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Olea R.A.,U.S. Geological Survey | Pardo-Iguzquiza E.,Instituto Geologico Y Minero Of Espana Igme
Mathematical Geosciences | Year: 2011

The semivariogram and its related function, the covariance, play a central role in classical geostatistics for modeling the average continuity of spatially correlated attributes. Whereas all methods are formulated in terms of the true semivariogram, in practice what can be used are estimated semivariograms and models based on samples. A generalized form of the bootstrap method to properly model spatially correlated data is used to advance knowledge about the reliability of empirical semivariograms and semivariogram models based on a single sample. Among several methods available to generate spatially correlated resamples, we selected a method based on the LU decomposition and used several examples to illustrate the approach. The first one is a synthetic, isotropic, exhaustive sample following a normal distribution, the second example is also a synthetic but following a non-Gaussian random field, and a third empirical sample consists of actual raingauge measurements. Results show wider confidence intervals than those found previously by others with inadequate application of the bootstrap. Also, even for the Gaussian example, distributions for estimated semivariogram values and model parameters are positively skewed. In this sense, bootstrap percentile confidence intervals, which are not centered around the empirical semivariogram and do not require distributional assumptions for its construction, provide an achieved coverage similar to the nominal coverage. The latter cannot be achieved by symmetrical confidence intervals based on the standard error, regardless if the standard error is estimated from a parametric equation or from bootstrap. © 2010 International Association for Mathematical Geosciences.


Garcia-Garizabal I.,CSIC - Centro de Investigacion y Tecnologia Agroalimentaria | Causape J.,Instituto Geologico Y Minero Of Espana Igme
Journal of Hydrology | Year: 2010

Irrigated agriculture is working towards environmental sustainability but more study is needed regarding the agro-systems' response to alterations imposed on them. This work analyzed the repercussions that alternative irrigation management can cause on water use and on the quality of irrigation return flows. The case of Bardenas Canal Irrigation District no. V (ID-V) was studied, by analyzing the change of salinity and nitrate concentration in the drainage network through monthly samplings, before (2000) and after (2007) ID-V implanted alternative flood irrigation management. The results showed that the electric conductivity (25 °C) and nitrate concentration in the drainage ditches increased in 2007 (0.99 dS/m and 62 mg/l) with respect to 2000 (0.86 dS/m and 57 mg/l). Nevertheless, the decrease in irrigation drainage in 2007 (88% lower in 2007 when compared to that of 2000) was a result of the decrease in water requirement (594 mm in 2007 against 752 mm in 2000) and of the increase in irrigation efficiency (93% in 2007 and 67% in 2000), causing the Riguel River to present a lower flow (13% inferior when compared with 2000), lower salinity (1.08 dS/m in 2007 and 1.18 dS/m in 2000), and lower nitrate concentration (29 mg/l in 2007 and 33 mg/l in 2000) when exiting ID-V in 2007. In summary, simple alternatives in irrigation management achieved an increment of 26% in water use, decreasing by 20% and 24% the salt and nitrate masses exported, respectively, ameliorating the quality of the system receiving the irrigation return flows (Riguel River). © 2010 Elsevier B.V. All rights reserved.


Pardo-Iguzquiza E.,Instituto Geologico Y Minero Of Espana Igme | Rodriguez-Tovar F.J.,University of Granada
Geo-Marine Letters | Year: 2011

One important handicap when working with stratigraphic sequences is the discontinuous character of the sedimentary record, especially relevant in cyclostratigraphic analysis. Uneven palaeoclimatic/palaeoceanographic time series are common, their cyclostratigraphic analysis being comparatively difficult because most spectral methodologies are appropriate only when working with even sampling. As a means to solve this problem, a program for calculating the smoothed Lomb-Scargle periodogram and cross-periodogram, which additionally evaluates the statistical confidence of the estimated power spectrum through a Monte Carlo procedure (the permutation test), has been developed. The spectral analysis of a short uneven time series calls for assessment of the statistical significance of the spectral peaks, since a periodogram can always be calculated but the main challenge resides in identifying true spectral features. To demonstrate the effectiveness of this program, two case studies are presented: the one deals with synthetic data and the other with paleoceanographic/palaeoclimatic proxies. On a simulated time series of 500 data, two uneven time series (with 100 and 25 data) were generated by selecting data at random. Comparative analysis between the power spectra from the simulated series and from the two uneven time series demonstrates the usefulness of the smoothed Lomb-Scargle periodogram for uneven sequences, making it possible to distinguish between statistically significant and spurious spectral peaks. Fragmentary time series of Cd/Ca ratios and δ18O from core AII107-131 of SPECMAP were analysed as a real case study. The efficiency of the direct and cross Lomb-Scargle periodogram in recognizing Milankovitch and sub-Milankovitch signals related to palaeoclimatic/palaeoceanographic changes is demonstrated. As implemented, the Lomb-Scargle periodogram may be applied to any palaeoclimatic/palaeoceanographic proxies, including those usually recovered from contourites, and it holds special interest in the context of centennial- to millennial-scale climatic changes affecting contouritic currents. © 2011 Springer-Verlag.


Martin-Gonzalez F.,Rey Juan Carlos University | Heredia N.,Instituto Geologico Y Minero Of Espana Igme
Tectonophysics | Year: 2011

The tectonic and tectonostratigaphic evolution of foreland basins and related Tertiary depressions are the key to investigate deformation history and the uplifting of the continental lithosphere of the Alpine-Pyrenean Orogeny. The northern part of the Duero basin is the foreland basin of the Cantabrian Mountains, which are, in turn, the western part of the Pyrenean Orogen. We have studied the western sharp end of the Duero foreland basin, and its relation to the Tertiary deposits of the NW Iberian Peninsula and the topography evolution. In order to propose a coherent tectonic and tectonosedimentary model that could explain all Tertiary deposits, we have analysed the depositional environment, stratigraphic sequences, paleocurrents and established a correlation of the main outcrops. Besides, a detailed structural mapping of the Alpine structures that limit and affect the main Tertiary outcrops has been carried out. The Tertiary deposits of the NW Iberian Peninsula depressions are affected and fragmented by Alpine structures that limit their extensions and locations. The stratigraphic succession is similar in the NW Tertiary outcrops; they are mainly terrigenous and carbonated continental deposits formed by assemblage of alluvial fans developed at the mountains front, in arid or semiarid conditions. Three formations can be identified in the main depressions: Toral Fm, Santalla Fm and Médulas Fm. The NW Tertiary outcrops were the western deposits of the Duero foreland basin that surrounded the lateral termination of the Pyrenean Orogen. These deposits were fragmented and eroded by the subsequent uplift of the Galaico-Leoneses Mountains and the NE-SW strike-slip faults activity (broken foreland basin). Only the latest stages of some of these outcrops can be considered as intramontane basins as traditionally have been interpreted. The sedimentation started in the northeast (Oviedo-Infiesto) during the Eocene and migrated to the west (As Pontes) during the Late Oligocene-Early Miocene times. Meanwhile, southern areas were far away from the tectonic activity (El Bierzo, O Barco, Monforte, Sarria depressions). © 2010 Elsevier B.V.


Pardo-Iguzquiza E.,Instituto Geologico Y Minero Of Espana Igme | Olea R.A.,U.S. Geological Survey
Computers and Geosciences | Year: 2012

In applied geostatistics, the semivariogram is commonly estimated from experimental data, producing an empirical semivariogram for a specified number of discrete lags. In a second stage, a model defined by a few parameters is fitted to the empirical semivariogram. As the experimental data are usually few and sparsely located, there is considerable uncertainty about the calculated semivariogram values (uncertainty of the empirical semivariogram) and about the parameters of any model fitted to them (uncertainty of the estimated model parameters). In this paper, the uncertainty in the modeling of the empirical semivariogram is numerically assessed by the generalized bootstrap, which is an extension of the classic bootstrap procedure modified for spatially correlated data. A computer program is described and provided for the assessment of those uncertainties. In particular, the program provides for the empirical semivariogram: the standard errors, the bootstrap percentile confidence intervals, the complete variance-covariance matrix, standard deviation correlation matrix. A public domain, natural dataset is used to illustrate the performance of the program. A promising result is that, for any distance, the median of the bootstrap distribution for the empirical semivariogram approximates more closely the underlying semivariogram than the estimate derived from the empirical sample. © 2011.

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