2.5.1. Varied RAW Threshold Irrigation Scenarios

Figure 4 presents the calculation process of varied RAW threshold irrigation scenarios.

**Figure 4.** Schematic representation of the crop response to varied RAW threshold irrigation scenarios simulated by AquaCrop (adjusted from [77]). RAW is readily available water content, TAW is total available water content, CC is the simulated canopy cover, CCo is initial canopy cover size, CGC is canopy growth coefficient in fraction per growing degree day (GDD), Kssto is the water stress for stomatal closure, KcTr is the crop transpiration coefficient (determined by CC and KcTr,x at maximum canopy cover), ETo is the reference evapotranspiration, Ksb is the stress coefficient for low-temperature effects on biomass production, fWP is the adjustment factor to account for differences in chemical composition of the vegetative biomass and harvestable organs, WP\* is the normalised water productivity.

These irrigation scenarios applied irrigation scheduling based on soil moisture depletion [86] by applying readily available water depletion in the default option in AquaCrop. The time and irrigation dose were calculated with the criteria below:


$$\text{ID} = \text{AD} \times \text{RAW} \tag{8}$$

where ID is irrigation depth (mm), RAW = p TAW = p 1000(FC − PWP)Zr, p is soil water depletion threshold, set to 0.3 for lettuce recommended by [69], and Zr is root depth (m). TAW is the amount of water that a crop can extract from its root zone [88]. FC is field capacity, that is, the amount of water well-drained soil should hold against gravitational forces (m3 m−3) [88]. PWP is permanent wilting point, referring to soil water content when a plant fails to recover its turgidity on watering (m<sup>3</sup> m<sup>−</sup>3) [88]. RAW is readily available soil water, referring to the fraction of TAW that a crop can extract from the root zone without suffering water stress [88]. AD is allowable depletion, defined as the percentage of RAW that can be depleted before irrigation water has to be applied.

Full irrigation scenarios with varied RAW thresholds were simulated by selecting allowable depletion levels at 0, 50, 80, 100% in AquaCrop, that avoid drought stress during the growth stage [41]. The irrigation schedule is generated by selecting a so-called "time" and "depth" criterion, with "back

to field capacity" and "allowable depletion", respectively. In other words, the different full irrigation scenarios result in decreasing irrigation frequency.

Deficit irrigation scenarios with varied RAW thresholds were similar to the full irrigation scenario criteria, but applied allowable depletion levels at 120, 130, 150, 180, and 200%. These levels result in drought stress during the growing stage, since soil moisture can decrease to a level below RAW before an irrigation event is triggered [41].

2.5.2. Varied Field Capacity Threshold Irrigation Scenarios

Figure 5 illustrated concept of the varied field capacity threshold irrigation scenarios.

**Figure 5.** Schematic illustration of the soil water reservoir concepts of varied irrigation depth under field capacity irrigation scenarios (adjusted from [89]). FC is field capacity, full ID is full irrigation depth.

The full irrigation scenario, based on a fixed irrigation frequency maintained the soil moisture in the root zone at field capacity on a daily basis, since the literature claims this is the optimal status to maximise lettuce yield [90]. The irrigation schedule was generated with a fixed time interval (daily) (time criteria) and refill to field capacity (depth criteria).

Deficit irrigation scenarios with varied field capacity threshold reduce the irrigation dose below the dose at field capacity but keeping the same irrigation frequency, as in full irrigation scenario. Daily generated irrigation doses obtained in full irrigation scenario were reduced by 70, 60, 50, and 40%.

Irrigation water productivity (IWP) was used to evaluate the irrigation scenarios for efficient irrigation water use [31,91]. IWP is the ratio between the yield and the irrigation water use [31].

$$\text{IWP} = \frac{\text{Y}}{\text{I}} \tag{9}$$

where IWP is irrigation water productivity (kg m<sup>−</sup>3), Y is simulated yield (kg ha−1) and interest yield in this study is biomass, and I is irrigation water use (mm).

The adjusted crop parameters obtained from the parameterisation process were used in the scenario simulation under the same weather conditions, using no soil surface cover in model field management, and no ground water at bottom soil profile boundary condition.
