Box model

A box model represents a system by finitely many internally uniform compartments. Balances of transported material and local sources give coupled ordinary differential equations. For a constituent's concentration $c_i$ in fixed box volume $V_i$, the stored amount is $V_i{c}_i$; changing volumes requires the balance of that product. A transport-only closed box model preserves total inventory, even when its transport matrix is nonsymmetric.