Water Holding Capacity: Sponge Model


"Dry" Soils Infiltration and Runoff Wetting and Saturation
Water Holding Capacity Percolation and Drainage Engineering and Bearing Capacity


Water Holding Capacity: Sponge Model


Gravitational water

This is the water which drains through the soil under the influence of gravity. Such drainage occurs through large soil pores. Small soil pores have the ability to hold the water against the pull of gravity through the process of capillarity.

The next series of images will address drainage of the water from the large pores in the sponge. 


Draining sponge, held flat - When the sponge is removed from the water, and held flat, water drains from it. This occurs rapidly at first, then slows to a stop. As it slows, notice the sponge is not equally wet from top to bottom. The top has more empty pores than the bottom.


Draining sponge, short vertical axis - If the sponge is turned from horizontal to vertical, the drainage begins again. As before, drainage will be rapid for a time, then slow to a stop.


Draining sponge, long vertical axis - If the sponge is turned one more time, the drainage will begin again, then slow to a stop. Careful observation of the side of the sponge from top to bottom will allow you to see which pores retain water, and which ones drain.


The only thing that changed as the sponge was turned is the distance over which gravity could work. In the first case (horizontal sponge), gravity only had about 4 cm over which it could pull water. In the second case (short vertical axis), gravity could work through about 10 cm. In the last case (long vertical axis), gravity could pull water for about 20 cm. Thus, each turn of the sponge provided more water to be moved by gravity. Upon careful observation, you noticed that water drained from the large pores in the sponge, but was retained in the smaller pores.


Field Capacity - When all the water that can has drained from the soil by gravity, the soil is at field capacity. A field definition is the amount of water remaining in the soil 2 to 3 days after a soaking rainfall or irrigation when evaporation from the surface has been prevented. A laboratory estimate places a saturated soil sample on a porous ceramic plate and applies 33 kPA (1/3 bar or 5 psi) of pressure to force some water through the soil pores, into the plate and out of the system.

This water exists in the soil capillaries, those pores that are small enough to hold the water against the pull of gravity.

Clays hold the most water at field capacity, while sands hold the least.

However, field capacity is affected strongly by soil structure. The amount of aggregation in the soil determines the amount of small pores available to hold water against gravity.


~ ~ ~


Capillary Water - This is the water held against gravity in the small soil pores, or capillaries. This water can be extracted by the plants, just as the next series of activities with the wet sponge indicates. 


Water extraction, wet sponge - Beginning with the sponge at field capacity, when all gravitational water had drained, begin to squeeze the sponge.
At first it takes little energy to extract water.

As the sponge becomes progressively drier, more energy is required to extract water. (It has to be squeezed harder.)

When the sponge is nearly dry, much more energy is required to extract just a little bit of water.

Likewise, not all of the capillary water held in the soil is equally available to the plants. Plants are able to extract water easily from soils that are near field capacity water contents. As the soil dries, the plants must work progressively harder to extract water, until finally, the soil is so dry that the plant can no longer expend enough energy to extract any more water. This is the concept of wilting point. But, wilting point is not the same for all plants. Some, like sunflowers, have the ability to extract more water from soil than others, like corn. 

~ ~ ~


Squeeze-dried sponge - It is important to note that, although no more water can be extracted from the sponge by squeezing it, it is not "dry". If the sponge is allowed to sit on the 

counter for a week, it will become drier as water evaporates from it. In the same way, bare soil in a field, especially after a tillage event, can become drier than wilting point.



Wilting Point, plant unavailable water - When no more water could be squeezed from the sponge, was it dry to the touch? The answer is no, it still retains water. This water is not absorbed in the pores between the sponge fibers, but adsorbed (or adhered) to the fibers of the sponge. The same is true in soil. All soils retain water that plants are unable to extract; clays hold the most unavailable water while sands hold the least.