The way water moves through the ground is the topic of this article, including an investigation the meaning of “porosity” and “permeability”.

To try to explain these terms I turn to the old and trusted “Landscape Processes, An Introduction to Geomorphology” by Darrel and Valerie Weyman (1977).

Remember the water cycle? – water evaporates and goes up, condenses, and comes down. What it lands on when it comes down is vital to a watershed. If rain lands on vegetation, it will be intercepted and some of it will then evaporate back to the atmosphere. Some rain, however, will drip down into the vegetation and some will trickle down further to the surface of the soil.

Soils vary greatly; leaves, roots, humus, gravel, sand, clay, rock. Within these soils are empty spaces which the rainwater will occupy. This process is called “infiltration” and the rate of infiltration is determined by the size and shape of the spaces and the amount of water already in those spaces.

So here is your first definition – The total volume of air spaces in a soil is called “porosity”. Soil porosity increases as the average size of particles in the soil decrease. This occurs because smaller particles pack themselves together to create a larger number of spaces for water to occupy.

One way to understand this idea is to fill a funnel with small gravel and pour 10 ml (tablespoon) of water through it and into a cup. Virtually all of the water will stream through the gravel. Then replace the gravel with sand and again pour the same amount of water through the funnel.

The water you poured onto the sand might not even make it through to the cup. It has been stopped by the much higher porosity of the sand. It filled up the empty air spaces created by the much smaller particles and different packing patterns between the grains, and there it is retained.

The rate at which water moves through the air spaces in the soil is called the “permeability” of the soil and depends less on the total amount of porosity than on the size of the individual air spaces.

Permeability increases as the porosity decreases. Because water moves quickly through large air spaces, the gravel in our experiment shows high permeability, but low porosity (little water is retained in the gravel). Conversely, the sand (many small spaces) did not let water drain through as quickly, so it has a lower level of permeability and it’s retention of the water shows it has high porosity.

Our water is pulled by gravity through the soil and deeper and deeper into the ground. As the weight of the overlying ground compacts the soils under it, they become less permeable to the water. Water may be deflected horizontally and start an “interflow” along the top of this non-permeable layer of ground. Water may also stop at this non-permeable layer and completely fill up all the spaces above it. As more water is pulled down the level of the filled spaces will push upward. Now the ground has become “saturated” with water filling all available space. The part of the rock which has become saturated is now called the “ground-water zone” and the upper surface of the ground-water zone is called the water table. Above the water table the rock is unsaturated and water can still percolate through until it reaches the saturated rock.

Drilling a well means that you must drill until you have entered the saturated rock below the water table. It is important to know what type of aquifer you are drilling into for your well. A gravel aquifer on top of a less permeable layer will fill up with rainwater quickly and provide a lot of water, however, as you now know gravel is highly permeable and that captured water can rapidly drain away from your well. It will all depend at what depth your water table is situated and on the stability of that level over time.