Comparison of three evapotranspiration models with eddy covariance measurements for a Populus euphratica Oliv.forest in an arid region of northwestern China

The accurate estimation of evapotranspiration (ET) in arid regions is important for improving the water use efficiency of vegetation. Based on successive observations from May to October of 2014, we estimated the ET of a Populus euphratica Oliv. forest during the growing season in an extremely arid region using the PM (Penman-Monteith), SW (Shuttleworth-Wallace) and SSW (an improved canopy transpiration model) models. Estimated ET values were compared with those of the eddy covariance measurements. Results indicated that the actual ET of the P. euphratica forest was always overestimated by the PM model. The accuracy of the SW model was higher than that of the PM model. However, some data were not easily obtained because of the complicated structure of the SW model. The newly proposed SSW model gave the most accurate ET values, and its accuracy was higher at hourly than at daily time scale. In conclusion, the SSW model is more suitable for sparse vegetation system at large scales in extremely arid regions.

Sensitivity of Penman-Monteith Reference Crop Evapotranspiration in Tao'er River Basin of Northeastern China

A non-dimensional relative sensitivity coefficient was employed to predict the responses of reference crop evapotranspiration (ET0) to perturbation of four climate variables in Tao'er River Basin of the northeastern China. Mean monthly ET0 and yearly ET0 from 1961 to 2005 were estimated with the FAO-56 Penman-Monteith Equation. A 45-year historical dataset of average monthly maximum/minimum air temperature, mean air temperature, wind speed, sunshine hours and relative humidity from 15 meteorological stations was used in the analysis. Results show that: 1) Sensitivity coefficients of wind speed, air temperature and sunshine hours were positive except for those of air tem- perature of Arxan Meteorological Station, while those of relative humidity were all negative. Relative humidity was the most sensitive variable in general for the Tao'er River Basin, followed by sunshine hours, wind speed and air tem- perature. 2) Similar to climate variable, monthly sensitivity coefficients exhibit large annual fluctuations. 3) Sensitivity coefficients for four climate variables all showed significant trends in seasonal/yearly series. Also, sensitivity coefficients of air temperature, sunshine hours and wind speed all showed significant trends in spring. 4) Among all sensitiv- ity coefficients, the average yearly sensitivity coefficient of relative humidity was highest throughout the basin and showed largest spatial variability. Longitudinal distribution of sensitivity coefficients for air temperature, relative hu- midity and sunshine hours was also found, which was similar to the distribution of the three climate variables.

Comparison of two remote sensing models for estimating evapotranspiration: algorithm evaluation and application in seasonally arid ecosystems in South Africa

Remote sensing tools are becoming increasingly important for providing spatial information on water use by different ecosystems. Despite significant advances in remote sensing based evapotranspiration(ET) models in recent years, important information gaps still exist on the accuracy of the models particularly in arid and semi-arid environments. In this study, we evaluated the Penman-Monteith based MOD16 and the modified Priestley-Taylor(PT-JPL) models at the daily time step against three measured ET datasets. We used data from two summer and one winter rainfall sites in South Africa. One site was dominated by native broad leaf and the other by fine leafed deciduous savanna tree species and C4 grasses. The third site was in the winter rainfall Cape region and had shrubby fynbos vegetation. Actual ET was measured using open-path eddy covariance systems at the summer rainfall sites while a surface energy balance system utilizing the large aperture boundary layer scintillometer was used in the Cape. Model performance varied between sites and between years with the worst estimates(R2<0.50 and RMSE>0.80 mm/d) observed during years with prolonged mid-summer dry spells in the summer rainfall areas. Sensitivity tests on MOD16 showed that the leaf area index, surface conductance and radiation budget parameters had the largest effect on simulated ET. MOD16 ET predictions were improved by:(1) reformulating the emissivity expressions in the net radiation equation;(2) incorporating representative surface conductance values;and(3) including a soil moisture stress function in the transpiration sub-model. Implementing these changes increased the accuracy of MOD16 daily ET predictions at all sites. However, similar adjustments to the PT-JPL model yielded minimal improvements. We conclude that the MOD16 ET model has the potential to accurately predict water use in arid environments provided soil water stress and accurate biome-specific parameters are incorporated.

SimET: An open-source tool for estimating crop evapotranspiration and soil water balance for plants with multiple growth cycles

Accurate estimation of crop evapotranspiration(ETc) and soil water balance, which is vital for optimizing water management strategy in crop production, can be performed by simulation. But existing software has many deficiencies, including complex operation, limited scalability, lack of batch processing, and a single ETc model. Here we present simET, an open-source software package written in the R programming language. Many concepts involved in crop ETc simulation are condensed into functions in the package. It includes three widely used crop ETc models built on these functions: the single-crop coefficient,double-crop coefficient, and Shuttleworth–Wallace models, along with tools for preparing model data and comparing estimates. SimET supports ETc simulation in crops with repeated growth cycles such as alfalfa, a perennial forage crop that is cut multiple times annually.

Comparison of Ten Potential Evapotranspiration Models and Their Attribution Analyses for Ten Chinese Drainage Basins

Potential evapotranspiration(EPET)is usually calculated by empirical methods from surface meteorological variables,such as temperature,radiation and wind speed.The in-situ measured pan evaporation(ETpan)can also be used as a proxy for EPET.In this study,EPET values computed from ten models are compared with observed ETpan data in ten Chinese river basins for the period 1961−2013.The daily observed meteorological variables at 2267 stations are used as the input to those models,and a ranking scheme is applied to rank the statistical quantities(ratio of standard deviations,correlation coefficient,and ratio of trends)between ETpan and modeled EPET in different river basins.There are large deviations between the modeled EPET and the ETpan in both the magnitude and the annual trend at most stations.In eight of the basins(except for Southeast and Southwest China),ETpan shows decreasing trends with magnitudes ranging between−0.01 mm d−1 yr−1 and−0.03 mm d−1 yr−1,while the decreasing trends in modeled EPET are less than−0.01 mm d−1 yr−1.Inter comparisons among different models in different river basins suggest that PETHam1 is the best model in the Pearl River basin,PETHam2 outperforms other models in the Huaihe River,Yangtze River and Yellow River basins,and PETFAO is the best model for the remaining basins.Sensitivity analyses reveal that wind speed and sunshine duration are two important factors for decreasing EPET in most basins except in Southeast and Southwest China.The increasing EPET trend in Southeast China is mainly attributed to the reduced relative humidity.

Estimation of the Actual Evapotranspiration by the SEBAL Method in the Irrigated Rice Perimeter of Zatta (Yamoussoukro—Côte d’Ivoire)

In this study, the SEBAL (Surface Energy Balance Algorithm for Land) model was used to map the spatio-temporal distribution of actual evapotranspiration in the Yamoussoukro department (Côte d’Ivoire). Like other regions of the country, the Yamoussoukro district is confronted with the phenomenon of evapotranspiration (ET). This is a very important component that comes into play in the water balance but also in the calculation of the water needs of agricultural crops. Consequently, its estimation is of paramount importance in research related to the rational management of water resources, particularly agricultural water. The objective of this study was to analyze the spatio-temporal distribution of actual evapotranspiration (AET) as a function of land cover and land use. The methodology used is based on the SEBAL model which uses remote sensing (Landsat 8_OLI/TIRS) and climatic data to estimate actual evapotranspiration and analyze the spatio-temporal distribution of AET. The results reveal that the AET varied from 0 to 5.44 mm/day over the period from December 2019 to February 2020 with an average value of 4.92 mm/day. The highest average values occurred for water bodies (4.90 mm/day) and flooded vegetation (4.88 mm/day) while the lowest values occurred in residential areas (2.04 mm/day). Furthermore, the results show that the difference between the SEBAL model and the FAO-Penman-Monteith method is minimal with an average RMSE of 0.36 mm/day for all the satellite images. This study demonstrates the considerable potential of remote sensing for the characterization and estimation of spatial evapotranspiration in the Zatta irrigated rice-growing area.

Estimation of land surface evapotranspiration using the METRIC model in Nepal

Climatic and atmospheric properties vary significantly within a small area for a topographically diverse region like Nepal.Remote sensing can be used for large-scale monitoring of atmospheric parameters in such diverse terrains.This work evaluates the Landsat-based METRIC(Mapping Evapotranspiration at High Resolution with Internalized Calibration)model for estimating Evapotranspiration(ET)in Nepal.The slope and aspect of terrain are accounted for in our implementation,making the model suitable for regions with topographical variations.The estimations obtained from the model were compared with ground-based measurements.The root-meansquare error for hourly ET(daily ET)was 0.06 mm h-1(1.24 mm d-1),while the mean bias error was0.03 mm h-1(0.29 mm d-1).These results are comparable with results from other studies in the literature that have used the METRIC model for different regions of the world.Thus,this work validates the applicability of the METRIC model for ET estimation in a mountainous area like Nepal.Further,this implementation provides ET estimation at a very high resolution of 30 m compared to the best available resolution of 5 km in earlier works,without compromising on the accuracy.ET estimation with high resolution over a large region in Nepal has applications in agricultural planning and monitoring,among others.

Ensemble Simulation of Land Evapotranspiration in China Based on a Multi-Forcing and Multi-Model Approach

In order to reduce the uncertainty of offline land surface model （LSM） simulations of land evapotranspiration （ET）, we used ensemble simulations based on three meteorological forcing datasets [Princeton, ITPCAS （Institute of Tibetan Plateau Research, Chinese Academy of Sciences）, Qian] and four LSMs （BATS, VIC, CLM3.0 and CLM3.5）, to explore the trends and spatiotemporal characteristics of ET, as well as the spatiotemporal pattern of ET in response to climate factors over China's Mainland during 1982-2007. The results showed that various simulations of each member and their arithmetic mean （EnsAVlean） could capture the spatial distribution and seasonal pattern of ET sufficiently well, where they exhibited more significant spatial and seasonal variation in the ET compared with observation-based ET estimates （Obs_MTE）. For the mean annual ET, we found that the BATS forced by Princeton forcing overestimated the annual mean ET compared with Obs_MTE for most of the basins in China, whereas the VIC forced by Princeton forcing showed underestimations. By contrast, the Ens_Mean was closer to Obs_MTE, although the results were underestimated over Southeast China. Furthermore, both the Obs_MTE and Ens_Mean exhibited a significant increasing trend during 1982-98; whereas after 1998, when the last big EI Nifio event occurred, the Ens_Mean tended to decrease significantly between 1999 and 2007, although the change was not significant for Obs_MTE. Changes in air temperature and shortwave radiation played key roles in the long-term variation in ET over the humid area of China, but precipitation mainly controlled the long-term variation in ET in arid and semi-arid areas of China.

基于辐射改进Penman-Monteith模型估算粮食主产区参考作物蒸散量

为进一步提高Penman-Monteith模型估算参考作物蒸散量(Reference crop evapotranspiration,ET0)的精度,以中国粮食主产区为研究对象,将其划分为温带湿润半湿润地区(THSZ)、温带干旱半干旱地区(TASZ)、暖温带半湿润地区(WTSZ)和亚热带湿润地区(SHZ),基于32个气象站点1994-2020年长序列实测逐日气象数据,将猎豹算法(CO)、沙猫算法(SCSO)、野狗算法(DOA)优化的时间卷积神经网络模型(TCN)和3种基于日照时数、3种基于温度的经验模型估算的辐射(R_(s))值与PM模型进行融合,得到改进PM模型。以均方根误差(RMSE)、决定系数(R^(2))、平均绝对误差(MAE)和效率系数(E_(NS))为精度评价体系,找出了粮食主产区不同分区的ET0最优估算模型,结果表明:基于日照时数模型的计算精度要优于温度模型,其中CO-TCN模型在全区内均表现出了较高的精度,在不同分区的RMSE、MAE、R^(2)和E_(NS)中位数取值分别为0.099~0.171 mm/d、0.057~0.111mm/d、0.984~0.998、0.983~0.997,由此可将CO-TCN模型估算的辐射值与PM模型融合,作为标准值用于估算粮食主产区ET0。

Stochastic Modelling of Actual Black Gram Evapotranspiration

The study was undertaken to develop and evaluate evapotranspiration model for black gram (Vigna Mungo L.) crop under climatic conditions of Udaipur, India. Pan evaporation data for the duration of twenty three years (1978-2001) and measured black gram evapotranspiration data by electronic lysimeter for duration of kharif season of 2001 were used for analysis. Black gram is an important crop of Udaipur region. No sys-tematic study on modelling of black gram evapotranspiration was conducted in past under above said cli-matic conditions. Therefore, stochastic model was developed for the estimation of daily black gram evapotranspiration using 24 years data. Validation of the developed models was done by the comparison of the estimated values with the measured values. The developed stochastic model for black gram evapotran-spiration was found to predict the daily black gram evapotranspiration very accurately.

Evapotranspiration Characteristics of a Lowland Dry Evergreen Forest in Central Cambodia Examined Using a Multilayer Model

Development pressure has led to serious deforestation on the Indochina Peninsula. Particularly rapid defor-estation has occurred in easily accessible lowland areas, and it is thus important to accumulate knowledge about these forests immediately. We measured evapotranspiration rates for a lowland dry evergreen forest in Kampong Thom Province, central Cambodia, using the energy balance Bowen ratio (EBBR) method based on meteorological data collected from a 60-m-high observation tower. Daily evapotranspiration was higher during the dry season than during the rainy season of the Asian monsoon climate. The seasonal variation in evapotranspiration generally corresponded to the seasonal difference in the vapor pressure deficit. A multi-layer model was used to simulate the seasonal variation in evapotranspiration. The multilayer model also reproduced the larger evapotranspiration rate in the dry season than in the rainy season. However, observed values substantially exceeded model-calculated results during certain periods at the beginning of the dry season and in the late dry season. Moreover, during the rainy season, the model tended to overestimate evapotranspiration. The differences between these observed and simulated values may have been caused by seasonal characteristics of photosynthesis and transpiration in the lowland dry evergreen forest that were not considered in the model simulation.

Sensitivity Analysis of the Penman-Monteith Reference Evapotranspiration to Climatic Variables: Case of Burkina Faso

Sensitivity analysis is important in understanding the effect of each climatic input variable on the variation of the evapotranspiration which is a significant element for hydrological modelling, irrigation programs, and water resources management. This study investigates the responses of evapotranspiration to the variation of maximum and minimum temperature, solar radiation, wind speed and maximum and minimum relative humidity. Daily data from 1998 to 2012 of eight stations are used. For each climatic variable, a variation of 5% up to ±25% was performed in order to evaluate the sensitivity of the evapotranspiration to the input variables. Results show that evapotranspiration is more sensitive to the variation of solar radiation, maximal temperature and wind speed, respectively.

Effect of Temporal Resolution of NDVI on Potential Evapotranspiration Estimation and Hydrological Model Performance

Normalized difference vegetation index (NDVI) data, obtained from remote sensing information, are essential in the Shuttleworth-Wallace (S-W) model for estimation of evapotranspiration. In order to study the effect of temporal resolution of NDVI on potential evapotranspiration (PET) estimation and hydrological model performance, monthly and 10-day NDVI data set were used to estimate potential evapotranspiration from January 1985 to December 1987 in Huangnizhuang catchment, Anhui Province, China. The differences of the two calculation results were analyzed and used to drive the block-wise use of the TOPMODEL with the Muskingum-Cunge routing (BTOPMC) model to test the effect on model performance. The results show that both annual and monthly PETs estimated by 10-day NDVI are lower than those estimated by monthly NDVI. Annual PET from the vegetation root zone (PETr) lowers 9.77%-13.64% and monthly PETr lowers 3.28%-17.44% in the whole basin. PET from the vegetation interception (PETi) shows the same trend as PETr. In addition, temporal resolution of NDVI has more effect on PETr in summer and on PETi in winter. The correlation between PETr as estimated by 10-day NDVI and pan measurement (R2= 0.835) is better than that between monthly NDVI and pan measurement (R2 = 0.775). The two potential evapotranspiration estimates were used to drive the BTOPMC model and calibrate parameters, and model performance was found to be similar. In summary, the effect of temporal resolution of NDVI on potential evapotranspiration estimation is significant, but trivial on hydrological model performance.

Application of MODIS-Based Monthly Evapotranspiration Rates in Runoff Modeling: A Case Study in Nebraska,USA

Daily and monthly flow-rates of the Little Nemaha River in Nebraska were simulated by the lumped-parameter Jakeman-Hornberger as well as a distributed-parameter water-balance accounting procedure for the 2003-2008 and 2000-2009 periods, respectively, with and without the help of the MODIS-based monthly estimates of evapotranspiration (ET) rates. While the daily lumped-parameter model simulation accuracy remained practically unchanged with the inclusion of the monthly MODIS-based ET rates interpolated into daily values (R2 of 0.66 vs 0.68, simulated to measured runoff ratio remaining the same 96%), the monthly water-balance accounting model outcomes did improve to some extent (from an R2 of 0.67 to 0.7 with simulated to measured runoff ratio of 72% vs 115%). In both cases the models had to be slightly modified for accommodation of the ET rates as predefined input values, not present in the original model setups. These results indicate the potential practical usefulness of satellite-derived ET estimates (CREMAP values in the present case) in monthly water-balance modeling. CREMAP is a calibration-free ET estimation method based on MODIS-derived daytime surface temperature values in combination of basic climatic variables, such as air temperature, humidity and solar radiation within a Complementary Relationship framework of evaporation.

Application of Spatially Distributed Calibrated Hydrological Model in Evapotranspiration Simulation of Three Gorges Reservoir Area of China:A Case Study in the Madu River Basin

Evapotranspiration(ET)is the key to the water cycle process and an important factor for studying near-surface water and heat balance.Accurately estimating ET is significant for hydrology,meteorology,ecology,agriculture,etc..This paper simulates ET in the Madu River Basin of Three Gorges Reservoir Area of China during 2009-2018 based on the Soil and Water Assessment Tool(SWAT)model,which was calibrated and validated using the MODIS(Moderate-resolution Imaging Spectroradiometer)/Terra Net ET 8-Day L4 Global 500 m SIN Grid(MOD16A2)dataset and measured ET.Two calibration strategies(lumped calibration(LC)and spatially distributed calibration(SDC))were used.The basin was divided into 34 sub-basins,and the coefficient of determination(R^(2))and NashSutcliffe efficiency coefficient(NSE)of each sub-basin were greater than 0.6 in both the calibration and validation periods.The R2 and NSE were higher in the validation period than those in the calibration period.Compared with the measured ET,the accuracy of the model on the daily scale is:R^(2)=0.704 and NSE=0.759(SDC results).The model simulation accuracy of LC and SDC for the sub-basin scale was R^(2)=0.857,R^(2)=0.862(monthly)and R^(2)=0.227,R^(2)=0.404(annually),respectively;for the whole basin scale was R^(2)=0.902,R^(2)=0.900(monthly)and R^(2)=0.507 and R^(2)=0.519(annually),respectively.The model performed acceptably,and SDC performed the best,indicating that remote sensing data can be used for SWAT model calibration.During 2009-2018,ET generally increased in the Madu River Basin(SDC results,7.21 mm/yr),with a multiyear average value of 734.37 mm/yr.The annual ET change rate for the sub-basin was relatively low upstream and downstream.The linear correlation analysis between ET and meteorological factors shows that on the monthly scale,precipitation,solar radiation and daily maximum and minimum temperature were significantly correlated with ET;annually,solar radiation and wind speed had a moderate correlation with ET.The correlation between maximum temperature and ET is best on the monthly scale(Pearson correlation coefficient R=0.945),which may means that the increasing ET originating from increasing temperature(global warming).However,the sub-basins near Shennongjia Nature Reserve that are in upstream have a negative ET change rate,which means that ET decreases in these sub-basins,indicating that the’Evaporation Paradox’exists in these sub-basins.This study explored the potential of remote-sensing-based ET data for hydrological model calibration and provides a decision-making reference for water resource management in the Madu River Basin.

The Spatial Sensitivity Analysis of Evapotranspiration using Penman-Monteith Method at Grid Scale

The need to allocate the existing water in a sustainable manner, even with the projected population growth, has made to assess the consumptive use or evapotranspiration (ET), which determines the irrigation demand. As underscored in the literature, Penman-Monteith method which is a combination of aerodynamic and energy balance method is widely used and accepted as the method of estimation of ET. However, the application of Penman-Monteith relies on many climate parameters such as relative humidity, solar radiation, temperature, and wind speed. Therefore, there exists a need to determine the parameters that are most sensitive and correlated with dependent variable (i.e., ET), to strengthen the knowledge base. However, the sensitivity of ET using Penman-Monteith is oftentimes estimated using meteorological data from climate stations. Such estimation of sensitivity may vary spatially and thus there exists a need to estimate sensitivity of ET spatially. Thus, in this paper, based on One-AT-A-Time (OAT) method, a spatial sensitivity tool that can geographically encompass all the best available climate datasets to produce ET and its sensitivity at different spatial scales is developed. The spatial tool is developed as a Python toolbox in ArcGIS using Python, an open source programming language, and the ArcPy site-package of ArcGIS. The developed spatial tool is demonstrated using the meteorological data from Automated Weather Data Network in Nebraska in 2010. To summarize the outcome of the sensitivity analysis using OAT method, sensitivity indices are developed for each raster cell. The demonstration of the tool shows that, among the considered parameters, the computed ET using Penman-Monteith is highly sensitive to solar radiation followed by temperature for the state of Nebraska, as depicted by the sensitivity index. The computed sensitivity index of wind speed and the relative humidity are not that significant compared to the sensitivity index of solar radiation and temperature.

A Modelling Stndy of Potential Evapotranspiration and the Estimation of Chinese Soil Moistnre Regimes

Based on the conception of fluid mechanics, the paper propounds a model for monthly potential evapo-transpiration ETi （mm）: where i is the number of a month, Pi the mean monthly atmospheric pressure （hpa）, ti the mean monthlyair temperature （° C）, di the number of days in the month, Ui the mean monthly wind velocity measured atheight 10-12 m （m/s）, woi the saturated water vapour pressure at ti（mmHg, 1 mmHg=133.322 Pa）, and hithe mean monthly relative humidity. The annual aridity K is:where ra is the mean annual precipitation （mm）. Based upon the data of 669 sites in China during 1951-1980, the relations among the soil moisture regime, the vegetation and the value of K may be illustrated asfollows:K Soil moisture regime VegetationK 【 1 Udic Forest Semi-udic Forest, forest-steppe and steppeSemi-aridic Arid steppe Aridic Desert-steppe Very aridic Desert

Analysis of Reference Evapotranspiration Change and Its Impact Factors in Australia

[ Objective] To discuss regional response of reference evapotranspiration （ ETo ） to global climate change and its influencing factors. [Method] Penman-Monteith equation was adopted to calculate ET0 in Australia from 1998 to 2007. Spatiotemporal change characteristics were analyzed by using GIS spatial analysis tools and relationships between ETo and main climate factors were also analyzed. [ Resultsl The results showed that multi-year average ET0 increased from the east and south part to the northwest part and inland, and its distribution was consistent with climate zones. Multi-year average ETo of the whole region was 1 750 mm, obtaining minimum and maximum values in 2000 and 2002 respectively. Regional ETo decreased in the order of summer, spring, autumn and winter; January and December got the highest monthly ET0, while June got the lowest value 79.55 mm. ETo positively correlated with mean temperature and solar radiation, R2 for each were 0.83 and 0.94, while the relationship between ETo and average relative humidity was negative, and precipitation had no significant relationship with ET0. [ Conclusion] This research could provide important reference for crop water requirement study and making irrigation method for Australia.

Spatial and temporal variations and controlling factors of potential evapotranspiration in China:1956-2000

Based on the climatic data of 580 stations in China during 1956 and 2000, potential evapotranspiration are calculated using the Penman-Monteith Method recommended by FAO. The spatial and temporal distributions of the potential evapotranspiration over China and the temporal trends of the regional means for 10 major river basins and whole China are analyzed. Through a partial correlation analysis, the major climate factors which affect the temporal change of the potential evapotranspiration are analyzed. Major results are drawn as follows： 1） The seasonal and annual potential evapotranspiration for China as a whole and for most basins show decline tendencies during the past 45 years; for the Songhua River Basin there appears a slightly increasing trend. 2） Consequently, the annual potential evapotranspirations averaged over 1980-2000 are lower than those for the first water resources assessment （1956-1979） in most parts of China. Exceptions are found in some areas of Shandong Peninsula, western and middle basins of the rivers in Southwest China, Ningxia Hui Autonomous Region as well as the source regions of the Yangtze and Yellow rivers, which may have brought about disadvantages to the exploration and utilization of water resources. 3） Generally, sunshine duration, wind speed and relative humidity have greater impact on the potential evapotranspiration than temperature. Decline tendencies of sunshine duration and/or wind speed in the same period appear to be the major causes for the negative trend of the potential evapotranspiration in most areas.

Study on the modified three-temperature model for spatial extrapolation of evapotranspiration based on individual urban vegetation evapotranspiration data

The evapotranspiration(ET)from urban vegetation is recognized as an effective way to improve the urban thermal environment.However,the traditional ET measurement approaches,such as the sap flow method,are limited in providing urban ET for specific areas.Consequently,establishing quantitative relationship between cooling effects and ET becomes challenging,which is crucial for understanding the role of urban greenery in mitigating urban heat.To overcome this challenge,an estimation model based on the three-temperature(3T)model was developed for accurately estimating urban ET by employing a reference tree with accurate ET data.In contrast to the 3T model,this study proposed a new method(modified 3T model)that considers the influence of the three-dimensional crown structure on the energy balance.Additionally,it retains the advantage over traditional ET models by not requiring the calculation of resistance parameters.This reduces the impact of uncertainty in the estimated resistance parameters on the ET results.The modified 3T model was validated based on the sap flow method and Bowen ratio system,where the results indicated good agreement between the modified 3T model and measured ET data,with root mean square errors of 22.2 W·m^(−2) and 26.4 W·m^(−2),respectively.Furthermore,the absolute relative error of ET estimation was affected by energy-related factors,such as solar radiation,air temperature,and relative humidity.The modified 3T model exhibited low sensitivity to surface temperatures,where the average sensitivity coefficients were below±0.15 from 0600 local time(LT)to 1700 LT.Considering the low accuracy of surface temperatures measured by infrared cameras(±2℃),it would be beneficial to combine the modified 3T model with unmanned aerial vehicles and infrared remote sensing to measure urban ET.