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Abstract The use of the bootstrap technique to estimate the reference level of137 Cs in an uneroded site is tested. The analysis is developed using 137Cs measurements made in a small experimental Sicilian basin. In the reference area the 137Cs activity is normally distributed with a known sample mean value, m equal to 94.4 mBq cm?2. The influence of137 Cs reference site sampling was determined generating samples having a fixed size, N and six different values of the sample coefficient of variation, CV, by a Monte Carlo technique. Then, for each size N, 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 loss E i, and the basin value E b are 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 (high CV of the 137Cs activity of the field samples), using the bootstrap technique for generating sequences of reference values having known mean value m (the mean value of the 137Cs activity of the drawn field samples) and large sample size (N = 50). 相似文献
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Abstract The caesium-137 technique affords both an alternative to conventional measurement methods and an effective quantitative estimate of soil redistribution at the basin scale. Among the available calibration relationships which link the degree of increase or depletion of the 137Cs activity relative to the baseline 137Cs input and sediment yield, the mass balance approach has received increased application for its physical basis. First, the applicability of the refined simplified point-based mass balance (RSPMB) model of Zhang et al. (1999) at the scale of the morphological unit is proposed herein. The 137Cs spatial distribution measured in a small Sicilian basin and the spatial distribution of the sediment yield calculated by a sediment delivery distributed approach are used to estimate values of the two key parameters of the RSPMB model, φ1 and φ2, the fraction of 137Cs fallout incorporated into soil and a particle size correction factor, respectively. Finally, the best procedure for experimental testing of a distributed sediment yield model by using caesium-137 measurements is investigated. 相似文献
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Abstract This paper reviews simplified methods for evaluating the rainfall erosivity index and proposes two relationships for estimating the annual value of Wischmeier's rainfall erosivity index at sites equipped with recording and non-recording raingauges. For the Sicilian region, the FAO index was also found to represent the erosion risk. A regional relationship to estimate the standard deviation of the annual erosivity index is proposed. The isoerosivity map is plotted by using 41 values of the rainfall erosivity index calculated by Wischmeier's procedure and 128 values estimated according to a relationship proposed by the authors. Finally, for each of 169 sites, an erosion risk index is calculated and an erosion risk map is plotted. 相似文献
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Abstract Since eroded sediments are produced from different sources distributed throughout a basin, sediment delivery processes at basin scale have to be modelled by a spatially distributed approach. In this paper a new theoretically based relationship is proposed for evaluating the sediment delivery ratio, SDRi, of each morphological unit, i, into which a basin is divided. Then, using the sediment balance equation written for the basin outlet, a relationship between the basin sediment delivery ratio, SDRW and the SDRi is deduced. This relationship is shown to be independent of the soil erosion model used. Finally, a morphological criterion for estimating a coefficient, β, is proposed. 相似文献
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McCARTHY D. STOLOVY S. KERN S. SCHNEIDER G. FERRO A. SPINRAD H. BLACK J. SMITH B. 《Earth, Moon, and Planets》1997,78(1-3):243-249
Near-infrared images of comet Hale-Bopp (C/1995 O1) were obtained from NICMOS/HST on UT August 27–28, 1997, when the comet
emerged from the 50 degree solar elongation limit at 2.99 AU from Earth. Diffraction-limited images were obtained with camera
2 filters centered at 1.87, 1.90, 2.04 and 2.22 μm with ∼0.2″ resolution (0.076″/pixel; 165 km/pixel). Over the 1.7-hour baseline
of observation, a recent (<7 hours) outburst is seen in the form of an expanding spiral arm with a projected expansion velocity
of ∼80 m/s. Other asymmetric features include a jet emanating from the nucleus and several static linear features. Comparisons
of the flux distribution in the 2.04 and 2.22 μm filters indicate that the region near the nucleus exhibits a slight, ∼3%,
water ice absorption.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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