An in-situ measurement method of evapotranspiration from typical LID facilities based on the three-temperature model

Low impact development (LID) facilities have ability to increase evapotranspiration (ET), and thus moderate the hydrological cycle and mitigate urban heat island (UHI) effects. However, it is difficult to measure ET of LID facilities by traditional methods (e.g. weighing lysimeter, Bowen ratio system and eddy covariance system) in urban environment. A recently developed method, called three-temperature model (3T model), has been used to measure actual ET of natural underlying surfaces in situ. However, its applicability in ET measurement for LID facilities has not been investigated. In order to investigate the accuracy of this method to estimate ET of LID facilities and its influencing factors, the experimental devices for vegetated LID (green roof with Zoysia tenuifoli (GRZ) and green roof with Callisia repens (GRC)) and non-vegetated LID (permeable concrete pavement (PCP) and permeable brick pavement (PBP)) were installed. The method was validated by comparing the ET measured by 3T model with that measured by weighing platform (hourly data, n = 223–302 for each facility). After validation, the method was used to measure ET variations of typical LID facilities under moist sub-tropical weather condition in Shenzhen, China. The results indicate the coefficient of determination (R²) between ET measured by 3T model and weighing platform is 0.63, 0.60, 0.70 and 0.65 for GRZ, GRC, PCP and PBP, respectively, and the slopes of regression equations are close to 1 and their intercepts are close to 0. Furthermore, the 3T model has higher absolute relative error (ARE) of ET estimation in the early morning and the late afternoon than that in the rest of daytime. This is mainly due to the effect of energy related factors, such as solar radiation, air temperature and the difference between reference surface temperature and air temperature. In addition, ET rates of LID facilities can maintain at a relatively high level in the first few days after rainfall. However, ET of PCP decreases faster than that of other LID facilities during the drying period. These results demonstrate the ability and accuracy of 3T model to measure variations of ET of LID facilities in situ.