*2.2. Irrigation Trials*

Two adjacent hybrid bermudagrass ('Latitude 36' *Cynodon dactylon* (L.) Pers. × *C. transvaalensis* Burtt-Davy) and tall fescue (A blend of 'PennRK4', 'Rebel XLR', + 'Firecracker SLS' *Schedonorus arundinaceus* (Schreb.) Dumort.) irrigation trials were arranged in a factorial randomized complete block design with repeated measures of canopy temperature and NDVI. Each trial consisted of 18 plots (3.7 m × 3.7 m) forming three blocks (replications) to impose six irrigation treatments. To minimize evaporative loss and wind drift, irrigation was done overnight and early morning. In addition, the smart controller performed an automatic run/soak schedule to eliminate runoff and provide enough soak time. All three replications for each treatment were irrigated at the same time by wiring to the same zone on the controller. Each plot was equipped with a TORO 252 Series solenoid valve (Toro Co., Bloomington, MN, USA) supplying water to four Toro O-T-12-QP corner-pop-up 6 sprinkler heads (152 mm tall). The sprinkler nozzles had an operating pressure range and flow rate of 276–517 kPa and 0.02–9.08 L min−1, respectively. To achieve steady water application, the sprinklers were equipped with factory-installed pressure-compensating discs (Toro Co., Bloomington, MN, USA).

The irrigation treatments consisted of three ETo-based irrigation levels and two irrigation frequencies (Table 1). A Weathermatic Smartline (SL) 4800 controller (Telsco Industries, Inc., Garland, TX, USA) was used to control irrigation treatments and schedule irrigation throughout the study autonomously. The controller used an onsite temperature sensor and latitude-based solar radiation information to calculate ETo using the Hargreaves and Samani equation [30]. Irrigation efficiency (i.e., low half distribution uniformity) of 0.78 and irrigation precipitation rate of 23 mm h−<sup>1</sup> was calculated using a catch-cans test performed before conducting the trial in year 1. Irrigation was non-limiting to ensure actively growing, non-stress turfgrass prior to initiating irrigation treatments. The experiment started on 4 May 2018, and data collection ended on 11 September 2018. All plots were switched back to the uniform non-limiting irrigation for recovery before starting the trial on 22 June 2019. On 26 August 2019, the main irrigation pipe broke and flooded the field, forcing the research team to terminate the trial. More information about the irrigation system characteristics and establishment of the plots is provided in the companion paper [23].

**Table 1.** Irrigation treatments throughout the 2-year tall fescue and hybrid bermudagrass irrigation research experiments at the University of California Kearney Research and Extension Center.

2018 Trial, Start: 4 May 2018 | End: 11 September 2018

Target Irrigation Levels (% ETo): Tall Fescue: 50%, 65%, 80% | Hybrid Bermudagrass: 40%, 50%, 60% Irrigation Efficiency: 78%

Watering Days: 2 days per week, 3 days per week

2019 Trial, Start: 22 June 2019 | End: 26 August 2019

Target Irrigation Levels (% ETo): Tall Fescue: 50%, 65%, 80% | Hybrid Bermudagrass: 40%, 50%, 60% Irrigation Efficiency: 78%

Watering Days: 3 days per week, 7 days per week (no restriction)

The controller used the user-defined "plant type" information to convert ETo to irrigation application (irrigation application = plant type × ETo). For each treatment, the plant type was calculated as the irrigation levels (% ETo) divided by the irrigation efficiency of the system.

> Table 2 summarizes the irrigation application data. All treatments were over irrigated mainly due to the inaccurate estimation of the irrigation precipitation rate using the catchcans method at the beginning of the trial. The actual applied irrigation rate was calculated at the end of the trial based on the revised irrigation precipitation rate of 18 mm h−1. The performance of the smart controller is discussed in detail in the companion paper [23].

> **Table 2.** Target irrigation treatments (T1–T3) versus programmed and applied irrigation levels throughout the 2-year tall fescue and hybrid bermudagrass irrigation research experiments conducted at the University of California Kearney Research and Extension Center.


Programmed irrigation levels are equal to target treatment levels divided by the irrigation efficiency of 0.78 (i.e., the low half distribution uniformity of the irrigation system). Applied irrigation levels were recalculated based on the irrigation run time data retrieved from the controller and precipitation rate of 28 mm day−<sup>1</sup> measured for the system at the end of the trial.

#### *2.3. Data Collection and Statistical Analysis*

Figure 3 illustrates an overview of the sensors and tools used in this study and the companion paper [8]. The active light source optical GreenSeeker handheld sensor (Trimble Inc., Sunnyvale, CA, USA) was used to collect NDVI data. The sensor has a measurement range of 0 to 0.99 and a roughly 51 cm wide oval field of view when held 122 cm above the ground. Canopy temperature was recorded using the Fluke 64 Max Infrared Thermometer (Fluke Corporation, Everett, WA, USA). According to the manufacturer, the thermometer has a measurement range of −30 to 500 ◦C and a spectral band of 8–14 microns with an accuracy of 1.5 ◦C or 1.5% of the reading. The resolution of the thermometer was 0.1 ◦C with an 87 mm field of view when held 150 cm above the ground. During the data collection, both sensors were held at approximately 1 m height and moved over the center of each plot (≈3–4 m2) while the trigger remained engaged to continuously scan and obtain an average representative value for each plot. The air temperature and relative humidity were recorded using the Fluke 971 Temperature Humidity Meter (Fluke Corporation, Everett, WA, USA) over each experimental plot during the data collection process. According to the manufacturer, the Fluke 971 handheld sensor has a temperature measurement accuracy of ±0.5 ◦C in the 0 to 45 ◦C range and an RH measurement accuracy of ±2.5% in the 10% to 90% RH range. The resolution of the temperature/humidity meter was 0.1 ◦C and 0.1% RH. The measurements were done within two hours of solar noon under cloud-free conditions. The handheld data were collected roughly once a week throughout the trial, 21 times in 2018 (from 30 April to 22 September), and 10 times in 2019 (from 24 June to 26 August).

**Figure 3.** An overview of the sensors and tools used in this study and the companion paper [23].

The NDVI and canopy temperature data were statistically analyzed using PROC GLIMMIX in SAS 9.4 software package (SAS Institute, 2014). The irrigation frequency and duration of the experiment differed in years 1 and 2. Therefore, each year was independently analyzed for the treatment effects to accommodate differences in experimental duration and irrigation regimes. The fixed effects were the irrigation levels, irrigation frequencies, and the date of data collection. The random effects were block and its interaction with irrigation levels and irrigation frequencies. The treatment effects were considered significant at *p*-values ≤ 0.05. The plotting software package Veusz 3.3.1 (https://veusz.github.io/, accessed on 27 August 2021) [31] was used to create all graphs. The NDVI data were compared against the turfgrass visual rating (VR) values presented in the companion study [23]. The rating was based on The National Turfgrass Evaluation Program (NTEP) standards [32], with the minimum and maximum scores of 1 and 9 assigned to dead and ideal turfgrass, respectively.

## *2.4. Crop Water Stress Index (CWSI)*

The CWSI relies on the temperature difference between the canopy and air, *dt* ◦C (*Tc*–*Ta*), and it is defined as:

$$\text{CVSI} = \frac{dt\_m - dt\_{lb}}{dt\_{ub} - dt\_{lb}} \tag{1}$$

where *m*, *lb*, and *ub* indicate the measured, lower baseline (non-water-stressed), and upper baseline (non-transpiring) of *dt*, respectively.

The empirical CWSI is based on the linear relationship between the lower baseline temperature difference and vapor pressure deficit (VPD) [22,33]:

$$dt\_{lb} = a(\text{VPD}) + b \tag{2}$$

The VPD is calculated as follows:

$$\text{VPD} = \varepsilon\_{\text{\textquotedblleft}} - \varepsilon\_{\text{\textquotedblleft}} \tag{3}$$

where *es* is the saturation vapor pressure (kPa) and *ea* is the actual vapor pressure (kPa) calculated as:

$$e\_s = 0.6108 \ast \exp\left(\frac{17.27 \times T\_a}{237.3 + T\_a}\right) \tag{4}$$

$$
\varepsilon\_{\mathfrak{a}} = \left(RH/100\right) \* \varepsilon\_{\mathfrak{s}} \tag{5}
$$

The CWSI was calculated for each species separately in this study. The mean canopy temperature data obtained from the highest irrigation levels were used to estimate the lower baselines over all the well-watered plots of each species. The mean air temperature and RH values collected over all the plots using the handheld Fluke 971 m were used to calculate VPD. The upper baseline was calculated as the mean temperature difference between air and severely stressed tall fescue grass [17]. The baseline was established using the canopy temperature data collected in both years from a plot of non-irrigated tall fescue turfgrass adjacent to the study field. The non-transpiring canopy temperature was assumed to be similar between tall fescue and hybrid bermudagrass species. This assumption is based on the data collected by the research team from side-by-side non-transpiring tall fescue and hybrid bermudagrass plots sprayed with glyphosate in southern California (data not published). Different baselines were established for each year to determine their stability over time.

#### **3. Results**

#### *3.1. NDVI*

Figure 4 shows the response of both species (dynamics of NDVI values) to the applied irrigation treatments in 2018 and 2019. Table 3 summarizes the results of the statistical analysis for both species in the years 2018 and 2019. For tall fescue, the NDVI values ranged between 0.30 and 0.80 in 2018 and between 0.23 and 0.69 in 2019. The irrigation level (*p* < 0.001) and frequency (*p* < 0.05) had significant effects on NDVI values in 2018. In 2019, only the irrigation level had a significant effect (*p* < 0.01) on turfgrass quality. The interaction between irrigation levels and frequency was not significant in neither of the years. In 2018, the dynamics of NDVI values over time for all treatments were somewhat similar, showing a slight decline as the trial progressed. However, for 83% ETo treatment (2 d wk−<sup>1</sup> frequency), a more noticeable reduction in NDVI values was observed toward the end of the experimental period. In 2019, the NDVI values for the lowest irrigation application of 83% ETo started to decline around mid-July for both 3 and 7dw−<sup>1</sup> irrigation frequency. The NDVI values showed no substantial change for the other irrigation treatments.

For hybrid bermudagrass, the NDVI valued varied between 0.53 and 0.76 in 2018 and between 0.34 and 0.80 in 2019. The irrigation levels had no significant effect on NDVI values in 2018 and 2019 (Table 3). The impact of irrigation frequency was only significant (*p* < 0.01) in 2019. The interaction between irrigation levels and frequency was not significant in either of the years. In 2018, the NDVI for all treatments stayed fairly stable with no substantial fluctuations over time. In 2019, NDVI values increased over time with no noticeable differences between the irrigation treatments (Figure 4).

**Figure 4.** NDVI values showing the response of hybrid bermudagrass and tall fescue turfgrass to varying irrigation levels (ETo%) and frequency (d/wk: days per week) imposed in 2018 and 2019. TF: tall fescue, B: hybrid bermudagrass.



NS, \*\*\*, \*\*, and \* are non-significant or significant at *p* ≤ 0.001, 0.01, and 0.05, respectively. Means sharing a similar letter are not significantly different, based on Turkey's test at α = 0.05. I, F, and T in the table refer to irrigation levels, frequency, and time (i.e., repeated measures of visual rating each year over time), respectively.

### *3.2. Canopy Temperature and CWSI*

Figure 5 illustrates the tall fescue and hybrid bermudagrass canopy temperature fluctuations over time in response to the irrigation treatments imposed in 2018 and 2019. Table 3 summarizes the results of the statistical analysis for both species in the years 2018 and 2019. The canopy temperature readings were very similar in early and mid-trials in both years for both species across the treatments, while the fluctuations in canopy temperature values over time were more pronounced in 2018. The non-irrigated plot of turfgrass adjacent to the study field had on average 19 ◦C and 17 ◦C higher canopy temperature than the irrigated plots in 2018 and 2019, respectively.

**Figure 5.** The canopy temperature dynamics of hybrid bermudagrass and tall fescue turfgrass plots under varying irrigation scenarios imposed in 2018 and 2019. d/wk: indicates irrigation frequency in days per week.

For the tall fescue plots, the minimum and maximum canopy temperature values were 24 ◦C and 49 ◦C in 2018 and 33 ◦C and 43 ◦C in 2019. Irrigation levels significantly impacted the canopy temperature in 2018 (*p* < 0.01) and in 2019 (*p* < 0.05) (Table 3). On average, there was a 1.2 ◦C in 2018 and 0.9 ◦C in 2019 temperature difference between the highest (129% ETo) and lowest (83% ETo) irrigation levels. The irrigation frequency had a significant effect in 2018 (*p* < 0.05) but not in 2019. However, in both years, the canopy temperature was slightly lower for the greater irrigation frequencies. The interaction of the irrigation level and irrigation frequency had no significant effect on canopy temperature values. In both years, 83% ETo treatment started showing higher temperature values toward the end of the trial compared to the other irrigation treatments (Figure 5).

For the hybrid bermudagrass plots, the minimum and maximum canopy temperature values were 23 ◦C and 42 ◦C in 2018 and 36 ◦C and 43 ◦C in 2019. The irrigation levels had no significant effect on canopy temperature in neither of the years (Table 3). The irrigation frequency significantly impacted the canopy temperature (*p* < 0.05) in both years, such that more frequent irrigation reduced the canopy temperature. The mean canopy temperature was 0.5 ◦C in 2018 and 0.1 ◦C in 2019 lower in 101% ETo than in 65% ETo treatment. The interaction of the irrigation level and irrigation frequency had no significant effect on canopy temperature values. The 65% ETo treatment in 2018 started showing higher canopy temperature values toward the end of the trial compared to other irrigation levels (Figure 5). However, in 2019, the 65% ETo treatment showed lower canopy temperature early in the trial, but the temperature values were similar across the treatments toward the end of the experiment.

Figure 6 illustrates the lower and upper CWSI baselines established for tall fescue and hybrid bermudagrass in 2018 and 2019. Table 4 summarizes the coefficients for the lower baselines. For the tall fescue plots and 129% ETo treatment, *dt* (i.e., canopy minus air temperature) varied between −7.1 and 5.2 ◦C in 2018 and between −0.6 and 7.9 ◦C in 2019. For the 83% ETo treatment, *dt* varied between −3.8 and 6.4 ◦C in 2018 and −0.6 ◦C to 9.2 ◦C in 2019. There was a moderate correlation of 0.64 in 2018 and 0.88 in 2019 between *dt* and VPD for the lower baseline. The slope of the lower baseline was −2.52 in 2018 and −4.22 in 2019. The intercept was two times higher in 2019 compared to 2018.

**Figure 6.** Graphical illustration of the lower (LB) and upper (UB) baselines of canopy temperature (Tc) minus air temperature (Ta) difference versus vapor pressure deficit for hybrid bermudagrass and tall fescue species in central California.

**Table 4.** Lower CWSI baselines for tall fescue and hybrid bermudagrass.


*r*: correlation coefficient; a: slope, b: intercept.

For hybrid bermudagrass, *dt* varied between −2.0 and 9.2 ◦C for the 101% ETo level in 2018 and between −0.9 and 7.8 ◦C in 2019. For the 65% ETo treatment, *dt* varied between 0.5 and 8.9 ◦C in 2018 and between 0.6 and 7.7 ◦C in 2019. The correlation between *dt* and VPD for hybrid bermudagrass was 0.69 in 2018 and 0.64 in 2019. The lower CWSI baselines had a somewhat similar slope in both years, but the intercept was 3 ◦C higher in 2019.

The upper baseline (set to mean *dt* for the severely stressed non-irrigated tall fescue plot) was equal to 21.9 ± 4.7 ◦C (mean ± standard deviation, SD) in 2018 and 19.7 ± 5.3 ◦C in 2019. The combined upper baseline data for both years had a mean ± standard deviation *dt* of 21.1 ± 4.9 ◦C.

Figure 7 depicts the dynamics of tall fescue and hybrid bermudagrass CWSI over time in response to the irrigation treatments imposed in 2018 and 2019. The CWSI dynamics over time for both species are similar to the canopy temperature fluctuations (Figure 8), with minor differences mainly toward the end of 2019. For tall fescue, the CWSI values varied between −0.3 and 0.29 in 2018 and between −0.59 and 0.27 in 2019. For hybrid bermudagrass, the CWSI values ranged from −0.34 to 0.24 in 2018 and from −0.26 to 0.56 in 2019.

**Figure 7.** The crop water stress index (CWSI) dynamics for the hybrid bermudagrass (B) and tall fescue (TF) turfgrass plots under varying irrigation scenarios imposed in 2018 and 2019. d/wk: indicated irrigation frequency in days per week.

**Figure 8.** Relationship between visual rating (VR) and NDVI data collected in 2018 and 2019 from hybrid bermudagrass (B) and tall fescue (T) irrigation trials conducted at the University of California Kearney Research and Extension Center.

#### **4. Discussion**

#### *4.1. NDVI and Visual Rating*

For tall fescue, the NDVI values ranged between 0.30 and 0.80 in 2018 and between 0.23 and 0.69 in 2019. The lower NDVI values in 2019 agree with the lower VR values reported in the companion paper [23]. This is attributed to the minimal fertilizer application in 2019, diminishing growth and greenness of tall fescue. NDVI and VR were well correlated for tall fescue in 2018 (*r* = 0.92) and in 2019 (*r* = 0.83). Bremer et al. [27] conducted a 3-year study near Manhattan, Kansas and reported *r*-value of 0.75 between VR and NDVI values of 'Dynasty' tall fescue. The slope of the intercept of fitted regression lines in our study differed between 2018 and 2019. Bremer et al. [27] also obtained different models for each turfgrass in each year of their study and mentioned that as a potential practical limitation to estimate visual quality using NDVI values.

For hybrid bermudagrass, the NDVI values varied between 0.53 and 0.76 in 2018 and between 0.34 and 0.8 in 2019. In 2019, the NDVI values increased as the trial progressed in response to the late green-up of hybrid bermudagrass through June and a fertilizer application in mid-July. This trend agrees with an increase in VR values reported in the companion paper [23]. The correlation between NDVI and VR was moderate (*r* = 0.72) in 2019, which is on the lower end of the reported values in the literature. We [8] obtained *r* = 0.84 between NDVI and VR values for hybrid bermudagrass based on a 3-year composite dataset in Riverside, California. Bell et al. [34] conducted a two-year study and reported an annual *r* of 0.8 between NDVI and VR of hybrid bermudagrass (49 cultivars) in Stillwater, Oklahoma. Trenholm et al. [35] studied three hybrid bermudagrass cultivars in Griffin, Georgia and reported *r*-values ranging from 0.70 to 0.90 between turfgrass VR and NDVI.

In 2018, no meaningful correlation was observed between NDVI and VR values, which is attributed to the narrow NDVI range of variation (0.53–0.76). However, the range of VR values was relatively wide (4–9) in 2018 [23]. We also noticed (data not presented in this study) the mean coefficient of variation between replications of the same irrigation treatment on average was 60% higher for VR values than NDVI values. The quality variation among replications of the same irrigation treatment is expected to be minimal. These results suggest that NDVI can be a more stable and repeatable parameter than VR to assess the overall response of the turfgrass to irrigation regimes.

Our results suggest the NDVI values of 0.6–0.65 for tall fescue and 0.5 for hybrid bermudagrass to maintain acceptable quality (VR = 6) in the central California region. We [8] obtained NDVI of 6 as the minimum threshold for hybrid bermudagrass in inland southern California. Further investigation is needed to verify the thresholds obtained in this study, particularly for hybrid bermudagrass, since the recommendation is only based on 2019 data.
