The total discharge, formula_52 (having units of volume per time, e.g., ft³/s or m³/s), is proportional to the intrinsic permeability, formula_53, the cross sectional area, formula_54, and rate of pore pressure change with distance, formula_55, and inversely proportional to the dynamic viscosity of the fluid, formula_56. The negative sign is needed because fluids flow from high pressure to low pressure. So if the change in pressure is negative (in the formula_57-direction) then the flow will be positive (in the formula_57-direction). The above equation works well for a horizontal tube, but if the tube was inclined so that point b was a different elevation than point a, the equation would not work. The effect of elevation is accounted for by replacing the pore pressure by "excess pore pressure", formula_59 defined as: