*3.3. Impact of Irrigation Treatments on Near-Surface Soil Moisture Dynamics*

The near-surface soil volumetric water content dynamics across the irrigation treatments for both turfgrass species are shown in Figure 5. For tall fescue in 2018, the minimum and maximum moisture values were 17.8% and 43.8%, respectively. In 2019, the minimum and maximum moisture values were 21.1% and 36.0%, respectively. In 2018, the soil moisture values showed some reduction at the beginning of the trial across treatments. The soil moisture for the 83% ETo was noticeably lower than the other treatments. This difference was more pronounced for the 3 d wk−<sup>1</sup> treatments compared with the 2 d wk−<sup>1</sup> treatments. However, the soil moisture trends for the 129% ETo and 108% ETo treatments were similar; they increased (more pronounced for the 3 d wk−<sup>1</sup> frequency restriction) early in the trial and then stabilized with some decline toward the end of the experiment. In 2019, the greatest and smallest soil moisture values belonged to the highest and lowest irrigation treatments, respectively, for both frequency restrictions scenarios, as expected. The two lesser irrigation levels caused a gradual decline in soil moisture over time, which was more pronounced for the 7 d wk−<sup>1</sup> treatment (no frequency restriction).

For hybrid bermudagrass in 2018, the minimum and maximum moisture values were 15.3% and 35.7%, respectively. In 2019, the minimum and maximum moisture values were 11.5% and 33.4%, respectively. In 2018, soil moisture showed no substantial fluctuations over time, except for an initial decrease in all treatments at the beginning of the trial (more pronounced for the 3 d wk−<sup>1</sup> irrigation restriction treatment). After that, the soil moisture showed minor fluctuations across treatments. In 2019, the soil moisture showed a gradual decrease over time for both watering restriction scenarios. The greatest and smallest irrigation levels caused the highest and lowest near-surface soil moisture, respectively, as expected.

**Figure 5.** The near-surface (12 cm) volumetric soil water content (VWC) dynamics in 2018 and 2019 across the irrigation treatments (percentages of ETo). TF: tall fescue; B: hybrid bermudagrass; d/wk: days per week.

### *3.4. Turfgrass Water Response Function (TWRF)*

The TWRFs developed using the combined two years of data for tall fescue (Equation (6)) and hybrid bermudagrass (Equation (7)) species are as follows:

$$VR = 9.81 - 0.02(CET\_o) - 0.56(F) + 7.08 \times 10^{-6} \left(CET\_o^2\right) + 0.01(I \times CET\_o) + 1.06 \times 10^{-3} (I \times F) \tag{6}$$

$$VR = 9.11 - 0.01(\text{C}ET\_o) - 0.46(F) + 2.23 \times 10^{-6} \left(\text{C}ET\_o^2\right) + 0.01(I \times \text{C}ET\_o) + 1.57 \times 10^{-3} (I \times F) \tag{7}$$

where *VR* is the visual rating, *CETo* is the cumulative ETo over time (mm), *F* is the irrigation frequency restriction (days wk<sup>−</sup>1), and *I* is the irrigation level (ETo percentages).

Figure 6 shows the performance of the fitted TWRFs developed for tall fescue and hybrid bermudagrass species. Table 4 summarizes the performance statistics for the fitted TWRFs.

**Table 4.** Performance statistics for the turfgrass water response functions developed for tall fescue and hybrid bermudagrass species.


<sup>1</sup> *r* = correlation coefficient; RMSE: root means square error; MBE: mean bias error; MAE: mean absolute error.

The TWRFs estimated VR with acceptable accuracy for both species, as illustrated by well-scattered data points around the 1:1 line (Figure 6). The RMSE, MAE, and *r* values were equal to 0.64, 0.59, and 0.79, respectively, for tall fescue and 0.37, 0.47, and 0.75, respectively, for hybrid bermudagrass species. No systematic bias was observed for any of the TWRFs, given the negligible MBE values of 0.0005 and −0.0032 for tall fescue and hybrid bermudagrass species, respectively.

**Figure 6.** Performance of turfgrass water response functions developed for tall fescue (**a**) and hybrid bermudagrass (**b**) species using a combination of 2018 and 2019 experimental data. VR: visual rating values.

Figure 7 illustrates the estimated impact of multiple irrigation scenarios ranging from 50% to 100% ETo on turfgrass (VR) using TWRF. The long-term mean ETo data were obtained from the CIMIS station #39. The estimated period is from May to August. Hybrid bermudagrass maintained its quality above minimum acceptable value for irrigation levels more than 70% ETo. Tall fescue held its rate above the threshold for approximately 40 and 55 days for the 60% and 100% ETo irrigation scenarios, respectively.

**Figure 7.** Response of tall fescue and hybrid bermudagrass to multiple irrigation levels based on long-term mean ETo data obtained from a weather station located nearby the experimental site. VR: visual rating values, DAI: days after initiation.

## **4. Discussion**
