• Irrigating Alfalfa in California with Limited Water Supplies
About 1,000,000 acres of alfalfa are irrigated in California. This large acreage coupled with a long growing season make alfalfa the largest agricultural user of water, with annual water applications of 4,000,000 to 5,500,000 acre-feet.
The crop water use or evapotranspiration (ET) is evaporation of water through leaves of the water uptaken by the plant and direct evaporation from the soil. Seasonal values of alfalfa ET range from about 33 inches in the Intermountain Area of northern California to about 60 inches in the Imperial Valley (Table 1). The different values, obtained from a recent study on alfalfa ET reflect climate characteristics of the various locations.
Table 1. Seasonal ET of fully-irrigated alfalfa for various locations in California
Alfalfa yield is directly related to ET with yield increasing in a straight-line manner as seasonal ET increases. Maximum yield occurs for maximum ET which depends on the climate characteristics. Insufficient soil moisture reduces the ET to values smaller than maximum ET and causes a yield loss.
Effect of a Limited Water Supply
Drought conditions can limit water supplies to levels smaller than needed for maximum yield. Several options are available for alfalfa growers to cope with a reduced water supply, but the bottom line is that yields will be reduced compared to normal water supply conditions.
Options for Limited Water Supplies
Reduce the irrigated acres (reduced acres option)
Table 2. Historical alfalfa crop evapotranspiration (inches per day).
Fully irrigate the early harvests; no irrigation on the remaining harvests (full / deficit option).
Distribute the limited water supply over the entire field throughout the crop season.
Reduce the amount of applied water per irrigation
Which Option is the Best?
The best option is the one which realizes the highest profit. Factors affecting the profit include the total alfalfa yield of the field, price of alfalfa, costs that vary with irrigated acres/number of cuttings such as harvest and irrigation costs, and fixed costs that do not vary with irrigated acres/number of cuttings, such as farm equipment, buildings, etc. since these costs were incurred prior to the conditions of limited water supplies.
The effect of the reduced acres and full/deficit options on yield and net returns was evaluated using yield and ET data from commercial fields located in the Imperial Valley, southern San Joaquin Valley, and southern Sacramento Valley. Results showed that either option was viable for the San Joaquin and Sacramento Valleys, while the full/deficit option was the best for the Imperial Valley.
The options of reduced number of irrigations and reduced applications per irrigation could not be evaluated because of the lack of data on the effect on yield of applying water amounts smaller than needed for fully irrigated conditions.
Stretching a Limited Water Supply
Applying water as efficiently as possible is needed to stretch a limited water supply. This means reducing surface runoff and deep percolation losses.
Surface runoff generally is the main loss with flood irrigation. Surface runoff occurs due to the large amount of water ponded on the soil surface during irrigation. After the irrigation water is terminated or cutoff, the ponded water continues to flow down the field. If the cutoff time is large, excessive runoff will occur.
Surface runoff can be greatly reduced by cutting off the irrigation water when the water reaches 80 to 90 percent of the field length. Surface runoff was reduced from 2.8 inches of water to 0.5 inches by reducing the cutoff time from 800 minutes to 600 minutes for a flood system that required 650 minutes for the water to reach the end of the field (figure 1).
Figure 1. Effect of reducing the irrigation water cutoff time on surface runoff. The check inflow rate was 950 gallons per minute. The time required for the water flowing across the field to reach the end of the field was 650 minutes. For the cutoff time of 600 minutes which was smaller than the time to reach the end of the field, surface runoff still occurred due to the amount of water ponded on the soil surface
Recovering and reusing the surface runoff should also be considered. This involves installing a tailwater ditch and pond at the end of the field, collecting the runoff in the pond, and then pumping the tailwater back to the head of the field. The pump tailwater should be used to irrigated border checks not be irrigated by the main irrigation water supply. Another approach is to collect the tailwater in a pond and then use the water to irrigate another field. For more information on tailwater return systems, see Reducing Runoff from Irrigated Lands: Tailwater Return Systems, ANR publication 8225, at http://anrcatalog.ucdavis.edu/SoilWaterIrrigation/8225.aspx
Surface runoff generally is not a problem with sprinkle irrigation. Thus, the sprinkle irrigation system must be managed to reduce deep percolation. This involves reducing the irrigation set time such that the amount of applied water reflects the amount of soil moisture depletion between irrigations.
In some areas, excessive salt accumulation in the root zone is a concern. Salinity problems can be aggravated by implementing measures that reduce deep percolation. Thus, periodic irrigations that leach the salt out of the root zone may be needed to control soil salinity.
Blaine Hanson, Irrigation and Drainage Specialist
Support for this web site was provided in part by a grant from the RREA (Renewable Resources Extension Act)