4. Water movement in soils
(1)
Saturated flow (Soil texture, structure, organic matter, hardpan, water content, temperature, etc.) :
Darcy's Law: Q / t = (A Ksat Δψ) / L
A = cross sectional area of the column; Ksat = saturated hydraulic conductivity
Δψ/L = water potential gradient, or hydraulic gradient
For typical Ksat values of some soil types. But please note the preferential flow, and the 104 power relationship with the pore radius changes.
(2) Unsaturated flow (Driven by matric potential gradient, or the thickness of water films on particles, <-10 kPa)
(3) Infiltration: the process by which water enters the soil pore space.
(4) Percolation: the process of water moving downward into the profile after infiltration, which involves both saturated flow and unsaturated flow.
(5) Water vapor movement:
Moist →Dry; Cool →Warm
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Saturated flow (percolation) in a column of soil with cross-sectional area A, cm2. All soil pores are filled with water. At lower right, water is shown running off into a container to indicate that water is actually moving down the column. The force driving the water through the soil is the water potential gradient, ψ1- ψ2/L, where both water potentials and length are expressed in cm |
The effect of land management and soil texture on saturated conductivity (Ksat) of three soils in Canada. Soils under native woodlots had higher Ksat values, apparently due to higher organic matter contents and to preferential flow channels provided by decayed roots and burrowing animals. Tillage practices had little effect on conductivity in sand, but in loam and clay loam soils, conductivity was higher where no-tillage systems had been used, suggesting that no-till had increased the proportion of larger, water-conducting pores. [Drawn from averages of three methods of measuring Ksat in Reynolds et al. (2000)]
Generalized relationship between matric potential and hydraulic conductivity for a sandy soil and a clay soil (note log scales). Saturated flow takes place at or near zero potential, while much of the unsaturated flow occurs at a potential of -0.1 bar (-10 kPa) or below.