UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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Conference: Bucharest University Faculty of Physics 2006 Meeting


Section: Atmosphere and Earth Science; Environment Protection


Title:
The method EOF (Empirical Orthogonal Functions): algorithm and interpretation for the variability of phreatic levels at basinal scale.


Authors:
Elena Tanase


Affiliation:
National Institute of Hydrology and Water Management

Sos. Bucuresti-Ploiesti 96

Bucuresti, ROMANIA


E-mail


Keywords:
EOFs method, phreatic levels, eigenvalues, eigenvectors


Abstract:
The main purpose of the Paper is the characterization of the annual variability of phreatic levels during 1979-1993 at a basinal scale, in Somes and Crisuri River Basins. For this purpose it is applied the statistical multivariate method EOF (Empirical Orthogonal Functions) to a data set formed by 96 drills phreatic levels which will form a input matrix Y[96, 15]. Before EOFs computations, the data set was standardized by division at the standard deviation with the aim of removing possible wrong values.The EOF method is used to extract dominant time variations and spatial patterns of multi-dimensional data.For a given variable X(t) = [Xi(t)], i=1, 2…I, where i is the space index and I the total number of space points, for n =15 cases the point number 96 (X1, X2, …Xp) it is applied a linear transformation to a new set of variables Y1, Y2, … (named EOFs, and the temporal coefficients Principal Components or PCs).[Xi(t)] includes all phreatic levels data , i=1,2…96 the number of space points.Y1 = a11X1 + a21X2 + ... + ap1Xp.It is found that the main natural variability is due to the well-known NAO, interpreting the first three EOFs. The EOF1 is associated to large scale interactions of the system-the NAO oscillation mode- during winter season, the EOF2 is associated to the mean state conditions of the system in the frame of regional context and the EOF3 is resembling with the snow cover duration map for the analyzed region. The interpretation was possible using different correlations between the EOFs and various circulation indices, like NAO DJFM.When the index of NAO phase is high, the main regional characteristics are the drought conditions, the phreatic depth is high, and the evolution of the phreatic system is under (drought) risk conditions.