mesas.py v1.0: a flexible Python package for modeling solute transport and transit times using StorAge Selection functions

Abstract

StorAge Selection (SAS) transport theory has recently emerged as a framework for representing material transport through a control volume. It can be seen as a generalization of transit time theories and lumped-parameter models to allow for arbitrary temporal variability in the rate of material flow in and out of the control volume, and in the transport dynamics. SAS is currently the state-of-the-art approach to interpreting tracer transport. Here, we present mesas.py, a Python package implementing the SAS framework. mesas.py allows SAS functions to be specified using several built-in common distributions, as a piecewise linear cumulative distribution function (CDF), or as a weighted sum of any number of such distributions. The distribution parameters and weights used to combine them can be allowed to vary in time, permitting SAS functions of arbitrary complexity to be specified. mesas.py simulates tracer transport using a novel mass-tracking scheme and can account for first-order reactions and fractionation. We present a number of analytical solutions to the governing equations and use these to validate the code. For a benchmark problem the time-step-averaging approach of the mesas.py implementation provides a reduction in mass balance errors of up to 15 times in some cases compared with a previous implementation of SAS.