Research Bulletin 520, Department of Agronomy , Iowa State University. Variation of soybean ET factor over the growing season. The actual soil water content also influences crop water use. As soil dries, it becomes more difficult for the plant to extract water from the soil. When the soil water content drops below field capacity, plants use less water.
Crops differ in their response to water stress at a given growth stage.
Different crops have different water requirements and respond differently to water stress. The range of water use for crops varies from one area to another. However, we can use some guidelines to determine crop water use. For soybeans, the factor would be 0.6 at 40 days after planting. Then water use drops off.