## 4 Discussion
### Irrigation Strategies to Reduce Water Use  
Deficit irrigation strategies in alfalfa are primarily feasible due to the seasonal yield pattern (Figures 7 and 14). The cumulative percentage of alfalfa yields at the sites A-D over three growing seasons suggested that approximately 73-74% of desert alfalfa production occurred by mid-July (Figure 14). The fact that substantial yields are achieved by mid-summer enables decisions to be made to either water fully, or conserve water for the remainder of the season. Deficit irrigation strategies could affect production over three harvest cycles of late July through late September-mid October, the timeframe that desert alfalfa typically produces about 21-22% of seasonal yields. This provides some guidance to estimate yield reductions should more severe deficit irrigation strategies be required. Figure 13: Seasonal trends of alfalfa crop coefficient values in the low desert. The average harvest cycle crop coefficient values four alfalfa sites of A–D over three years (a total of 96 harvest cycles). The derived polynomial regression may provide the most accurate crop coefficient values for specific harvest cycles in different times of year to estimate alfalfa crop water needs over the crop season in the region. A combination of triage (eliminating non-productive fields), and partial-season production is recommended when faced with curtailment of water supplies. In this low desert environment, the MDI strategies are likely to be the most viable approach of deficit irrigation of alfalfa fields. The findings of the deficit irrigation treatments suggested an average of 4-6% seasonal yield reductions resulting in up to 40% less water applied (0.124 (ha-m) ha\({}^{-1}\)) than standard farmer practice (Figures 11 and 12). Severe deficit irrigation strategy with no irrigation events over a three-month summer period of July-September, may cause greater yield loss. A recent three-year study at the University of California Desert Research and Extension Center (alfalfa trials under SDI) suggested that cutting 75% crop water demands during the summer (a 90 day period) may reduce an average of 13% yields and about 9% plant stand densities [37]. Yield and plant stand losses, soil water depletion, and salt build-up due to the summer water deficit were meaningful at this research trial. Salinity build-up is less likely to occur if the field was a surface irrigated field. Considering the seasonal yield pattern (Figure 14), a severe water deficit (likely 0.234-0.246 (ha-m) ha\({}^{-1}\) water cuts during the summer period) may cause 19-21% seasonal yield losses (nearly 5.1 Mg ha\({}^{-1}\)) in the desert region. This prediction is in a good agreement with the results of a one-year study conducted in the Palo Verde Valley in 2010 [38]. It is important to state, however, that a portion of the water saved may need to be applied in the fall-winter to refill the soil profile and maintain salts accumulated in the soil. The amount of this excessive water depends on soil type and condition, and contribution to the crop water needs from the water table (as capillary movement of water upward from the water table). This amount could be nearly 25% (or even more) of the total water cuts. Although policy is not always clear on this, salinity management with good drainage and excess water applications in desert conditions is a key beneficial use to maintain soil health and crop productivity in desert environments. The developed alfalfa crop water use-production function across the experimental fields (Figure 15) indicates that it takes about 59 mm of ETa to make 1 Mg ha\({}^{-1}\) of alfalfa dry matter (range 1433 \(\leq\) seasonal ETa \(\leq\) 1595). An earlier study found that it takes about 53 mm of ETc (crop evapotranspiration under standard conditions) to make 1 Mg ha\({}^{-1}\) of alfalfa DM yield in the San Joaquin Valley, California [39]. Figure 14: Cumulative percentage of alfalfa yield over the season in the low desert. The three-year yield data (2019–2021) from sites A–D were adopted for this analysis.