3.3.2. Crop Evapotranspiration and Biomass Growth Response

Figures 10 and 11 illustrate the cumulative crop evapotranspiration (ETc) and cumulative biomass of lettuce, respectively, under various irrigation scenarios simulated with AquaCrop calibrated for lettuce.

In varied RAW threshold irrigation scenarios, total simulated ETc ranged from 60 to 100 mm in site S1, and from 53 to 85 mm in site S2 (Figure 10a,c). The main reason for the higher ETc yield in site S1 is the higher adjusted transpiration characteristic of lettuce in sand soil as compared to loam soil. The simulated values of ETc fall within the range reported by Abdullah et al. [97] for lettuce, which varied from 43 mm to 285 mm in response to their different irrigation applications between 0 and 267 mm for open surface soil.

In varied field capacity threshold irrigation scenarios, simulated total crop evapotranspiration ranged from 77 to 205 mm in site S1, and from 83 to 211 mm in site S2 (Figure 10b,d). In both irrigation scenario classes, it was noted that while reducing irrigation events, crop evapotranspiration decreased simultaneously.

Figure 11 shows the response of biomass to the different irrigation scenarios. The varied RAW threshold irrigation scenarios (Figure 11a,c) resulted in biomass yield range from 0.88–1.77 ton ha−<sup>1</sup> at site S1, and 0.44–0.91 ton ha−<sup>1</sup> at site S2. By definition, biomass growth is closely related to crop evapotranspiration. Thus, the difference between biomass yields in the two experimental sites is due to the difference in the KcTr,x (coefficient for maximum crop transpiration) and CCx (maximum canopy cover) parameters between both sites.

**Figure 10.** Crop evapotranspiration accumulation responses to different scenarios: (**a**) varied RAW threshold irrigation scenarios at site S1 (sand soil); (**b**) varied field capacity threshold irrigation scenarios at site S1; (**c**) varied RAW threshold irrigation scenarios at site S2 (loam soil); (**d**) varied field capacity threshold irrigation scenarios at site S2. RAW is readily available water content, S0RAW-S200RAW refers to irrigation scenarios with irrigation at 0–200% of RAW for sand soil. L0RAW-L200RAW refers to irrigation scenarios with irrigation at 0–200% of RAW 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 11.** Biomass accumulation responses to different scenarios: (**a**) varied RAW threshold irrigation scenarios at site S1 (sand soil); (**b**) varied field capacity threshold irrigation scenarios at site S1; (**c**) varied RAW threshold irrigation scenarios at site S2 (loam soil); (**d**) varied field capacity threshold irrigation scenarios at site S2. RAW is readily available water content, S0RAW-S200RAW refers to irrigation scenarios with irrigation at 0–200% of RAW for sand soil. L0RAW-L200RAW refers to irrigation scenarios with irrigation at 0–200% of RAW 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.

As expected, in varied RAW threshold irrigation scenarios, the simulations maintained biomass yield at 1.77 ton ha−<sup>1</sup> at site S1 and 0.90 ton ha−<sup>1</sup> at site S2 in the full irrigation scenarios with allowable depletion from 0–100% of RAW (e.g., S0RAW to S100RAW for site S1 and L0RAW to L100RAW for site S2), that is due to no-water stress condition. As the water stress started below the RAW line [41], with available depletion from 120–200% of RAW thresholds, the biomass yields decreased up to 50% in the S200RAW (200% of RAW threshold) scenario at site S1 and 52% in L200RAW scenario at site S2.

In varied field capacity threshold irrigation scenarios (Figure 11b,d), biomass yields ranged from 0.85 to 1.77 ton ha−<sup>1</sup> at site S1, and 0.89 to 0.90 ton ha−<sup>1</sup> at site S2. At site S1, reducing deficit irrigation at 50% of field capacity (S50FC scenario), the biomass yield started to decrease with 22% and deficit irrigation at 40% of field capacity (S40FC scenario), biomass yields decreased up to 51% compared to full irrigation scenario (S100FC). For site 2, deficit irrigation up to 40% of field capacity (L40FC) did not affect biomass yield.
