# Multiprocessing for the Particle Tracking Model MODPATH

## TL;DR

This paper enhances the MODPATH particle tracking model by implementing parallel processing with OpenMP, significantly reducing simulation runtimes especially for large particle numbers and complex flow models.

## Contribution

It introduces parallel processing capabilities to MODPATH using OpenMP, improving computational efficiency for large-scale particle tracking simulations.

## Key findings

- Dynamic thread scheduling benefits heterogeneous flow simulations.
- Parallel processing reduces runtimes for large particle counts.
- Simulation runtimes become independent of grid complexity at high particle numbers.

## Abstract

Particle tracking has several important applications for solute transport studies in aquifer systems. Travel time distribution at observation points, particle coordinates in time and streamlines are some practical results providing information of expected transport patterns and interaction with boundary conditions. However, flow model complexity and simultaneous displacement of multiple particle groups leads to rapid increase of computational requirements. MODPATH is a particle tracking engine for MODFLOW models and source code displays potential for parallel processing of particles. This article addresses the implementation of this feature with the OpenMP library. Two synthetic aquifer applications are employed for performance tests on a desktop computer with increasing number of particles. Speed up analysis shows that dynamic thread scheduling is preferable for highly heterogeneous flows, providing processing adaptivity to the presence of slow particles. In simulations writing particles position in time, thread exclusive output files lead to higher speed up factors. Results show that above a threshold number of particles, simulation runtimes become independent of flow model grid complexity and are controlled by the large number of particles, then parallel processing reduces simulation runtimes for the particle tracking model MODPATH.