*3.1. Plant Growth and Soil Moisture Status*

Figure 6 shows both the lettuce growth measurement and simulation by AquaCrop. Biomass accumulated at a very low rate during the first two weeks of the growing season, and increased sharply in the final week. This trend accords with results obtained by Gallardo et al. [73].

The measured canopy cover and biomass yields were 34% and 0.11 ton ha<sup>−</sup>1, respectively, at site S1 with sand soil, and 18.5% and 0.11 ton ha<sup>−</sup>1, respectively, at site S2, which has loam soil. The measured

results are comparable with Fazilah et al. [92], who found observed canopy cover of 33% and biomass yields of 0.22 ton ha−<sup>1</sup> for lettuce under similar tropical conditions. Zhang et al. [93] found higher measured biomass for lettuce with a range of 0.33 to 0.63 ton ha−<sup>1</sup> under lower temperatures of 20–25 ◦C. Thus, high day temperatures above 23 ◦C often limit lettuce production [94]. Optimum growth for lettuce occurs between 15–20 ◦C [12]. Unfavourable weather conditions, of high average temperature 33/25 ◦C (day/night) during the experiment, can be the reason of the low measured biomass yields for this study.

**Figure 6.** Observed (Obs) data and simulation (Sim) of lettuce growth of AquaCrop: (**a**) canopy cover at site S1 (CC-S) (sand soil) and site S2 (CC-L) (loam soil); (**b**) aboveground biomass at site S1 (B-S) and site S2 (B-S). The error bars were based on 10 biomass samples, except the last observed, which was based on 60 samples at harvest time. Sim CC-S is simulated canopy cover at site S1, Sim CC-L is simulated canopy cover at site S2, Obs CC-S is observed canopy cover at site S1, Obs CC-L is observed canopy cover at site S2, Sim B-S is simulated biomass at site S1, Sim B-L is simulated biomass at site S2, Obs B-S is observed biomass at site S1, Obs B-L is observed biomass at site S2.
