When stressed, alfalfa draws moisture from throughout its root zone. However, it will typically draw 80 percent of the water it uses for growth from the upper five feet of an unrestricted root zone. For this reason, and to retain some reserve storage for the retention of rainfall, irrigation scheduling decisions for alfalfa are frequently based on the assumption of an effective rooting depth of five feet. Capacity of the soil to retain water determines how much is held within the root zone. The soil water storage capacity is largely a function of texture.
Coarse soils are able to store less available water than fine textured ones.
Available water refers to the amount of water stored between the point at which gravity causes water to drain from the soil, and the point at which plants are no longer capable of extracting water from the dried soil. Typical values of available soil water storage capacity for various soil types can be found in the USDA County Soil Surveys. These values range from about 0.8 inches of water per foot of soil depth for fine sands, to 2.5 inches per foot of soil for silt loams. Typical values are shown in Table 7. Typical Irrigation Characteristics for Agricultural Soils.