*4.2. Canopy Temperature and CWSI*

The reported results in our companion paper [23] show that when water conservation is concerned, hybrid bermudagrass is the superior species since it can sustain its quality better than tall fescue when irrigation is limited, as expected. The tall fescue treatments received more water (equal to roughly 20% ETo) than the hybrid bermudagrass and still could not continuously maintain the same VR values during the summer months in central California. Culpepper et al. [36] conducted two greenhouse trials in Texas, USA, to compare the response of bermudagrass, buffalograss (*Buchloe dactyloides* (Nutt.) Engelm.), and tall fescue to water deficit. They reported that tall fescue (a C3 cool-season turfgrass) under heat and drought stress showed the most rapid decline in quality and photosynthetic rates compared to the C4 warm-season grasses, demonstrating the benefits of the C4 versus C3 photosynthetic pathway.

On the other hand, the result of this study showed that the mean canopy temperature was higher for hybrid bermudagrass than tall fescue across the treatments in both years. On average, in both years, the hybrid bermudagrass plots were 1.6 ◦C warmer than tall fescue plots when they received the same irrigation treatments of 83–84% ETo. Further comparative studies are needed to evaluate the potential water conservation and cooling benefits of irrigated warm-season and cool-season species versus alternative groundcover species in California.

For the same irrigation levels, increasing irrigation frequency (number of watering days) slightly (0.6 ◦C on average) but consistently decreased canopy temperature without compromising the turfgrass quality. We attribute this to a more pronounced evaporative cooling associated with higher irrigation frequencies while minimizing runoff and deep percolation. Consequently, as suggested in the companion paper [23], when ET-based smart controllers are used, we recommend no watering restrictions so the controller can adjust the watering days based on the actual weather conditions and evaporative demand. Then, a minimum deficit threshold should be programmed to avoid unnecessary evaporative loss due to light irrigation applications.

The correlation between *dt* and VPD for well-watered treatments (lower baselines of CWSI) was moderate in this study. We observed *r*-values of 0.64 and 0.88 for tall fescue and 0.69 and 0.64 for hybrid bermudagrass in two years of our research. Jalali-Farahani et al. [37] reported *r* = 0.87 between VPD and *dt* for well-watered hybrid bermudagrass in Tucson, Arizona. Payero et al. [38] observed *r*-values ranging from 0.92 to 0.95 between *dt* and VPD at different solar radiation levels for tall fescue grass at Kimberly, Idaho. Taghvaiean et al. [14] conducted a field study in Berthoud, Colorado, on multiple turfgrass species and mentioned that the effect of solar radiation is negligible when *dt* data are collected under the clear sky and close to solar noon. On the other hand, multiple studies reported a high correlation between CWSI and solar radiation [37,38]. We leave it to future studies to determine the potential improvement in hybrid bermudagrass and tall fescue CWSI lower baselines developed in this study when additional weather parameters such as solar radiation are considered.

The CWSI values are expected to vary from 0 to 1, representing no transpiration and maximum transpiration rates, respectively. In our study, the CWSI values ranged from −0.34 to 0.56 for hybrid bermudagrass and −0.59 to 0.29 for tall fescue. Jalali-Farahani [37] reported violations of the theoretical range one-fourth of the time. In addition, Al-Faraj et al. [39] studied the CWSI of 'Falcon' tall fescue in a controlled environment and concluded that there is no easy way to ensure that the empirical CWSI consistently stays within the theoretical range of zero (no stress) and one (severe water stress).

In our study, the mean ± SD CWSI values were 0 ± 0.13 and 0 ± 0.10 for wellwatered tall fescue (129% ETo) and hybrid bermudagrass (101% ETo), respectively. Jalali-Farahani [37] reported a mean CWSI value of −0.02 and an SD of 0.28 for well-watered hybrid bermudagrass. In our study, for the 65% ETo treatment, the mean ± SD hybrid bermudagrass CWSI was 0.1 ± 0.12. Somewhat similar to our results, Emekli et al. [15] reported 0.09 and 0.10 as seasonal mean CWSI of hybrid bermudagrass under 100% pan evaporation (≈75% ETo) and 75% pan evaporation (≈56% ETo) irrigation treatments in Antalya, Turkey. They reported a good relationship between VR and CWSI and suggested a CWSI of approximately 0.1 to maintain hybrid bermudagrass quality. However, we can not recommend a CWSI threshold to maintain turf quality in the acceptable range because of the high variability of CWSI values over time and their low correlation with VR values (not reported in this study). The canopy temperature and CWSI dynamics were very similar in this study, and CWSI did not provide much extra information regarding the dynamic impact of irrigation regimes on turfgrass quality.
