Maximum plant available water is defined as:

Maximum plant available water is defined as the amount of water that can be readily absorbed by plant roots and is often calculated as the difference between field capacity and the permanent wilting point.

Field capacity refers to the quantity of water that soil can hold against the force of gravity, which is available for uptake by plants. On the other hand, the permanent wilting point is the moisture level at which plants can no longer extract sufficient water to maintain their physiological functions, leading to wilting.

Therefore, the difference between field capacity and the permanent wilting point directly gives us the range of water that plants can utilize effectively, which constitutes the maximum plant available water. Understanding this concept is vital for effective water management in agriculture, as it influences irrigation practices and soil health.

While runoff and drainage are important factors in water movement within the soil, they do not directly define the amount of water available to plants in the context of this question.