Because the seasonal consumptive water use requirements of alfalfa are directly related to the length of the growing season, the total water requirement of alfalfa will vary. During a normal growing year in Oklahoma, the amount of irrigation required to achieve the maximum potential yield is about 17 inches in the southeast and 23 inches in the Panhandle. The balance of the water requirement is supplied by preseason soil water and effective rainfall during the growing season. The effective rainfall during a normal growing season decreases steadily as you travel across the state in a northwesterly direction. It ranges from a high of over 24 inches in the southeast down to less than 12 inches in the Panhandle.

Water requirements and scheduling
The rate at which alfalfa consumes water varies considerably depending upon time of year and stage of growth. Daily use rate is less than 0.1 inches per day early in the season and, on occasion, can exceed 0.35 inches per day for a fully developed canopy in midsummer. Weather conditions have a great effect on water use rate of alfalfa. Hot, dry, windy conditions necessitate considerably more water usage than cooler, humid, calm conditions for the same crop growth stage.

Irrigation systems are normally sized to meet peak water use demand of the crop during the driest period of the growing season. Systems may be sized to supply the complete water needs of the crop, assuring maximum production even during sustained drought conditions. More commonly, they are sized to supplement normal growing-season rainfall and assure maximum potential production in approximately six years out of 10. An irrigation system designed to supplement normal rainfall has a smaller water supply and smaller equipment components than a system that can supply total water needs, and is normally less expensive.

Alfalfa’s total water demand peaks in July at an average daily demand of about 0.29 inches per day. For an irrigation system that operates 18 hours per day with a 75 percent application efficiency, a water supply of 9.7 gallons per minute must be available for every acre to be irrigated. This means that to meet the total water requirement of 40 acres of alfalfa when there is no rainfall, 390 gallons per minute must be supplied by the system.

The frequency of irrigation and the depth of water to apply is a function of the soil in which the alfalfa is growing, the prevailing weather conditions and the crop rooting depth. Alfalfa is a deeply rooted crop. Feeder roots of mature stands may extend deeper than 12 feet in unrestricted soils. A more typical rooting depth is in the range of 4 to 8 feet. The crop can be sustained only by water stored in the soil profile within reach of its root system, so rooting depth has a significant effect on the frequency and amount of irrigation.

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 5 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 5 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 are shown in Table 1*.

As an example, alfalfa growing on a loam soil, capable of storing 2 inches of water per foot of soil depth, would have 10 inches of available water stored in its 5-foot effective rooting depth. However, the crop cannot deplete all of this water without becoming stressed to such a degree that growth is retarded. Normally, irrigation should occur when no more than 50 to 60 percent of the available soil water has been depleted from the effective root zone. This means that the crop should be irrigated after 5 to 6 inches of water have been used. This is also the net amount of water to be replaced at each irrigation.

The period of time that elapses before this amount of water is used varies during the season according to the weather conditions. If the peak daily water use during the growth cycle is 0.29 inches per day, 5 inches of water should be sufficient to supply alfalfa for the first 18 to 22 days after harvest and removal of the hay, without subjecting the crop to significant water stress.

An additional 2.5 inches of irrigation water is normally needed to see the crop through to cutting time, unless some rainfall occurs. Smaller, more frequent irrigations may be applied. However, this normally leads to less efficient water use. This is because immediately after irrigation the water use rate is elevated due to surface soil and vegetation wetness. The more often irrigation occurs, the more significant this excess evaporation becomes.

Though the water use rate immediately after harvest is greatly reduced because of the reduced amount of vegetation, research has shown this is the most critical period for water stress. Application of irrigation water to assure adequate water availability in the first few days after harvest stimulates a more rapid regrowth of vegetation than delaying irrigation to the second or third week after harvest.

Irrigation system types
The soil type also affects the type of irrigation system that can be used to apply water and the rate at which water is applied. The permeability or intake rate of the soil determines the rate at which water can be applied. The intake rate of a soil is largely determined by its texture. Typical intake rates for various soil textures are given in Table 1*.

With sprinkler systems, the precipitation rate of the system should not exceed the ultimate intake rate of the soil. If the system applies water too rapidly, water will pond on the soil surface and run off of sloping fields. The precipitation rate of a hand-moved or side-roll system is determined by the discharge of the individual sprinklers and the spacing between sprinklers. For a continuously moving system, such as a center pivot or lateral move system, the precipitation rate is determined by the individual sprinkler discharge, the sprinkler spacing and the speed of movement of the system.

Alfalfa is adaptable to virtually all types of sprinkler systems. Side-roll and center pivot systems are the most commonly used systems, but hand-moved and volume-gun systems may be used also. Alfalfa can also be irrigated by surface irrigation methods. Border irrigation can be practiced on smooth, level or uniformly sloping fields. Small cross-section furrows, or corrugations, can be used to control the movement of water across less uniform surfaces. To achieve an acceptable level of application efficiency, the permeability of the soil must fall within fairly narrow limits. If the intake rate is too high, the water cannot be spread over any but the shortest length of run before its advance will be completely absorbed. If the intake rate is too low, extremely low flow rates must be used with very long application times to prevent excessive runoff losses.

Water quality
As with most crops, yield increases realized when irrigating alfalfa can be significantly affected by the quality of the irrigation water. The presence of dissolved mineral salts in the water can result in an increase in the energy level required for plants to remove water from the soil pores. Some crops are more readily affected by salinity problems than others. Alfalfa is rated as being moderately sensitive to salinity effects.

Certain mineral elements have toxic effects on crops. Alfalfa is tolerant to the toxic effects of sodium in the soil and water. In non-saline conditions, alfalfa suffers no adverse effects until the exchangeable sodium percentage of the soil reaches a value of 40 to 60. Alfalfa is likewise tolerant to boron content in irrigation water. No visible toxic effects are observed until the boron content of irrigation water reaches 3 milligrams per liter.

Before considering development of an irrigation system, it is advisable to have the water supply tested to determine its suitability for irrigation purposes.  FG

*References have been omitted due to space, email office@progressivepublish.com to request.

—From Oklahoma State University Extension website

Mike Kizer Extension Agricultural Engineer, Oklahoma State University