ABSTRACT
Darcy’s law is the basic law governing the flow of water (or other liquids) in permeable materials, and it tells us that the flow velocity q (m sec1) at any point (e.g., in soil, porous rock, concrete, timber, or other material) is proportional to the gradient of the water potential at that point. However, Darcy’s law tells us only about the flow at individual points and is sufficient to describe only steady flow processes. To model unsteady flows (where the moisture distribution changes with time), we must also know the relationship between velocities at neighboring points. If the neighboring velocities are unequal, their ‘mismatch’ must be compensated by a filling or emptying of pores between the points. The additional equation required to complete the mathematical description of flow is the so-called continuity equation. Basically, this equation ensures that matter is not created or destroyed, and so is also called the conservation equation. When Darcy’s law is combined with the continuity equation, we obtain Richards’ equation, first derived by the physicist Lorenzo Adolph Richards in 1931.[1]
Richards was a pioneering soil physicist who contributed enormously to soil water physics in the United States in the period c.1930-1960.[2,3] His contributions to theory included the conceptual extension of Darcy’s Law to unsaturated flow,[1] as part of his development of Richards’ equation. On the experimental side, he: invented the tensiometer;[2,4] developed the pressureplate apparatus[5] to measure water desorption from soil; developed the thermocouple method for measuring the vapor pressure (or ‘‘water activity’’) in soil or biological materials; and helped establish the relationship between the permanent wilting point for plants and the soil water content at 15 bar suction. He also investigated salt-affected soils.
