**3. Results**

## *3.1. Method Verification*

The verification experiments showed quite a coincidence between the ET rates measured by the '3T + IR' method and BREB method. The correlation coefficient of the ET rates of the two methods was 0.958 (significant at the level of 0.01, by SPSS). Moreover, the linear regression demonstrated the consistency of the two methods (Figure 2). The distribution of the data was close to the 1:1 line and the regression line was ETB = 1.07ET3 – 0.06 (R<sup>2</sup> = 0.92), which means the rates measured by '3T + IR' were always close to the rates measured by BREB methods. The RMSE of the two rates was also just 0.03 mm h−1. This finding indicates that the '3T + IR' method could accurately measure the ET rate of urban grass and shrubs. Therefore, we applied this method directly in the field experiments on urban hedges in this study.

**Figure 2.** Comparison of the ET rates of urban vegetation estimated by the '3T + IR' method and BREB method. 3T + IR: three-temperature model + infrared remote sensing; BREB: Bowen ratio energy balance; ET3: ET rate measured by '3T + IR' method; ETB: ET rate measured by the BREB method.

#### *3.2. Characteristics of Meteorological Conditions*

Our field experiments were conducted on a sunny day in each season from 2015 to 2016. The typical days for each season were 22 August 2015 (Summer); 18 December 2015 (Autumn); 4 February 2016 (Winter); and 19 March 2016 (Spring). The season division is according to the Shenzhen Bureau of Meteorology [54]. The daily average temperature of the summer day was as high as 32.32 ◦C (Figure 3). The temperatures were still high even in the autumn day (14.55 ◦C) and winter day (17.92 ◦C). The solar radiation showed an almost single-peak variation in all days. It was the strongest in the summer day, when the daily average reached 555.98 W m<sup>−</sup>2. The air was the most humid on the spring day (89%) followed by the summer day (72%). The winter day had a low relative humidity (20%). The wind velocity was not high in any of the four days. The highest was during the autumn day, when its daily average was 0.78 m s<sup>−</sup>1.

**Figure 3.** Characteristics of weather in the typical sunny days during each season in the study area. (**a**) the air temperature; (**b**) the relative humidity; (**c**) the solar radiation; (**d**) the wind velocity. Data were measured by the Bowen ratio system in 22 August 2015 (Summer); 18 December 2015 (Autumn); 4 February 2016 (Winter); and 19 March 2016 (Spring). The air temperature and relative humidity are the average of the values measured at 1.5 m and 2.0 m.

#### *3.3. ET Characteristics of Urban Hedges*

#### 3.3.1. Surface Temperatures of the Urban Hedges

The infrared images of the hedges and the lawn were taken over four days. Subsequently, the ET rates were calculated by our software (Figure 4).

**Figure 4.** The surface temperatures (K) and ET rates (mm <sup>h</sup>−1) of the two hedges at 12:00 p.m. in 22 August 2015. (**a**) the surface temperature of the *L. quihoui* hedge; (**b**) the ET rates of the *L. quihoui* hedge; (**c**) the surface temperature of the *H. littoralis* hedge; (**d**) the ET rates of the *H. littoralis* hedge. The ET rates were calculated by our software based on the '3T + IR' method and plotted by ArcGIS.

As depicted in Figure 5, the surface temperatures of the two hedges also showed single-peak variations in all four days, much like the solar radiation. For most of the time during the four days, the surface temperature of the *L. quihoui* hedge was higher than that of the *H. littoralis* hedge. The daily average surface temperature of the *L. quihoui* hedge was 27.43, 34.43, 20.55, and 21.94 ◦C in each day. At the same time, the surface temperature of the *H. littoralis* hedge was 26.00, 33.66, 19.77, and 20.86 ◦C, respectively. The surface temperatures of the hedges were slightly higher than the air temperature. The *L. quihoui* hedge was 2.22, 2.11, 6.00, and 4.02 ◦C higher than the air temperature. The smallest difference between surface and air temperature occurred during the summer day, when the solar radiation and air temperature were the highest. The surface temperature of the lawn used for comparison was much warmer than that of the two hedges. The surface temperature differences between the lawn and the *H. littoralis* hedge were 4.85, 9.14, 5.83, and 3.65 ◦C over the four days.

**Figure 5.** Surface temperature of the hedges and the lawn for comparison over the four days (the temperatures were the average values of three images. (**a**) Spring: 19 March 2016; (**b**) Summer: 22August2015;(**c**)Autumn:18December2015;(**d**) Winter:4February2016).

#### 3.3.2. ET Rates of the Urban Hedges

The ET rates of the two hedges showed similar variation trends in the spring, autumn and winter days (Figure 6). They both increased from the morning and began to decrease after reaching peaks in the midday. The sudden drop at 11:00 a.m. during the summer day might be the result of the stomatal closure of the plants due to high surface temperatures. The ET rate was still quite high at 3:00 p.m. on the summer day. The ET of the *H. littoralis* hedge usually reached its maximum when the solar radiation was at its peak (Figure 3). However, the ET of the *L. quihoui* hedge rates reached their peaks at a different time compared to the *H. littoralis* hedge in the spring day. In particular, the ET rate of the *L. quihoui* hedge achieved another peak at 1:00 p.m. during the spring day. We also calculated the vapor pressure deficit (VPD) and found that it increased to its maximum at 1:00 p.m. during that day (data not shown).

**Figure 6.** ET rates of the hedges and the lawn for comparison on the four typical sunny days in four seasons. (**a**) Spring: 19 March 2016; (**b**) Summer: 22 August 2015; (**c**) Autumn: 18 December 2015; (**d**) Winter: 4 February 2016.

Figure 6 also showed that the ET rates of the hedges on the summer day were obviously stronger than those of the other three days. The daily average ET rate of the *H. littoralis* hedge was approximately 0.38 mm <sup>h</sup>−1, while the daily average ET rate of the *L. quihoui* hedge was 0.33 mm h−<sup>1</sup> (Table 2). Despite a lower level of solar radiation on the winter day, these data showed higher ET rates than on the autumn day, which may be attributed to the lowest relative humidity during this time. The ET rate was the lowest on the spring day with the lowest solar radiation and VPD. Meanwhile, we found that the ET rate of the *H. littoralis* hedge was higher than that of the *L. quihoui* hedge over the four days. The differences were 0.01, 0.05, 0.04, and 0.01 mm h−1. The ET rates of the hedges were always higher than the lawn, especially on the summer day, when the ET rate of the *H. littoralis* hedge was 0.20 mm h−<sup>1</sup> higher than the lawn. The difference was the smallest on the spring day, when all the three vegetation types had low ET rates.

**Table 2.** Average ET rates (mm <sup>h</sup>−1) of the hedges and the lawn for comparison on the four typical sunny days in four seasons. Spring: 19 March 2016; Summer: 22 August 2015; Autumn: 18 December 2015; Winter: 4 February 2016.


#### 3.3.3. The LE/Rn of the Urban Hedges

It is usually understood that green space could cool the surrounding area through latent heat flux. To reflect the diurnal course of energy exchange, the ratio of latent to net radiation (LE/Rn) was used to illuminate the cooling effect. As shown in Figure 7, the variation of LE/Rn showed different characteristics over the four days. The LE/Rn fluctuated through the day. At 8:00 a.m. on the summer day, the LE/Rn of the two hedges could reach approximately 90%. Their LE/Rn maintained a high value during the summer day, indicating that most of the net radiation was consumed by latent heat. On the autumn and winter day, their LE/Rn had obvious changes in the morning and afternoon because of low latent heat consumption at the beginning and ending of the day.

During the summer day, the hedges could consume over 60% of the net radiation through latent heat. The ET rate was the lowest on the spring day, the LE/Rn of the hedges during that day was also the lowest. Though the LE/Rn of the *L. quihoui* hedge exceeded 50% at 5:00 p.m. during the spring day, its cooling effect was still negligible because the latent heat was only 0.63 W m<sup>−</sup>2. The LE/Rn of the lawn for comparison had variation trends similar to the LE/Rn of the hedges except for the summer day, which began with a small LE/Rn and was still high at 5:00 p.m.

**Figure 7.** Latent heat flux of the hedges and the lawn for comparison on the four typical sunny days in four seasons. The latent heat flux was figured out directly from the three-temperature model. The LE/Rn is the proportion of the latent heat to the net radiation. (**a**) Spring: 19 March 2016; (**b**) Summer: 22 August 2015; (**c**) Autumn: 18 December 2015; (**d**) Winter: 4 February 2016.

Overall, the daily average LE/Rn of the *H. littoralis* hedge was still higher than that of the *L. quihoui* hedge during all days (Table 3). On the summer day, the *H. littoralis* hedge consumed 68.44% of the net radiation while for the *L. quihoui* hedge it was 60.81%. The LE/Rn of the lawn was lower than that of the two hedges. The largest differences appeared in the summer day and extended to 28.92%, suggesting that the hedges have much better cooling potential than the lawn.

**Table 3.** Daily average LE/Rn of the hedges and the lawn for comparison on the four typical sunny days in four seasons. Spring: 19 March 2016; Summer: 22 August 2015; Autumn: 18 December 2015; Winter: 4 February 2016.


#### *3.4. Cooling Effects of Urban Hedges*

#### 3.4.1. Cooling Effects on Air Temperature of the Urban Hedges

LE/Rn described the cooling effects of the vegetation in an indirect way. The temperature reduction was also calculated to intuitively evaluate the cooling effect of the hedges henceforth. The cooling effects of plants on air temperature or surface temperature have been widely studied in recent years [55–57]. Most studies on this topic were based on comparing the temperature differences between two sites. However, this method could not divide the cooling effects of the plants and show how much the ET specifically contributes to cooling. Here, we reference a method to calculate the cooling effect of the hedges through ET alone [58]. For the unit volume of air

$$
\Delta T\_a = 60 \ast L \,\mathrm{E} / \rho\_{\mathrm{air}} \mathrm{CV} \tag{7}
$$

where Δ*Ta* (◦C min−<sup>1</sup> m<sup>−</sup>2) is the cooling rate by ET of unit area hedges. *LE* is the latent heat (W·m<sup>−</sup>2) and has been analyzed using the '3T + IR' method. *C* is the specific heat capacity of air, which is 1005 J·kg−1·◦C−1. *V* is the volume of the air and equals 10 m<sup>3</sup> here, following the reference paper [58]. *ρair* is the air density (kg·m<sup>−</sup>3), and it is a function of air temperature (*Ta*),

$$
\rho\_{\dot{a}\dot{r}} = 1.2837 - 0.0039T\_a \tag{8}
$$

The variation of the cooling rates of the studied hedges always followed the variation of their ET rates (Figure 8). The hedges could cool the air most effectively when the ET rate and radiation reached their maximums. The cooling effects of the hedges were the most robust on the summer day and the weakest on the spring day. Though the cooling effects in the autumn day were stronger than on the spring day, the hedges had a shorter cooling period due to shorter radiation duration. The cooling effect of the *H. littoralis* hedge was slightly stronger than the *L. quihoui* hedge. The daily average cooling rates of the *H. littoralis* hedge were 0.12 ◦C min−<sup>1</sup> m<sup>−</sup>2, 1.29 ◦C min−<sup>1</sup> m<sup>−</sup>2, 0.42 ◦C min−<sup>1</sup> m −2, and 0.61 ◦C min−<sup>1</sup> m<sup>−</sup><sup>2</sup> over the four days and were 0.10 ◦C min−<sup>1</sup> m<sup>−</sup>2, 1.13 ◦C min−<sup>1</sup> m<sup>−</sup>2, 0.30 ◦C min−<sup>1</sup> m<sup>−</sup>2, and 0.56 ◦C min−<sup>1</sup> m<sup>−</sup><sup>2</sup> for the *L. quihoui* hedge. Both hedges had stronger cooling effects than the lawn, especially on the summer day. The cooling rates of the *Z. matrella* lawn were 0.05 ◦C min−<sup>1</sup> m<sup>−</sup>2, 0.63 ◦C min−<sup>1</sup> m<sup>−</sup>2, 0.21 ◦C min−<sup>1</sup> m<sup>−</sup>2, and 0.40 ◦C min−<sup>1</sup> m<sup>−</sup><sup>2</sup> over the four days.

**Figure 8.** The cooling rates of hedges and the lawn for comparison on air temperature on the four typical sunny days in four seasons. (**a**) Spring: 19 March 2016; (**b**) Summer: 22 August 2015; (**c**) Autumn: 18 December 2015; (**d**) Winter: 4 February 2016.

#### 3.4.2. Cooling Effects of the Urban Hedges on Surface Temperature

Surface temperature can easily be obtained using infrared remote sensing techniques and has therefore become the basis of most studies on the cooling effects of the vegetation. In this study, the cooling effects of the urban hedges on surface temperature at a small scale is discussed. The thermal imager simultaneously photographed the surface temperature of an asphalt road near the study site when the vegetation was photographed. The surface temperature of the asphalt road was always high, especially during the summer day (Figure 9). On that day, it could be as high as 62.73 ◦C at 3:00 p.m. and the daily average increased to 53.60 ◦C. The surface temperatures of the asphalt road during the other three days were similar, while the surface temperature of the hedges showed obvious differences (Figure 5). The average daily surface temperatures of the road were 30.22, 28.35, and 27.86 ◦C in the spring, autumn, and winter day.

**Figure 9.** Surface temperature of the asphalt road in the four typical days in four seasons. Spring: 19 March 2016; Summer: 22 August 2015; Autumn: 18 December 2015; Winter: 4 February 2016.

The surface temperature of the asphalt road was higher than the hedges most of the time (Figure 10). The cooling effects of the hedges were more evident in the mid-day, when the underlying surface temperatures were high. During the summer day, the cooling effects on surface temperature could even be over 20 ◦C between 11:00 a.m. and 4:00 p.m. This means the hedges could significantly reduce the peak surface temperature in a day. The daily average cooling effects of the two hedges during the summer day were 19.17–19.94 ◦C. They were much weaker on the other three days, especially in the spring day, when the two hedges could only cool the underlying surface by 2.80–4.22 ◦C. The surface temperature cooling effects were even negative in the morning of the spring day. The low ET rate of the *L. quihoui* hedge restricted its cooling effect at that time. In addition, the asphalt road dissipated heat in the night before becoming cooler in the morning [59]. As a result, the surface temperature of the road could be lower than the hedge.

The *H. littoralis* hedge had better cooling effects on underlying surface temperatures than the *L. quihoui* hedge. The *H. littoralis* hedge cooled the underlying surface temperature by 4.22, 19.94, 8.57, and 7.00 ◦C on the four days, respectively. Simultaneously, the *L. quihoui* hedge could cool the surface by 2.80, 19.17, 7.80, and 5.92 ◦C. The hedges always showed better cooling effects than the lawn. The cooling effects of the lawn were −0.62, 10.81, 2.75, and 3.36 ◦C on the four selected days. The most distinct differences of cooling effects between the hedges and the lawn were during the summer day. The hedges could cool the surface by 9 ◦C more than the lawn. The minimum differences occurred on the winter day, when the two hedges cooled by more 2.56 ◦C and 3.64 ◦C than the lawn.

**Figure 10.** Surface temperature differences between the hedges, lawn and asphalt pavement on the four typical sunny days in four seasons. (**a**) Spring: 19 March 2016; (**b**) Summer: 22 August 2015; (**c**) Autumn: 18 December 2015; (**d**) Winter: 4 February 2016.
