The physical law describing water movement through saturated porous materials in general and soils in particular was proposed by Darcy (1856) in his work concerned with the water supplies for the town of Dijon. He established the law from the results of experiments with water flowing down columns of sands in an experimental arrangement shown schematically in Fig. 1. Darcy found that the volume of water Q flowing per unit time was directly proportional to the crosssectional area A of the column and to the difference Dh in hydraulic head causing the flow as measured by the level of water in manometers, and inversely proportional to the length L of the column. Thus

KA DhQ (1) L

where the proportionality constant K is now known as the hydraulic conductivity of the porous material. The dimensions of K are those of a velocity, LT1. Typical values of K for soils of different textures are given in Table 1. Conversion factors relating various units are given in Table 2. Since the hydraulic conductivity of a soil is inversely proportional to the viscous drag of the water flowing between the soil particles, its value increases as the viscosity of water decreases with increasing temperature, by about 3% per C.