The Modular Aquatic Simulation System 1D (MASS1) simulates open channel hydrodynamics and transport in branched channel networks, using cross-section averaged forms of the continuity, momentum, and convection-diffusion equations. Thermal energy transport (temperature), including meteorological influences is supported. The thermodynamics of total dissolved gas (TDG) can be directly simulated.

MASS1 is one-dimensional and calculates cross-sectional average values of hydraulic and water quality conditions in the river and/or reservoir system. Thus, only single values of water surface elevation, discharge, velocity, concentration, temperature are computed at each point in the model at each time interval. Lateral and vertical variations of quantities are not simulated. The MASS1 model simulates a branched (tree-like) channel system. Looped channel systems cannot be simulated with the current version of MASS1.

Continuity and momentum equations are discretized using the classic Preissmann four-point implicit finite-difference scheme and the resulting system of nonlinear algebraic equations are solved using the double sweep method as described by Cunge (1980).

An explicit TVD scheme and split operator method is used to solve the transport equation. A time sub-cycling scheme is used to allow the hydrodynamics to run at the larger time steps allowed by the implicit scheme for hydrodynamics while using a smaller time step that satisfies the explicit stability criteria. The Courant number must be less operator than 1.0 to maintain stability in the explicit method.  Transport time step can be determined automatically. 

MASS1 is a command line application. There is no graphical user interface.  All input and output are plain text files. Some documentation is included in the source code.  Some pre- and post-processing tools are also included in the source.