*2.1. Experimental Design and Growth Conditions*

Two experiments were conducted in two consecutive growing seasons: the winter season (WS) and spring season (SS) of 2018–2019 at the Educational farm, King Saud University, Riyadh, which is in an arid area. A meteorological station was set up to constantly measure weather parameters, namely, air temperature, relative humidity, solar radiation, evapotranspiration, and rainfall throughout the WS and SS (Figures 1 and 2). Field preparations were made, including plowing, grading, and leveling. Then, the irrigation layout was implemented according to the experimental design, as shown in Figure 3.

**Figure 1.** Daily climate parameters in the winter and spring of 2018–2019 during the squash growing seasons: (**a**) Daily maximum and minimum temperature, (**b**) solar radiation, (**c**) relative humidity, and (**d**) wind speed.

**Figure 2.** Seasonal reference evapotranspiration (ETo) at the experimental field throughout the winter and spring growing seasons.

**Figure 3.** Schematic diagram of the experimental fields under mulch treatments (black mulch-BM, transparent mulch-WM and non-mulch- NM) and irrigation treatments (full irrigation-FI, partial root drying with 50% of evapotranspiration-PRD50, partial root drying with 70% of evapotranspiration-PRD70).

Soil physical and chemical analyses were conducted by taking soil samples every 0.1 m down to a depth of 0.5 m, as shown in Table 1. Soil physical parameters were determined, including the field capacity (FC), wilting point (WP), saturated hydraulic conductivity (ks), bulk density (ρb), and soil saturation (S). The experiment was conducted in a split-plot design (Figure 3). Treatments were allocated three levels of irrigation and three mulching treatments. The mulching treatments, transparent mulch (WM), black mulch (BM), and without mulch (NM), were assigned as main plots, and the irrigation treatments, FI with 100% of crop evapotranspiration (ETc), irrigation with 70% of ETc (PRD70), and irrigation with 50% of ETc (PRD50%) were allocated in subplots. The experimental plot area was 13 m in length by 0.70 m in row width (9.1 m2). A total of 27 plots were made by replicating each treatment three times.


**Table 1.** Soil physical and chemical properties.

FC: field capacity; WP: wilting point; ks: saturated hydraulic conductivity; S: soil saturation; ρb: bulk density.

#### *2.2. Applied Irrigation Water*

Drip pipes were buried 15 cm below the soil surface and had 26 inline emitters, which were spaced at intervals of 0.5 m, and had a discharge rate of8Lh−<sup>1</sup> at an operating pressure of 100 kPa. In the FI experimental plot, one lateral was installed adjacent to the crop rows, while in the PRD treatments, two laterals with two control valves were installed 0.4 m apart in each crop row. Irrigation in the PRD treatment was shifted between the two sides of plants every five days.

A weather station (WS-PRO LT Weather Station, Rain Bird) was launched in the experiment field. Daily reference evapotranspiration (*ETo*) was calculated from daily climate data according to Allen et al. [39] using Equation (1),

$$ET\_O = \frac{0.408\Delta (R\_H - G) + \gamma \frac{900}{T + 273} \mu\_2 \left(\varepsilon\_S - \varepsilon\_a\right)}{\Delta + \gamma (1 + 0.34\mu\_2)}\tag{1}$$

where *ETo* is reference crop evapotranspiration (mm day−1), *Rn* is net radiation at the crop surface (MJ m−<sup>2</sup> day−1), *G* is soil heat flux density (MJ m−<sup>2</sup> day−1), *T* is mean daily temperature at 2 m height (◦C), *u*<sup>2</sup> is wind speed at 2 m height (m s−1), *es* is saturation vapor pressure (kPa), *ea* is actual vapor pressure (kPa), Δ is the slope of the vapor pressure curve (kPa ◦C<sup>−</sup>1), and *γ* is a psychrometric constant (kPa ◦C<sup>−</sup>1).

Irrigation was conducted every day using an automatic controller (ESP-LXME controllers, Rain Bird Corporation, Tucson, AZ, USA), which was connected with a central control (IQ v2.0, Rain Bird Corporation, Azusa, CA, USA). The IQ-software monitored and adjusted watering schedules for the controller and site from a compatible Windows PC, which was connected with the weather station to schedule irrigation automatically based on *ETc.* The crop water requirements (*ETc*) were estimated using Equation (2),

$$ET\_{\mathbb{C}} = ET\_{\mathbb{O}} \times K\_{\mathbb{C}} \tag{2}$$

where *ETc* is the crop water requirement (crop evapotranspiration; mm day−1), and *Kc* is the crop coefficient. The crop growth stages, initial, development, mid, and late stage, were 20, 30, 25, and 15 days, respectively, and *Kc* of 0.6, 1.0, and 0.75 were used for the initial, mid, and late stage, respectively [39]. Moreover, the values of *Kc* were adjusted according to Allen et al. [39] based on the relative humidity, wind speed at 2 m, percentage of wetted soil surface in the experimental field using Equations (3) and (4),

$$K\_{\rm c\,ini} = \,\, f\_w \, K\_{\rm c\,ini}(\text{Table}) \tag{3}$$

where *Kc ini* is the adjusted value of initial *Kc*, *fw* is the fraction of surfaced wetted by irrigation, and *Kc ini(Table)* is the value of initial *Kc* from Allen et al. [39].

$$K\_{\text{c.mid } OR\text{ end}} = K\_{\text{c.mid } OR\text{ end}(Table)} + [0.04(\mu\_2 - 2) - 0.004(RH\_{\text{min}} - 45)] \left(\frac{h}{3}\right)^{0.5} \tag{4}$$

where *Kc mid OR end* is the adjusted values of mid *Kc* or end *Kc*, *Kc mid OR end(Table)* is the value of of mid *Kc* or end *Kc* from Allen et al. [39], *RHmin* is the mean value for daily minimum relative humidity during the mid-season growth stage or the end-season stage [%], and *h* is mean plant height during the mid-season stage or the end-season stage [m].

At 20 days after sowing (DAS), PRD70 and PRD50 were applied until harvesting.

#### *2.3. Plant Management*

Two seeds of zucchini squash, *Cucurbita pepo* L., were hand-sown 10 cm apart on both sides of the central line of the planting rows, and there was 0.5 m between plants within a row. Seeds were planted on November 18, 2018 and terminated on 15 February 2019 in the WS, and in the SS, seeds were planted on 23 March 2019 and terminated on 20 June 2019. Chemical fertilization was applied at the recommended rate for squash production in this area: 5.1 g N/plant, 5.1 g P2O5/plant, 16.8 g K2O/plant, 37.5 g Ca(NO3)2/plant, 28.5 mL H3PO4/plant, 14.52 mL HNO3/plant, and 1.41 g humic acid/plant. Pest management and disease control were conducted based on local squash protection procedures.

#### *2.4. Soil Moisture Measurements*

Capacitance probes (EnviroSCAN®, Sentek Sensor Technologies, Stepney, Australia) were used to measure soil moisture. Enviroscan probes were used to continuously monitor volumetric soil water content (*θv*) down to a depth of 0.5 m in the root zone of each irrigation treatment. Probes were installed vertically at a distance of 0.10 m from laterals and had five sensors at 0.10 m intervals. Soil frequencies (*Fs*) were converted into scaled frequencies (*Sf*) according to Equation (5) following Buss [40],

$$S\_f = \frac{F\_A - F\_\text{S}}{F\_A - F\_\text{W}} \tag{5}$$

where *FA* is the sensor reading in the air, *FS* is the sensor reading in the soil, and *FW* is the sensor reading in non-saline water. According to Vera et al. [41], *θ<sup>v</sup>* can be calculated using Equation (6),

$$\theta\_v = \left(\frac{S\_f - C}{A}\right)^{\frac{1}{p}} \tag{6}$$

where *A* = 0.1957, *b* = 0.404, and *C* = 0.02852. The determination coefficient value provided using standard default calibration was 0.97. One EnviroScan device per plot was installed in single lateral treatments (FI), while two EnviroScan devices were installed 0.6 m apart in the diagonal direction in the two lateral treatments: PRD70 and PRD50 (Figure 3).
