*3.2. Model Parameterisation and Evaluation*

The primary crop variables calibrated for daily lettuce growth were canopy cover and biomass, with the daily soil moisture simulated by AquaCrop, by adapting available physical soil data.

Table 5 presents the adjusted model parameters for canopy cover and biomass curve simulation of lettuce growth. The time to recovery of transplant, the time to reach the maximum canopy cover, the initial canopy cover (CCo), the maximum canopy cover growth coefficient (CCx), the coefficient for maximum crop transpiration (KcTr,x), and the normalised biomass water productivity (WP\*) were mainly calibrated.



WP\* was adjusted at 16 gm−<sup>2</sup> for both sites, within the recommended range. KcTr,x was adjusted at 0.65 and 0.5 for site S1 and S2, respectively. These adjusted KcTr,x are lower than crop coefficient for the mid-season (Kcb,mid = 1) proposed by FAO-56. The difference between the values proposed by FAO-56 and the adjusted KcTr,x values is due to the fact that the FAO crop coefficients were obtained for specific agroclimatic conditions, which are different from the conditions of this study [78].

In addition, KcTr,x is a major requisite for estimating crop transpiration and biomass. The low adjusted value of this parameter resulted in low simulated biomass yields to fit to measured values.

High temperature stress observed during the experiment could be the reason for the low observed lettuce biomass production [12]. This observation leads to a recommendation for further development of a heat stress factor in relation to canopy cover and biomass simulations for lettuce.

The minimum root depth cannot be adjusted under 0.1 m, while the root development of lettuce was under this limit. Thus, root development in the model requires further modification [91].

The crop growth simulation of canopy cover and biomass fitted the observed data well (Figure 6). The statistical values for model evaluation in Table 6 were satisfactory, resulting in *R*<sup>2</sup> = 0.99, RMSE < 0.8%, N < 4.6 for canopy cover, and *<sup>R</sup>*<sup>2</sup> > 0.98, RMSE < 0.01 ton ha<sup>−</sup>1,N< −0.07 for biomass. Thus, the model has ability to simulate well the growth of lettuce in both soil types at the two experimental sites.


**Table 6.** Statistical evaluation of model simulation.

The measured and simulated soil moisture, at both soil depths of 5 and 15 cm in both sites, also matched well (Figure 7). The soil moisture simulation resulted in good accuracy with low RMSE of 0.18 and 0.14 m3 m−<sup>3</sup> at depths of 5 and 15 cm, respectively, at site S1, and 0.05 and 0.06 m<sup>3</sup> m−<sup>3</sup> at depths of 5 and 15 cm, respectively, at site S2.

**Figure 7.** Simulated soil moisture and observed soil moisture data measured at depths of 5 cm (H1) and 15 cm (H2) using soil moisture sensor 10HS and soil potential MPS-2: (**a**) soil moisture at site S1; (**b**) soil potential at site S1; (**c**) soil moisture at site S2; (**d**) soil potential at site S2. DAP is day after planting, Sim SM is simulated soil moisture, Obs SM is observed soil moisture, IRRI is irrigation, h is soil potential.
