There are usually two strategies to mitigate the urban heat island. One is to increase the surface reflectance (albedo) of urban surface materials (building and pavement), which can reflect more short wave solar irradiance back to the space and reduce the net energy income of urban area [1
]. This can be achieved by using high reflectance building material, or coating and painting technologies [2
]. Another strategy is to increase evapotranspiration (ET) of urban area, which can use the absorbed solar energy by latent heat and then decrease air temperature [3
]. Actually, ET is one of the most important components of the water budget and energy balance in urban environments. It has been extensively researched and applied to water and environmental quality management in agricultural and natural ecosystems. However, few researchers focus on urban ET, and it may be the least-studied part of urban hydrology [4
]. According to Grimmond and Oke [4
], urban ET is usually regarded far lower than that in rural areas because ground of urban area is covered by buildings and paved roads. Therefore, ET in urban areas has been considered negligible in many cases [4
]. Moreover, the urban surface is composed of complex underlying covers, including buildings, roads, vegetation, and water bodies. The large spatial heterogeneity makes it extremely difficult to measure urban ET using many of the conventional methods.
The rapid development of global urbanization, however, is creating a series of ecological and environmental problems for the people living in urban areas [8
], such as increased risks of urban heat islands and floods, water shortages, and water pollution. Most of these challenges are closely related to the urban ET. In urban environments, large areas of permeable and moist soil are replaced by artificial impermeable surfaces (cement, asphalt, etc.), which greatly change the physical properties of the land surface. This trend is further worsened by human activities introduced changes to the water and energy cycles. These changes increase the intensity of urban heat islands [9
], urban rain islands, and waterlogging [11
]. Urban ET, as the only factor connecting the energy balance and water budget in urban areas, is an unavoidable factor in urban water management and living environment maintenance.
In fact, a few recent studies have showed that urban ET is quite large and is an important part of urban water budget [5
]. It is widely recognized that urban ET can relieve the heat island effect and reduce storm floods [17
]. Experimental and simulation studies on green roofs show that ET can significantly reduce urban temperature [20
]. In addition, as a source of water vapor, urban ET is very important for the urban climate [5
]. Actually, some pioneer cities have put ET into their environmental management evaluation framework. According to the reports of Philadelphia and New York City, urban green projects can possibly provide an added value to the urban environment, which may have worth of hundreds of millions to billions of dollars [22
]. Therefore, the accurate determination of urban ET is essential for urban hydrology, water management, and ecological and environmental planning and management [24
Despite its importance, up to now only few studies have focused on urban ET. Grimmond and Oke [4
] measured urban ET using the eddy covariance method and found that ET could reach approximately 1–3 mm day−1
. The related studies began to increase gradually after 2010. Various technologies—such as the sap flow, Bowen ratio, lysimeter, eddy covariance, and large aperture scintillometer—have gradually been used in urban ET measurement. Pataki et al. [27
] measured the transpiration rate of 15 species of trees using the sap flow method in Los Angeles. They found that ficus, United States shittim, and California sycamores had higher transpiration rates, whereas American redwood had lower transpiration rates. Peters et al. [28
] studied the seasonal change of urban ET using the sap flow, Bowen ratio, and eddy covariance methods. Their results showed that turf grasses used more water than trees in Minneapolis-Saint Paul area. DiGiovanni et al. [19
] measured ET at six locations in New York City using lysimeter and found that there were significant differences in ET among different green land types. Jacobs et al. [29
] measured the urban ET using the eddy covariance, sap flow, and large aperture scintillometers in Arnhem and Rotterdam in the Netherlands. The results showed that ET was 0.5–1.0 mm day−1
in the two cities, whereas the transpiration rate of the trees was on average 170 liters per day, which contributed considerably to the local ET. Jacobs also noted that there is no linear relationship between the urban reference ET and actual ET due to the complexity of the city ground surface [29
]. Ward et al. [30
] observed urban ET using a large aperture scintillometer in Swindon of Britain. In his study, the measured ET was 1–3 mm day−1
on average, which was greater than the eddy covariance measured urban ET (0.5–2.0 mm day−1
). At the 2015 AGU fall conference, more studies reported their results on urban ET, including ET estimation methods, time and space variations, and driving factors [3
Based on these carefully designed pioneer studies, it is clear that urban ET is a very important component of the urban water and energy cycles. It is also revealed that, although these conventional methods are mature, it is still a challenge to obtain accurate ET by using these methods in urban [5
]. This is because it is very difficult to meet their fetch requirements and their low spatial resolution. A fetch-free and high spatial resolution ET estimation method is highly needed for urban water and environmental management.
Hence, the goals of this study are to investigate the characteristics of urban ET and its main affecting factor and then to improve a fetch-free, high spatial resolution method for urban ET estimation.
4.1. Relationship between Urban ET and Precipitation (P)
In this study, the daily and monthly ET could reach 6 mm and 100 mm, respectively. Compared with the magnitude of ET observed in other landscapes (grasslands, farmlands, and forests), these values were not small (Yan and Qiu, 2016). The daily ET in our study could reach to 6 mm day−1
, much higher than 1–3 mm day−1
observed in other urban areas located in temperate zones [4
]. Urban ET is very obvious and cannot be neglected for water and environmental management.
Though the ratio of ET/P is an important parameter for water management and flood control in urban areas, this ratio is not well known because of a lack of experiment-based ET values. Although there are some urban ET values based on model simulations, the lack of verification makes it difficult to use these poorly tested results. Our results showed that during the whole year (1 August 2014 to 31 July 2015), ET was 361.36 mm and 624.72 mm in the dry and wet seasons, respectively, whereas the corresponding precipitation was 140.20 mm and 1033.10 mm.
As shown in Figure 8
, on a monthly basis, ET/P was usually greater than 1 in the dry season (October to March). It was even greater than 4 in October, November, February, and March. However, ET/P was usually less than 1 in the wet season (from April to September). The minimum ET/P was only 0.28 (May).
The average ET/P was 0.6 in the wet season, indicating 60% of precipitation evaporated as vapor. It was 2.6 in the dry season, indicating ET was 260% the precipitation in the same period. The large amount of ET in the dry season was due to added irrigation. Assuming that all rainfall in the dry season was used for lawn ET and the effect of irrigation on ET was not considered, we got ET/P = 0.65 over the year, indicating 65% of the annual precipitation evaporated in this humid city. For many cities located in the sub-humid and sub-arid areas, ET/P could be easily larger than 0.65, indicating most or all of their annual precipitation could be evaporated. There would be less or no surface runoff under these conditions if their vegetation was well established.
As mentioned above, ET could use large amount of solar radiation as latent heat and reduce the air temperature and urban heat island intensity. Our former measurements showed that the ground surface temperature difference between the lawn (with ET) and the nearby paved road (without ET) could be as much as 30 °C in middle summer days [41
]. Based on the air temperature measurement at the height of 1.5 m above ground surface, we found that the average urban island intensity could be 1.67 °C lower in this vegetated area than the nearby no vegetation area throughout a two-year period [42
]. Furthermore, our results also showed that the daily average transpiration rate (measured by sap-flow method) of a small-sized ficus tree in this field was 36–55 kg (in hot season) and its cooling effect is equivalent to a 1.6–2.4 kWh air conditioner working for all day (24 h) [3
]. The cooling effect of ET to urban area and urban heat island is very obvious. Increasing urban ET could be a useful way to mitigate climate warming, especially in urban areas.
Our results revealed that ET can take most of the annual precipitation, even if in a humid subtropical urban environment. This study was one of the very few studies on urban ET based on detailed experiments and observations. This result also showed that vegetation ET can greatly reduce urban runoff, which is important for flood control. In addition, ET can also use a large amount of solar energy and reduce the intensity of urban heat islands. Therefore, Urban ET and E/P are two of the most important parameters for ongoing sponge city design, low impact development, and urban thermal environmental management throughout the world.
4.2. Main Impact Factor of Urban ET
The relationships between ET and meteorological factors have been well studied for years in farmlands, grasslands, forests, and other landscapes. It is clear that there are three main factors controlling ET: the available energy (radiation), vapor diffusion condition (VPD and wind velocity), and soil water availability. However, the main fact controlling urban ET may differ with these landscapes because the lower wind velocity, higher temperature, and lower soil water availability (due to very thin soil layer). This may especially important in our study area because in addition to the lower wind velocity, VPD is lower, too. Therefore, it is interesting to investigate which factor is the most important one to control urban ET. For these purposes, we explored the influence of solar radiation (here we use PAR as representative), air temperature, relative humidity, and wind velocity on ET by using partial correlation analysis method.
We used the average data during a one-year period (1 August 2014 to 31 July 2015) to investigate their relationships. The partial correlation coefficients between urban lawn ET and PAR, relative humidity, wind velocity, and air temperature were 0.927, 0.079, 0.037, and 0.028, respectively, at a 99% confidence level. These results showed that urban ET was strongly related with solar radiation. The relationship between urban ET and relative humidity, wind velocity, and temperature was very weak.
Generally, ET is well related with radiation, humidity, and wind velocity in other landscapes. However, it does not fit with our results. To investigate whether seasonal changes reduced the influence of these factors, we carried out the same analysis using monthly average data. The results are shown in Table 3
. Again ET was highly related with PAR with the correlation coefficient varying from 0.65 to 0.92. The correlation coefficients between ET and temperature, humidity, and wind velocity were all very low.
Usually, VPD is an indicator of the combined effect of temperature and humidity. To investigate the effect of VDP on urban ET, we also carried out a partial correlation analysis between ET and PAR, VPD, and wind velocity. The correlation coefficients were 0.923, −0.039, and 0.028, respectively. This result showed that the role of VPD on ET under our conditions was limited.
These results were possibly due to the low wind velocity and constant humidity conditions in the study area. The mean wind velocity in the study site was only 0.3 m s−1 over the experimental period. The daily RH was also stably located in the range of 60% to 80%. Therefore, it was concluded that the urban ET was strongly related only with radiation. The effects of VPD, humidity, temperature, and wind velocity on urban ET were very weak under these urban conditions.
4.3. Evaluation of the ‘3T Model + Infrared RS’ Method for Urban ET Estimation
As discussed before, it is difficult to estimate urban ET by using many of the conventional methods because their fetch requirements cannot be met under the condition of high spatial heterogeneity in urban area. In addition, these methods are usually based on the measurements carried out at one point and then extend its results to the whole field. This also not work under the condition of high spatial heterogeneity in urban areas. In this study, we tried to use the approach of ‘3T model + infrared RS’ to overcome these challenges. We chose this approach because this method does not include wind velocity, VPD, humidity, and other parameters sensitive to spatial heterogeneity. The main input parameter of the proposed method is the surface temperature, which can be measured at over 100,000 points by thermal cameras on the ground, airplanes, and satellites. Up to now, it has been successfully applied under several spatial heterogeneous conditions [43
By verifying with Bowen ratio method, it was found that the results of two methods agreed with each other very well. The distribution of ET data were close to 1:1 line, the slope of regression line between them was close to 1 (1.12, 1.11, and 0.98 for the three days, respectively), intercept close to zero (0.03, 0.24, and 0.00 for the three days, respectively), and a high coefficient of determination (R2 = 0.93, 0.95, and 0.91 for the three days, respectively). Urban ET and its spatial heterogeneity can be well measured and revealed by the approach of ‘3T model + infrared RS’. Because wind velocity, VPD, humidity, and other spatial sensitive parameters are not required in this methodology, these advantages make it a suitable methodology for urban ET estimation.
The study area is a typical urban area characterized by plenty of sunshine and precipitation, high humidity in the wet season, and a lower wind velocity. These features are very common in subtropical urban and it is worth investigating its ET characteristics. Our studies show that urban ET is mainly affected by solar radiation under these conditions. The effects of air humidity, wind velocity, and air temperature are very weak.
The daily average ET of the urban lawn is 2.70 mm. The average monthly ET in the wet season (May to October) is over 100 mm, whereas it is less than 60 mm in the dry season (November to March). The annual total ET is approximately 990 mm. Compared with the magnitude of ET observed in other landscapes, these ET values are quite obvious. In addition, the average monthly precipitation is 23.37 mm in the dry season and 172.18 mm in the wet season. The ratio of ET/P is 0.65 in the wet season, 2.6 in the dry season, and 0.84 over the year, indicating that ET can take away most of the water from precipitation and much less runoff can be expected. The large amount of ET in the dry season was due to added irrigation. Assuming that all rainfall in the dry season was used for lawn ET and the effect of irrigation on ET was not considered, the ET/P was still 0.65 over the year. This means that 65% of the annual precipitation was evaporated. Urban ET is a highly important component of urban water and energy balances, and it cannot be neglected for sponge city design, low impact development, and urban thermal environmental management.
The fetch-free approach of ‘3T model + infrared RS’ is verified to be a reasonable method to estimate urban ET under highly spatial heterogeneous conditions. The verification results show that the coefficient of determination (R2) of urban lawn ET estimated by the suggested method and Bowen ratio method is greater than 0.93, and the root mean square error is less than 0.04 mm h−1. In addition, the spatial heterogeneity of ET can be reasonably reflected by this approach. Because wind velocity, VPD, humidity, and other spatial sensitive parameters are not required in this method, these advantages make it a suitable methodology for urban ET measurement.