The Design Document page provides a description of the algorithms, implementation and planned testing including unit, verification, validation and performance testing.
The first table in Design Document gives overview of this document, from this info the Design Documents Overview page is automatically created. In the table below, 4.Equ means Equations and Algorithms, 5.Ver means Verification, 6.Perf - Performance, 7. Val - Validation
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In the table below, 4.Equ means Equations and Algorithms, 5.Ver means Verification, 6.Perf - Performance, 7. Val - Validation, - completed, - in progress, - not done
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Groundwater, which accounts for 30% of freshwater reserves globally, is a vital source for human water supply. Climate change is expected to impact the quality and quantity of groundwater in the future. Numerous studies have shown a positive soil moisture-rainfall feedback through observational data, as well as, numerical simulations. Despite the obvious need to accurately represent soil moisture dynamics, the version-0 of the ACME Land Model (ALM) employs a non-unified treatment of hydrologic processes in the subsurface. Presently, ALM simulates transport of water in the subsurface via a theta-based Richards equation that is loosely coupled to an unconfined aquifer model to account of groundwater–soil water interaction. This ad hoc treatment of vadose–phreatic zone interaction at times results in unphysical model prediction of unsaturated soil layers below predicted water table.
A variably saturated flow model (VSFM) will overcome above-mentioned shortcoming by using pressure-based Richards equation that is valid in unsaturated and saturated zone. The VSFM solver will use finite volume spatial discretization and backward Euler time integration. The nonlinear equations resulting from spatial and temporal discretization will be solved using the Portable, Extensible Toolkit for Scientific Computation (PETSc).
The requirements for VSFM model include:
For each requirement, there is a design solution that is intended to meet that requirement. Design solutions can include detailed technical discussions of PDEs, algorithms, solvers and similar, as well as technical discussion of performance issues. In general, this section should steer away from a detailed discussion of low-level software issues such as variable declarations, interfaces and sequencing.
d/dt (rho * phi * sat) = - Div (rho * q) + Q ... (1)
where
rho = density of water, which is a function of pressure and temperature,
phi = porosity,
sat = liquid saturation
q = darcy flux
Q = source/sink of water (e.g. infiltration, evapotranspiration, bare soil evaporation)
Div = divergence operator
where
k = permeability,
kr = relative permeability.
mu = viscosity,
P = liquid pressure
Design and Implementation