Soil Water
Soil Water Movement
As discussed in Section 7.1, Hydrologic Cycle, water movement is the entry of free water into the soil at the soil–atmosphere interface. This is a pivotal process in landscape hydrology that greatly influences soil moisture content. The free water at the soil surface may be from rainfall, snowmelt, or irrigation. The terms infiltration and percolation are frequently interchangeably used even though they represent two different processes. Water infiltration is the movement of water from the soil surface into the soil profile. Water moves by gravity into the open pore spaces in the soil and is influenced by soil texture, surface characteristics, and slope.
Saturated Flow
Gravitational flow occurs only under saturated conditions, when matric forces cannot hold water against the force of gravity. At saturation large soil pores are full of water, and water flows readily through those large interconnected pores. Most water movement occurs in the macropores, and movement is driven by differences in gravitational potential. It should not be inferred that saturated flow occurs only down the profile. The hydraulic force can also cause lateral and even upward flow, as occurs when groundwater wells up under a stream.
Factors Influencing Saturated Flow
Soils dominated by large sand particles tend to have relatively large pore spaces and thus large values of saturated hydraulic conductivity. Soils dominated by small clay particles tend to have relatively small pores spaces and small values of saturated hydraulic conductivity. Some types of clay are especially conducive to fine pores. Soils high in montmorillonite (2:1) generally have low conductivities compared to soils with 1:1 type of clays. Likewise, soils with stable structure conduct water much more rapidly through interspersed cracks and pores than do those with unstable structural units, which break down upon being wetted.
Unsaturated Flow
When rain or irrigation water is added to the surface of dry, unsaturated soils, the immediate soil surface becomes water saturated. If water is allowed to drain from a saturated soil, water leaves the soil first and most rapidly via the largest pores. As drainage proceeds over time, water movement shifts to smaller and smaller pores or micropores, often referred to a as capillary pores, at moisture contents of field capacity or less.
Water Vapor Movement
When soils are unsaturated, some of the pore space is air filled and water moves through the pore space as vapor. The driving force is the difference in water potentials as expressed by differences in vapor pressure. There are mainly two soil conditions that affect the water vapor movement namely moisture regimes and thermal regimes. In addition to these regimes, there are various other factors which influence the moisture and thermal regimes of the soil like organic matter, vegetative cover, poor size, and soil color.
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