Evaluation of Selected Geostatistical Methods for Interpolating Hydraulic Conductivity in Shallow Alluvial Aquifers Using Cross-Validation Statistics

Abstract

Hydraulic conductivity (K) is essential for understanding groundwater movement in shallow alluvial aquifers. Upstream of the Siret–Moldova confluence, Romania, this parameter is poorly constrained, because available K values come from only 34 wells reported in existing hydrogeological documentation. This study provides the first directional geostatistical characterization of K in this shallow alluvial aquifer and evaluates the spatial reliability of the mapped zones. K values ranged from 9.96 to 171.73 m/day. Because the raw data were positively skewed, the values were log-transformed before geostatistical modelling. Spatial continuity and anisotropy were examined using omnidirectional and directional semivariograms. Three theoretical models were tested—Spherical, Gaussian and Exponential—and their performance was compared using cross-validation statistics and prediction standard-error maps. The Gaussian and Spherical models showed comparable performance; however, the Gaussian model was retained because it produced marginally lower errors and a smoother spatial pattern (ME = 0.0055, RMSE = 0.4870, RMSSE = 0.8697, R2 = 0.6712). The back-transformed K map shows three main zones: lower values in the northern and eastern sectors, intermediate values in the central sector, and the highest values toward the west. This pattern is consistent with coarser Moldova River alluvial deposits in the west and more heterogeneous lithological conditions in the central area. The prediction standard-error maps indicate the areas where the interpolated hydraulic conductivity (K) is more reliably constrained. The resulting map provides a preliminary spatial representation of the main hydraulic-conductivity zones and indicates where additional K measurements are needed before detailed groundwater-flow modelling or local groundwater-resource assessment.