By Bob Hutmacher, Richard Mead and Peter Shouse

Alfalfa grown for forage is a major crop in many areas of the western United States. In the arid and semi–arid west, irrigation is required to obtain economic alfalfa yields. Because alfalfa is a perennial crop with a potentially long growing season, it can use a substantial amount of water. Alfalfa production in Imperial County, CA, has been valued at more than $170 million dollars for the past several years, with over 250.000 acres in production.

Increasing competition for limited water supplies during California drought periods have focused concern on crops that are commonly perceived as "high water users". Alfalfa certainly fits this category. Numerous reports estimate that alfalfa has an annual evapotranspiration in desert regions at or in excess of 6.5 feet (1900 mm). Considering the large forage market in California and the amount of water used for alfalfa irrigation in the Imperial and Palo Verde Valley areas (1.5 million acre-feet)) and in California (estimated total of four million acre-feet), there certainly is an incentive to investigate improvements in water use efficiency in alfalfa production.

Through funding from several organizations in California, the USDA–ARS in 1990 began a five–year evaluation of alfalfa at the USDA–ARS Irrigated Desert Research Station in Brawley, CA. The purpose of the study was to examine alfalfa water requirements and the long-term influence of irrigation management on soil accumulations of salts. This experiment focused on:
1. the comparison of crop responses, irrigation water requirements and salinity accumulations as affected by subsurface drip versus furrow irrigation; and
2. the influence of two drip lateral spacings on the above characteristics. The soil at the experiment site is a heavy clay loam with low surface infiltration rates. All plots were 600 feet in length.

In bed-planted alfalfa, subsurface drip lateral spacing of 40–inch and SO–inch installed at an average depth of 16 inches below the bed centers was evaluated. Emitter spacing was 40 inches. Bed-planted alfalfa is commonly used in high temperature areas. because high temperatures in combination with flooded conditi'ons can cause crown and root damage.

During the first one and one-half year operation of the experiment, approximately 22 percent higher yields were obtained in the drip plots with 94 percent of the water application given to furrow plots. Problems with surface soil wetting were noted in both drip treatments, even with the drip laterals placed at a 16–inch depth during Phase I of the project (1991-1992).

These problems resulted in the need to reduce drip water applications during a "dry down" period during each harvest cycle. This allowed for harvest equipment traffic, but limited soil compaction and plant damage.

To provide an alternative method to deal with surface soil wetting problems, the alfalfa crop was terminated in late 1992 in all drip and furrow plots. The drip system was replaced at a 25– to 28–inch depth, an interim sudangrass crop grown in spring and summer of 1993, and alfalfa replanted in 1993. Phase II of the experiment was alfalfa grown in 1994 and 1995.

During this second phase, applied water and evapotranspiration were within five per-cent of each other in the drip– and furrow–irrigated plots. However. yields averaged 26 to 35 percent higher in SDI plots. Problems with surface soil wet areas were eliminated with the deeper drip lateral installations. Even during high water application periods in the summer (as much as 0.6 inches per day), surface wet areas did not develop. In both furrow and drip plots, root densities were greatest and most water uptake occurred in the upper 3 ½feet of soil.

Accumulations of salt within the root–zone of alfalfa can be a significant problem with the 800 to 900 ppm total salts in the water from the Colorado River. Locations and amounts of salt accumulation across the beds in Phase I of this study depended on lateral spacing and whether or not the wetted patterns of lateral water movement from adjacent beds met.

During Phase I of this study, some areas of the surface six inches of soil in the drip plots had high salinity levels. This was from the movement of water and salts up to the soil surface layers. Even these high concentrations of salts did not result in stunted plants, perhaps due to limited root activity in the upper parts of the soil surface. However, these surface concentrations do represent a potential threat if flushed down into the active rootzone by rain, or if too small an amount of water is applied for leaching.

Much of the within-bed variation and stratification in salinity levels developed during Phase I of the studv was eliminated with the large pre–plant water application (eight to ten inches of water) made in the spring of 1993. This was after installation of the new drip tubing and prior to sudangrass planting).

Soil samples were taken in fall 1993 and again during January 1995 to evaluate salinity distribution as affected by location within the bed and irrigation method. In general, furrow irrigation tends to produce higher salt accumulations in the center of the beds. With SDI, salts are directed toward the outer perimeter of the soil volume wetted by the emitters.

Results of salt accumulation data to date indicate that with Colorado River water and high evapotranspiration (ET) conditions of the Imperial Valley, irrigation for leaching purposes should be considered after two to three years. This will ensure that salinity is kept at non-limiting levels within the alfalfa rootzone.

In the Imperial Valley and under bed production conditions, the most effective leaching method is probably sprinklers. Sprinklers direct salts primarily down rather than also across the beds as occurs with furrow irrigation. The drip systems can effectively move the salts down below the depth of the emitters, but cannot effectively leach the more shallow soil zones.

No problems with excessive or low emitter flow rates have been detected to date in the second phase of the experiment. In addition, there was no evidence of root intrusion into the drip lines during Phase II. The absence of root intrusion in this study is under the following operating conditions:
1. use of drip tubing with turbulent-flow emitters and thick walls when compared with tapes;br 2. continuous injection of a five to seven percent phosphoric acid solution to maintain a concentration of ten to 20 mg P/L: and
3. weekly injection of chlorine (free chlorine level of five to ten mg/l) and N-phuric acid (to bring down the pH to about 3.5) for a duration of one to two hours per week.

There is no strong evidence that root intrusion would occur if the chemical treatments were not used. However, from the standpoint of prevention of chemical precipitate clogging, acid treatments of one type or another were considered necessary.

Choices of alternative drip tubing or tapes with closer lateral or emitter spacing might reduce the problems even at more shallow depths through reductions in localized water application rates. If a lower cost tape is used in place of these tubing materials, closer lateral spacing may be economically feasible.

Increases in water use efficiency (on the order of 20 percent plus) with SDI in this experiment largely came from increases in yield, not from reductions in applied water or ET. SDI allowed continued water applications during harvest periods. resulting in faster regrowth and larger yields than in furrow plots.

Short run lengths (600 feet) also resulted in relatively high water application uniformity in furrow plots compared with what would be expected with longer runs. In many soils and cultural conditions less conducive to uniform water applications, there may be greater opportunities to also save water with SDI systems through reductions in deep percolation losses.

In forage market areas where land values and water costs are low, even 20 percent potential yield increases may not warrant conversion to SDI. However, where water costs are increasing or total water or land availability for crop production are serious limitations, SDI has promise as a viable alternative to other irrigation methods. Growers in New Mexico, Texas, Nevada and California are already testing a range of drip tubing tapes, lateral spacings and lateral depths on forage crops in regions where water availability is the primary concern.

Bob Hutmacher, plant physiologist, and Richard Mead. soil scientist. are both with the USDA-ARS. Water Management Research Lab, located in Fresno, PA. Peter Shouse is a soil scientist at the USDA–ARS Salinity Laboratory. located in Riverside, CA.

Research was funded by the following organizations: Imperial Valley Conservation Research Center Committee, Imperial Irrigation District and the Metropolitan Water District of Southern California.