Lecture 15 Soil Water (2) Soil Water Movement (1) Concept of Hydraulic Head Soil moisture Characteristics Darcy’s Law Infiltration

Hydraulic Head Pressure: Force per unit area, e.g., (kg m/s 2 )/m 2 = kg/(ms 2 ) For water with density of 1000 kg/m 3 under the gravity of about 10 m/s 2, 1 m of water in height will create a pressure of the magnitude: 1000 kg/m 3  10 m/s 2 =10,000 Pascal = 100 mb=0.1 bar. Therefore, 10 m of water in height is equivalent of 1 bar, i.e., one atmospheric pressure. Because liquid water density does not vary and gravity can be taken as a constant, the pressure can be converted to water column height, which is often called hydraulic head.

Retention Curve Describes the suction required to extract water from soil at various soil water contents. Figure 6.2, WR Soil Moisture Characteristics Atm. pressure Wilting point ~15 bar or 15,000 cm)

Hysteresis Difference in the retention curve between wetting and drying processes in the soil. Figure 6.3, WR Soil Moisture Characteristics

Soil Water Movement Darcy’s Law: Where ν is the macroscopic velocity of water, K(  ) is the hydraulic conductivity (which, in the unsaturated conditions, varies with the water content (θ)  h/  l is the hydraulic gradient comprising the change in hydraulic head ( h) with a distance along the direction of flow ( l ).

Hydraulic Conductivity Hydraulic conductivity: the speed of macroscopic water movement in the soil, usually in units: cm/h. It depends on the porosity and pore size distribution. Figure 6.9, WR Soil Water Movement