18 Water Flow in Soil

Importance of soil water flow

Understanding types of water flow in soil and the factors affecting them helps us improve water management in various ways including irrigation system management, flood prevention and mitigation, minimizing leakage risks for hazardous wastes, and turfgrass maintenance. Water flow and wise management thereof is therefore integral to managing a wide array of systems, from agroecosystems to waste storage systems and golf courses.

From the previous chapters, we’ve learned that water flows from high to low water potential. Here we’ll learn what affects the rate of flow, and when.

Types of water flow

The rate of water flow through soil depends on the amount of water in the soil, specifically the water potential and, concurrently, the thickness of water films. Generally, the thicker the water films (and therefore the higher the water potential), the faster the water flow in units of volume per unit time. We therefore classify soil water flow into 3 types, based on key water potentials: saturated flow, unsaturated flow, and vapor flow.

Saturated flow

Saturated flow, as the name suggests, occurs when soil pores are completely filled with water. In other words, it occurs at saturation, i.e. when the soil matric potential is 0 (Ψ_m=0).

Saturated flow is affected by the same factors that influence permeability: texture, aggregation, and pore connectivity. Use the interactive below to explore the effect of texture.

Increasing aggregation increase the number of macropores found between aggregates (inter-aggregate pores), which, like a larger water pipe, increases permeability and allows for faster water flow. Then, pore connectivity refers to how many pores (especially larger ones) are found directly adjacent to each other so as to form a more direct water flow path through the soil, thereby increasing permeability and water flow rate. We can also think about this in terms of the opposite concept, tortuosity. Tortuosity refers to how winding and indirect a path that water (or air) must take to get through the soil. The more tortuous the soil, the longer it takes for water to flow through the soil, resulting in lower permeability, plus slower infiltration and drainage.

The exact rate of saturated flow can be calculated with Darcy’s Law, but this is largely beyond the scope of this introductory book. For now, note that flow rate increases with increasing overall permeability (a function of texture, aggregation and pore connectivity as detailed above), and with an increasing difference in water potentials.

Unsaturated flow

Unsaturated flow occurs after some water has drained from soil, and water moves as a liquid through the remaining water-filled pores and water films. In terms of matric potentials, this occurs between saturation at 0 kPa and the hydroscopic coefficient at -3100 kPa (-3100 < Ψ_m < 0). See the vapor flow section for more information on the hydroscopic coefficient.

Compared with saturated flow, unsaturated flow is slower because water can only flow through water-filled pores and water films, and therefore must flow around air-filled pores found in unsaturated soils. This increases the water flow path, slowing down water flow. As unsaturated soil dries, the water-filled pores diminish in number and the water films thin, making water flow paths more winding and less direct, slowing down water flow. Therefore, unsaturated flow decreases with decreasing water film thickness. In other words, unsaturated flow is directly correlated with matric potential and water film thickness. The closer the soil gets to saturation, the faster the unsaturated flow.

Use the below question to guess the effect of texture on unsaturated flow, and then expand the accordion menu below that to learn even more (this concept is intermediate in difficulty and may be optional in introductory courses).

Vapor flow

Vapor flow occurs when water moves as a gas through the soil, and this becomes the dominant form of water flow when water films are only ~5 or fewer molecules thick. Such thin layers of water occur at or below the hydroscopic coefficient, equivalent to a matric potential of -3100 kPa. When water films are so thin, each water molecule is so very strongly attracted to surfaces that it can only break free from the particle surface if it evaporates.

It is very difficult to measure water potential below -3100, so it becomes easier to predict the direction of vapor flow using humidity. As humidity decreases, water potential decreases. Therefore, water will flow as a vapor from areas of high to low humidity.

Vapor flow is important to desert-adapted plants called xerophytes, some of which are capable of taking up water as a gas, frequently by condensing it on contact with the roots. At such low water potentials, soil water content begins to depend more on soil surface area and soil particle surface chemistry. The more surface area available and the more hydrophilic (water-attracting) the surfaces are, the more soil will attract water vapor and the more water it can hold at Ψ_m < -3100 kPa.

Water Flow in Stratified Soils

Stratified soils are soils where there is an abrupt change in texture from one layer to the next. For example, when a sand layer is found above a loam layer, or a clay layer is found above a loam layer. Such strongly contrasting textures effectively create a barrier to flow.

When going from a coarse texture to a fine texture, the water flows quickly down through the coarse-textured layer, then slows and spreads out as it reaches the less permeable fine texture. When going from a fine texture to a coarse texture, the water is attracted to the numerous small micropores in the fine-textured layer, and spreads out in this layer instead of going down to the next layer. Frequently the water will only flow down to the coarser underlying layer after the upper, finer layer is saturated with water. At this point, the water potential in the fine layer is sufficiently high to overcome the effect of texture and allow water to flow down, from high to low water potential.

Check out this video for an example (it also appears in the infiltration and permeability chapter):

Note that in the video, the soil starts off dry so we are primarily viewing unsaturated flow. It is unclear whether any layers temporarily reach saturation as the are wetted.

Perched water tables and stratified soils

Very stratified soils, with extreme contrasts in textures, can bring water flow to a near-complete halt. This creates what is called an aquitard, a relatively impermeable layer. Above this layer, even saturated water cannot flow downwards. This causes groundwater to become “perched,” i.e. stuck, above this near-impermeable layer. Such groundwater bodies are called perched aquifers or perched water tables. Think of the water being perched above the near-impermeable layer, like a bird perched on a tree branch.

In wet regions, a perched water table can create a wetland or other poorly-drained zone. Wetlands have many benefits as explained in subsequent chapters, but the poor drainage would be problematic for growing crops.

In dry regions, a perched water table can be quite boon, as this makes it easier to dig wells and get water when water is scarce. This can even enable formation of ponds and lakes in the desert, making an oasis.

One such place is Benson, AZ, where the water table allowed the formation of several ponds and dense growth of vegetation, attracting people to live there as well. Unfortunately this area was contaminated with trichloroethylene (TCE), so the water became unsafe to drink, and hazardously close to the surface as well. In summation, stratified soils and other poorly permeable layers can profoundly change the overall hydrology of an area.