Laws of soil moisture

Observed regularities in the processes connected with the flow and distribution of water in soils may, if they are sufficiently general, be referred to as laws of soil moisture. The Darcy equation which expresses the proportionality between transmission velocity and the driving force for water in saturated soils is one such regularity that is commonly referred to as a law. Another regularity that applies without exception whenever outflow of free water occurs from soil, is of the nature of a boundary condition, but is here proposed as a law; namely, “Outflow of free water from soil will occur only if the pressure in the soil water exceeds atmospheric pressure.” Examples for the application of the outflow law are cited and discussed.