In this chapter a general model for the two-phase fluid flow in porous media is presented, together with its simplified form, known as the Richards equation, which is applicable (under specific assumptions) to describe water flow in the vadose zone. In each case the governing equations are formulated at the Darcy scale, using the capillary pressure–saturation relationship and an empirical exten...

Steady state multiphase flow models are used for the design and operation of pipelines that are used in the oil and gas industry. The predictions obtained with these models contain uncertainties, which arise from model assumptions and simplifications, and from uncertainties in the input parameters. In this thesis we investigate the effect of uncertainties in the input parameters (such as the wa...