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James Hartsig
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 – storm water
or natural.
Eventually, it returns to the atmosphere.
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| 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. |
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.
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| 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. |
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.
By James Hartsig, Soil Scientist, Duraroot Environmental Consulting, LLC
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https://soilsmatter.wordpress.com/2016/05/15/how-does-water-move-through-soil/
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