Head and Velocity Covariances Under Quasi‐Steady State flow and their Effects on Advective Transport

Abstract

Most stochastic models of solute transport assume flow to be at steady state. However, even locally uniform gradients tend to show seasonal fluctuations in magnitude and direction. Such transients affect the prediction of flow and plume migration and spread. The question is how and to what extent. We address this question by developing low‐order approximations for autocovariances and cross covariances of velocity, head, and log hydraulic conductivity under quasi‐steady state flow, then using a first‐order Lagrangian approach to examine their effect on advective transport. Our results show that whereas periodic temporal fluctuations in the magnitude of the mean velocity may either enhance or reduce dispersion, similar fluctuations in its direction always cause longitudinal dispersion to decrease and transverse dispersion to increase.