1DUSAT

1DUSAT is a flexible one-dimensional unsaturated flow and transport model. The initial version of this model was developed in 1991. In 2018, 1DUSAT was revised to include mobile/immobile porosity regions for transport, and a single site attachment/detachment model with a nonlinear Langmuirian blocking coefficient.

Assuming that the role of the air phase is insignificant, one-dimensional isothermal flow in an unsaturated/saturated porous medium is described in 1DUSAT by the modified form of Richards’ equation. Water saturation is assumed to be single-valued function of the water pressure head and relative permeability is assumed to be single-valued functions of the water saturation. 1DUSAT provides options for one of seven constitutive relationships. The transport domain or total porosity is divided into a mobile region and an immobile region, with the rate of mass exchange between these regions assumed to be first-order. An advection, dispersion and reaction equation is used to represent the mobile regions, and a reaction equation for the immobile regions.

The transient nonlinear unsaturated flow equation is solved using a fully-implicit mass conservative finite or control volume (CV) approach. Fluid mass is conserved by the direct application of the CV approach to the mixed form of the flow equation. A full Newton-Raphson iterative scheme is employed to converge to the required solution at each time step. The advection, dispersion and reaction equations were solved using a similar CV approach for the spatial domain along with a standard theta differencing scheme for the temporal domain. A central or upstream scheme is available for the spatial weighting of the advective term. Mass balance calculations are provided at the end of each time step.

1DUSAT has the provision to use the dynamically dimensioned search (DDS) optimization method within the OSTRICH toolkit as the calibration tool.

The User’s Guide, and executable and sample input files are available upon request.