How does water move through soil?

In the basic water cycle, water falls on the land in some type of precipitation (rain or snow). It either is soaked into the ground or runs off into a body of water – stormwater or natural. Eventually, it returns to the atmosphere.

After a large rain event, even a good soil mixture can get saturated. Excess water runs over the soil – or city streets – and into retention ponds. Cities have learned to create these ponds to hold the water until it can soak through the soil at the bottom of the pond. This helps the water flow naturally back into the groundwater table, instead of surface waters like streams and lakes. Credit: SV Fisk

But, the part of the story about water movement in soil is complex. Soil scientists call this topic “soil hydrology.” Let’s cover some basics first.

Soil texture, soil structure, and gravity influence water movement. Each of these factors is critical in how we understand soil hydrology concepts. Once we understand them, we can then use them for agriculture, construction, and environmental sustainability purposes.

Soil particles are either sand, silt, or clay. Sand-sized particles are the largest of the three and are typically held loosely together. Clay-sized particles are the smallest particles and tightly bound together. Silt-sized particles are sized in between sand and clay. The relative amount of sand, silt and clay in a given area makes up the “soil texture.”

“Soil porosity” describes the amount of macro- and micropores in the soil. These pores exist in gaps where soils particles come together. The macropore space in a sand-dominated soil, where the particles are larger, would be much more than the micropore space in a clay-dominated soil, where particles are smaller and held together tightly.

Water will move in and out of these pores if they are connected to one another. These pores also allow water to enter the soil surface through infiltration, where it starts moving both laterally and vertically.

The term soil scientists use for the connectivity of soil macro- and micropores and how quickly water moves through them is “soil permeability”. High soil permeability means that the pore space in the soil is well-connected and that the pores are found throughout the soil. Beach sand is highly permeable.

Soils with low permeability may have several pores but those pores may not be connected. Or there may be very few pores. However, once water reaches pores in low-permeability soils, it has to move down the soil profile via gravity or laterally via capillary action. Water will move laterally in the soil profile if there is enough pore space in that soil. This movement is aided by the capillary action of water in small spaces. The water will bind to the edges of the pores and slowly move laterally and even upward if the voids are small enough. You might see a puddle of water in this instance.

Soil particles clump together – in what soil scientists call aggregates. These structures can also affect water’s ability to move in soil. This is a close up of aggregated soil particles with both subangular blocky and granular soil structures. Credit: Arina Sukor

There are several types of “soil structures” in the soil environment, and they affect the rate at which water moves through soil profiles. Soil structures that allow water to move easily through the soil profile are granular or crumb shaped. These types of soil structures form clumps in a way that allows for abundant connected void space. Granular soil structures are an ideal surface soil (topsoil) structure as it allows for both the vertical and lateral movement of water immediately upon infiltration. Granular soil structures have plenty of space for water to flow around the soil, which will then begin to move downward once the topsoil horizon becomes saturated.

Soil structures that inhibit the vertical movement of water down the soil profile are “plate-like” and “massive” (in this case, we don’t mean large, we just mean the structure has no form!) As clays accumulate and bind together, they tend to form hard subsurface layers. Water will have difficulty moving past these layers and can result in standing water – puddles, ponding, even swampy areas.

It is often recommend that soils with low water permeability be aerated to allow surface flow. If the soil has trouble moving water down the soil profile, it will then move laterally or above the soil surface.

If water can successfully move down the soil profile, it will eventually reach the groundwater table. The pathway to get to the groundwater table is often very complex and incorporates several soil physical characteristics. Healthy soils tend to be those with moderate soil porosity, a mixture of clayey and sandy soil textures, well-aggregated soil structures like granular and blocky, and active gravimetric forces.

Soils with poor hydrology typically have hardened, clayey soil textures with little to no porosity. These types of soils exhibit issues (surface ponding, perched water tables) for agriculture, construction, and environmental sustainability.

By identifying and evaluating the soil physical characteristics of a given soil profile, soil scientists can determine the rate of water movement and if measures need to be taken to improve it.

Read more here about water movement through soil called “preferential flow.”

By James Hartsig, Soil Scientist, Duraroot Environmental Consulting, LLC

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