand six different values of the sample coefficient of variation,CV, by a Monte Carlo technique. Then, for each sizeN, the probability distribution of the mean μ of the sequences generated by Monte Carlo technique is defined. The soil redistribution is determined, both at morphological unit and basin scale, using the proportional method of Martz & de Jong for calculating the net soil loss. The analysis showed that the spatial distribution of the net soil lossEi, and the basin valueEbare independent of the sample size,N, and the coefficient of variation,CV, of the samples drawn from the reference area, if the bootstrap technique is applied for estimating the mean μ(μ) to use as reference value. The soil redistribution is also examined using as reference value the quantiles μ2.5, μ25, μ75, μ97.5 corresponding to a frequency F(μ) equal to 2.5, 25, 75 and 97.5%, respectively. In conclusion, the analysis established that a robust estimate of the reference value can be obtained even in fields where a small number of samples was drawn (highCVof the137Cs activity of the field samples), using the bootstrap technique for generating sequences of reference values having known mean valuem(the mean value of the137Cs activity of the drawn field samples) and large sample size (N= 50).

ISSN:0262-6667    

DOI:10.1080/02626660009492318

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Spatial rainfall patterns and seasonal variability were assessed for a mountainous river basin with monsoon climate. Factors were identified that could explain this variability, and a GIS-supported method to determine the areal distribution of precipitation was developed. To find acceptable regression equations, a division had to be made between rainfall stations dominated by the southwest-monsoons and the northeast-monsoons, respectively. Distance to the southwestern border was the main explaining factor for precipitation at southwest-monsoon dominated stations. For northeast-monsoon dominated stations, altitude and slope were the most important factors. The basin was divided into pixels with characteristics typical for northeast- or southwest-monsoon dominated rainfall stations to allow calculation of spatial rainfall. The difference when comparing regression-based estimates with Thiessen-based estimates was small when considering the annual estimates for the whole basin. However, when analysing seasonal rainfall or sub-catchments, the differences between Thiessen-based and regression-based estimates were significant.

ISSN:0262-6667    

DOI:10.1080/02626660009492319

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The problem of transformation of rainfall data from one scale to another has been gaining considerable importance in recent years. Though the application of the concept of fractal theory, in the studies conducted thus far, nearly unanimously points at the possibility of such a transformation, the suitability of the theory to the highly variable rainfall in time and space has very often been questioned. A preliminary attempt is made herein to address this issue by investigating the existence of temporal scaling behaviour in rainfall data observed in two different climatic regions: (a) a subtropical climatic region (Leaf River basin, Mississippi, USA) and (b) an equatorial climatic region (Singapore). Rainfall data of three different resolutions, six-hourly, daily, and weekly, observed over a period of 25 years, are investigated. A mono- or simple-scaling method (box dimension method) is employed. The results achieved for the different data sets clearly indicate the existence of temporal scaling in rainfall observed in the two regions, an encouraging news on the suitability of fractal theory in understanding and modelling the rainfall process. However, the insufficiency of a single dimension to characterize the rainfall behaviour is realized, as the dimension depends on the rainfall intensity level, which, in turn, may be related to the rainfall generating mechanisms. A comparison of the box-dimension results obtained for data of different resolutions, from each of the regions, seems to indicate a possible connection between them, a prospect of tremendous practical importance. Another interesting observation is the similarity between the box dimension results obtained for rainfall data from Leaf River basin and Singapore, but this is also clearly related to the intensity level. The dependence of the dimension on the intensity threshold suggests the use of a multi-dimensional fractal approach, where the process is characterized by more than one dimension (or a dimension function) instead of one single dimension. On the basis of the present results, some potential areas for further study are identified.

ISSN:0262-6667    

DOI:10.1080/02626660009492320

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The uncertainty in mean discharge from two large South American rivers (the Amazonas at Obidos, Brazil, and the Paraná at Corrientes, Argentina) resulting from variation in the data used to fit rating curves at the two sites is evaluated. For the Amazonas at Obidos, where mean annual discharge over the 27-year period 1970–1996 was 164 200 m3s−1uncertainty arising from rating curve variability amounted to 2256 m3s−1equivalent to about 16% of year-to-year variability. For the Paraná at Corrientes, where mean annual discharge over the 24-year period 1961–1985 was 17597 m3s−1, the uncertainty arising from rating curve variability was 245.3 m3s−1, equivalent to 4.4% of year-to-year variability in mean discharge. Where volumes of freshwater delivered to the oceans by large rivers must be estimated for climatemodelling or other purposes, ill-defined rating curves contribute to the uncertainty in mean annual discharges, but become of secondary importance where land-use change and other factors cause changes in discharge; and some evidence is advanced to suggest that such changes have occurred in the Parana basin since 1960.

ISSN:0262-6667    

DOI:10.1080/02626660009492321

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The data-based mechanistic (DBM) modelling methodology is applied to the study of reservoir sedimentation. A lumped-parameter, discrete-time model has been developed which directly relates rainfall to suspended sediment load (SSL) at the reservoir outflow from the two years of measured data at Wyresdale Park Reservoir (Lancashire, UK). This nonlinear DBM model comprises two components: a rainfall to SSL model and a second model, relating the SSL at the reservoir inflow to the SSL at the reservoir spillway. Using a daily measured rainfall series as the input, this model is used to reconstruct daily deposition rates between 1911 and 1996. This synthetic sediment accretion sequence is compared with the variations in sand content within sediment cores collected from the reservoir floor. These profiles show good general agreement, reflecting the importance of low reoccurrence, high magnitude events. This preliminary study highlights the potential of this DBM approach, which could be readily applied to other sites.

ISSN:0262-6667    

DOI:10.1080/02626660009492322

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The response of monthly 7-day low flow, monthly instantaneous peak flow, and monthly frequency of flood events to El Niño and La Niña episodes is investigated for 18 rivers that represent a diverse range of climatic types throughout New Zealand. A significant positive or negative deviation from the long-term average was observed in over half the possible combinations of river, streamflow index, and type of ENSO episode; significant deviations were most frequent in the case of low flow, especially during La Niña episodes. Patterns of streamflow response differ widely between rivers, and the response of a given river to individual ENSO episodes is very variable. The patterns of streamflow response to ENSO are consistent to some extent with the climatic effects of ENSO already identified by meteorologists. Two core regions can be defined in which streamflow tends to respond in the same way. These are in the northeast of the North Island, and in the axial ranges of the South Island, where there are significant effects of ENSO on the frequency and duration of rain-bearing northeasterly and westerly winds respectively. The patterns of response strongly reflect topography, and the exposure of catchments to predominant air masses.

ISSN:0262-6667    

DOI:10.1080/02626660009492323

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A modelling scheme is developed for real-time flood forecasting. It is composed of (a) a rainfall forecasting model, (b) a conceptual rainfall-runoff model, and (c) a stochastic error model of the ARMA family for forecast error correction. Initialization of the rainfall-runoff model is based on running this model on a daily basis for a certain period prior to the flood onset while parameters of the error model are updated through the Recursive Least Squares algorithm. The scheme is suitable for the early stages of operation of flood forecasting systems in the presence of inadequate historical data. A validation framework is set up which simulates real-time flood forecasting conditions. Thus, the effects of the procedures for rainfall-runoff model initialization, forecast error correction and rainfall forecasting are assessed. Two well-known conceptual rainfall-runoff models (the Soil Moisture Accounting model of the US National Weather Service River Forecast Service—SMA-NWSRFS and TANK) together with data from a Greek basin are used for illustration purposes.

ISSN:0262-6667    

DOI:10.1080/02626660009492324

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

An investigation of hydrological droughts has been conducted, based on the truncation level approach: each drought event is characterized by its duration and deficit volume. The truncation level is defined to reflect the expected natural water availability and, therefore, is evaluated monthly as a fixed percentile of the monthly flow-duration curve. Thus, the problem of expected zero flow of ephemeral rivers during the dry season is taken care of. The data material consists of daily discharge data from ten Zimbabwean rivers, and both ephemeral and perennial rivers are included in the analysis. The partial duration series approach is used to predict the severity of future droughts, i.e. theT-year events. The two-component exponential distribution is adopted as exceedence distribution for both duration and deficit volume. The parameters of the two-component exponential distribution are estimated using the maximum likelihood method. A method for calculating theT-year event and an approximate expression of the uncertainty of theT-year events have been developed. An observed problem of underestimation of observed deficit volumes is reduced by the introduction of censoring in the partial duration series. A better description of the observed events has been obtained by censoring the duration and deficit volume series. A relationship between optimal censoring and the coefficient of variation of the drought series has been indicated.

ISSN:0262-6667    

DOI:10.1080/02626660009492325

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The planning of water resources depends on the type and size of projects, the ecological factors involved, etc. Emphasis is placed on presenting an overview of water resources through meteorological, hydrological, ecological and economic data. Economic data include all costs and benefits, specifically those hitherto under estimated, environmental social costs and benefits. This study was carried out on the Bedthi and Aghnashini rivers in the Uttara Kannada district of the Western Ghats region, Karnataka State, India. It is estimated that 720 and 510 million kWh of electricity can be generated in Bedthi and Aghnashini River basins, respectively, if all the streams are harnessed. Focusing on land submergence impact, a model is proposed to minimize submergence and maximize net energy in a region with seasonal power generation, reservoir storage capacity (to meet the region's demand during all seasons) and installed generation capacity as the decision variables. Net energy analyses incorporating biomass energy lost in submergence show that maximization in net energy at a site is possible if the hydroelectric generation capacity is adjusted according to the seasonal variations in the river's water discharge. A Decision Support System (DSS) used for optimal design of hydroelectric projects in Uttara Kannada district is discussed.

ISSN:0262-6667    

DOI:10.1080/02626660009492326

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A procedure is presented for using the bivariate normal distribution to describe the joint distribution of storm peaks (maximum rainfall intensities) and amounts which are mutually correlated. The Box-Cox transformation method is used to normalize original marginal distributions of storm peaks and amounts regardless of the original forms of these distributions. The transformation parameter is estimated using the maximum likelihood method. The joint cumulative distribution function, the conditional cumulative distribution function, and the associated return periods can be readily obtained based on the bivariate normal distribution. The method is tested and validated using two rainfall data sets from two meteorological stations that are located in different climatic regions of Japan. The theoretical distributions show a good fit to observed ones.

ISSN:0262-6667    

DOI:10.1080/02626660009492327

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor