36.1.2.3. Degree of Saturation

Degree of saturation is an important concept in groundwater hydrology, referring to the ratio of the volume of water in the soil to the volume of voids available in the soil. It is expressed as a percentage and indicates how saturated a particular soil or rock layer is with water. Degrees of saturation can range from 0% (completely dry) to 100% (fully saturated).

Degree of saturation helps us understand how much water the soil can hold compared to how much it currently has. This is crucial for various applications, including agriculture and engineering, as it affects the soil's physical properties. When degree of saturation is at 100%, it means all the void spaces are occupied by water, making it fully saturated; at 0%, those voids contain only air. The degree of saturation affects how groundwater moves through the soil. For example, in fully saturated soils, water can flow freely, whereas in partially saturated soils, flow can be impeded.

Think of a sponge. When it is dry, it has lots of empty spaces that can fill up with water, indicated by a degree of saturation of 0%. When you soak the sponge, it reaches a point where every space is filled with water, which would be a degree of saturation of 100%. How well the sponge absorbs water and how water moves through it changes depending on whether it is dry, damp, or fully saturated.

The degree of saturation is influenced by various factors, including:

• Soil type: Different soils have varying capacities to hold water.
• Weather conditions: Rainfall or drought can change the moisture content rapidly.
• Depth of the water table: The closer the water table is to the surface, the higher the degree of saturation.

Different types of soil have unique characteristics that affect how much water they can hold. For example, sandy soils have larger pores and can drain quickly, often resulting in lower degrees of saturation after rain compared to clay soils, which hold water tighter because of their smaller pores. Similarly, previous weather conditions such as heavy rain or extended dry periods will directly influence how much water is available in the soil. Lastly, the depth of the water table determines how saturated a soil can get; if the water table is shallow, the degree of saturation can be higher because the surrounding soil is more likely to absorb up the water.

Imagine two different kinds of kitchen sponges. One is a coarse, open-cell sponge (like sandy soil), and the other is a dense, tightly-woven sponge (like clay). The coarse sponge will release water quickly and might not stay saturated for long after you squeeze it out. In contrast, the dense sponge holds onto water tightly, remaining wet longer after being soaked. Furthermore, if you place both sponges in a basin of water (the water table), the sponge that sits deeper may not absorb any liquid if the water table is at the very bottom of the basin and isn’t close to the sponges.

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