How is 'soil moisture' defined in irrigation terms?

In irrigation terms, 'soil moisture' refers to the amount of water that is held in the soil and is available for plant uptake. This definition focuses on the water that exists in the soil's pore spaces, which is crucial for supporting plant growth and development. Soil moisture is essential for maintaining appropriate moisture conditions, allowing roots to absorb water and nutrients effectively.

The other choices do not adequately capture the concept of soil moisture. The total amount of rainfall does not account for factors such as evaporation and infiltration that affect the water's availability to plants. The level of groundwater is related but refers specifically to water found below the ground surface and not the moisture that plants can access. Meanwhile, the moisture content in the air deals with atmospheric humidity rather than the water present in the soil itself. Therefore, the correct choice emphasizes the crucial relationship between the water contained in the soil and the needs of plants for that water.