41.2.4. Available Water

Available Water is the difference between field capacity and wilting point.

Available Water = θ_FC − θ_WP
Where FC and WP are volumetric moisture contents.

Available water refers to the water that plants can actually use to grow. It is calculated by taking the amount of water the soil retains at field capacity (when the soil is ideally moist) and subtracting the amount of water that remains when plants begin to wilt (the permanent wilting point). This difference gives us the volume of moisture that is available for plants to extract from the soil. Field capacity and wilting point are two key concepts in understanding soil moisture dynamics.

Imagine a sponge soaked in water. The sponge retains water (like soil at field capacity), but if you squeeze it, some water comes out, and if you continue to squeeze, you reach a point where no more water comes out (permanent wilting point). The water that is retained in the sponge without being squeezed represents the available water for the plants.

Field Capacity is the amount of water retained in soil after gravitational water has drained.

It represents optimal moisture for plant uptake.

Field capacity occurs after soil has been saturated and excess water has drained away due to gravity. At this point, the remaining water is held in the micropores of the soil, which is available for plants. This condition is crucial because it signifies the ideal moisture level that allows plants to absorb water easily through their roots without being waterlogged.

Think of a garden hose watering a plant. Initially, water flows out quickly and floods the soil (saturation). After a little while, the soil can’t hold any more water, and the excess drains away. Once the soil settles, what’s left (without the risk of root rot) is the moisture level at field capacity, which is just right for healthy plant growth.

Permanent Wilting Point is the moisture level at which plants cannot extract water and begin to wilt.

Water is held too tightly in micropores.

The permanent wilting point represents a critical threshold for plants; it's the level at which the soil moisture is so low that plants cannot extract it. This is primarily because the remaining water is held tightly in the soil's tiny pores, making it unavailable to roots. When plants reach this point, they start to show signs of stress, wilting ultimately leading to potential death if water isn't made available soon.

Imagine trying to drink water from a sponge that’s too dry. Even if the sponge still has some moisture, you can’t get any liquid out when it's that dry. Similarly, once the soil reaches its wilting point, plants can't access the water they need to survive, just like you can’t drink from that sponge.

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