3.3.3. Relationship between Water Productivity and Irrigation Scenarios

The responses of biomass yield and irrigation water productivity to irrigation depths in various scenarios are presented in Figure 12. Simulated water productivity of varied RAW threshold irrigation scenarios ranged from 1.5 to 2.1 kg m−<sup>3</sup> for site S1 and 0.9 to 1.4 kg m−<sup>3</sup> for site S2. In varied field capacity irrigation scenarios, simulated irrigation water productivity (IWP) ranged from 0.8 to 1.36 kg m−<sup>3</sup> for site S1 and 0.43–1.08 kg m−<sup>3</sup> for site S2. The simulated irrigation water productivity results are comparable with other studies found in the literature. For instance, Gallardo et al. [98] found a measured IWP for lettuce dry matter of 1.86 kg m<sup>−</sup>3.

**Figure 12.** Comparison of biomass and water productivity response (IWP) to different irrigation scenarios. RAW is readily available water content. S0RAW-S200RAW refers to irrigation at 0–200% of RAW threshold irrigation scenarios for sand soil. L0RAW-L200RAW refers to refers to irrigation at 0–200% of RAW threshold irrigation scenarios for loam soil. S40FC-S100FC refers to deficit irrigation at 40–100% of field capacity for sand soil. L40FC-L100FC refers to deficit irrigation at 40–100% of field capacity for loam soil.

Figure 13 shows the relationship curves of biomass yield and irrigation water productivity response to irrigation scenarios. As expected, irrigation water productivity curve response to irrigation depths had parabolic relationships for both soil types in varied RAW threshold irrigation scenarios. Increasing water use efficiency can be enhanced by decreasing the irrigation to an optimum point. The optimum point, which resulted in 22% water saving for site S1, was found at the scenario with depletion of 150% of RAW (S150RAW), resulting in the irrigation water productivity = 2.07 kg m<sup>−</sup>3, irrigation depth = 81 mm, and biomass yield = 1.68 ton ha−1. For site S2, the optimum irrigation water productivity was at 130% of RAW scenario (L130RAW), resulting in irrigation water productivity = 1.42 kg m−3, irrigation depth = 60 mm, and biomass yield = 0.85 ton ha<sup>−</sup>1.

**Figure 13.** Relationship between biomass and irrigation water productivity responses to different scenarios: (**a**) at site S1 (sand soil) and (**b**) at site S2 (loam soil). I is irrigation, B is biomass, IWP is irrigation water productivity, RAW-IS is varied readily available water content threshold irrigation scenarios, FC-IS is varied field capacity threshold irrigation scenarios.

In varied field capacity threshold irrigation scenarios, for site S1, the optimum irrigation water productivity with 39% water saving was found at deficit irrigation at 60% of field capacity (S60FC) with irrigation water productivity = 1.36 kg m−3, irrigation depth = 130 mm, and biomass yield = 1.77 ton ha−1. For site S2, the optimum water productivity resulted in 60% water saving, which was found at deficit irrigation at 40% of field capacity (L40FC scenario) with irrigation water productivity = 1.08 kg m<sup>−</sup>3, irrigation depth = 83 mm, and biomass yield = 0.89 ton ha−1.

The varied RAW threshold irrigation scenarios resulted in higher simulated higher irrigation water productivity than the varied field capacity threshold scenarios in this study. Overall, deficit irrigation simulation scenarios in both irrigation scenario classes can provide a remarkable improvement in irrigation water productivity for water saving strategies.
