A re-examination of two methods for estimating daily evapotranspiration from remotely sensed instantaneous observations Ronglin Tang, Zhao-Liang Li, Xing Huo, Yazhen Jiang, Bohui Tang, Hua Wu To cite this version: Ronglin Tang, Zhao-Liang Li, Xing Huo, Yazhen Jiang, Bohui Tang, et al.. A re-examination of two methods for estimating daily evapotranspiration from remotely sensed instantaneous observa- tions. International Journal of Remote Sensing, Taylor & Francis, 2019, Fifth International Sym- posium on Recent Advances in Quantitative Remote Sensing (RAQRS), 40 (5-6), pp.1981-1995. 10.1080/01431161.2018.1465615. hal-02377731 HAL Id: hal-02377731 https://hal.archives-ouvertes.fr/hal-02377731 Submitted on 4 Dec 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. A re-examination of two methods for estimating daily evapotranspiration from remotely sensed instantaneous observations Ronglin Tang, Zhao-Liang Li, Xing Huo, Yazhen Jiang, Bohui Tang, Hua Wu To cite this version: Ronglin Tang, Zhao-Liang Li, Xing Huo, Yazhen Jiang, Bohui Tang, et al.. A re-examination of two methods for estimating daily evapotranspiration from remotely sensed instantaneous observa- tions. International Journal of Remote Sensing, Taylor & Francis, 2019, 40 5-6, pp.1981 - 1995. 10.1080/01431161.2018.1465615. hal-02377731 HAL Id: hal-02377731 https://hal.archives-ouvertes.fr/hal-02377731 Submitted on 4 Dec 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. A re-examination of two methods for estimating daily evapotranspiration from remotely sensed instantaneous observations Ronglin Tang a,b,Zhao-LiangLic,d,XingHuoe,f, Yazhen Jiangb, Bohui Tang a,b and Hua Wua,b aState Key Laboratory of Resources and Environment Information System, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China; bCollege of Resources and Environment, University of Chinese Academy of Sciences, Beijing, China; cKey Laboratory of Agricultural Remote Sensing, Ministry of Agriculture/Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing, China; dICube, UdS, CNRS, Illkirch, France; eSchool of Mathematics, Hefei University of Technology, Hefei, China; fSchool of Engineering Science, University of Science and Technology of China, Hefei, China ABSTRACT ARTICLE HISTORY The significance of estimating land surface evapotranspiration (ET) Received 19 October 2017 has been widely recognized in the fields of hydrology, meteorol- Accepted 5 April 2018 ogy, agriculture, and global change. This article compares two ET conversion methods, the constant decoupling factor (Ω) method and the constant reference evaporative fraction (EFr) method, that estimate daily ETs from instantaneous values. The daily ET was estimated either by (1) summing multiple half-hourly or hourly ET estimates that were derived through an application of the two ET conversion methods to half-hourly or hourly meteorological variables (i.e. aggregating the ET outputs) or by (2) directly applying the two ET conversion methods to daily meteorological variables (i.e. aggregating the meteorological inputs). The compar- ison was made using ground-based eddy covariance (EC) system measurements and the moderate resolution imaging spectroradi- ometer (MODIS)-based latent heat flux (LE) datasets collected from April 2009 to late October 2011 at the Yucheng station over the North China Plain. The results show that both the constant Ω method and the constant EFr method produced daily latent evaporation (LE) estimates that were in agreement with the ground-based EC measurements. When the two methods were applied to the MODIS-based LE datasets that have a small bias of –8Wm−2 and an root mean square error (RMSE) <60 W m−2, the validation results of the estimated daily LE against the ground-based EC measurements showed a relative bias of <7% and a relative RMSE of <20%. For both ET conversion methods, aggregating the ET outputs produced better agreement with the ground-based EC measurements than directly obtaining the daily ET by aggregating the meteorological inputs did. No significant difference was observed in the model performance between the constant Ω method and the constant EFr method. CONTACT Zhao-Liang Li [email protected] Key Laboratory of Agricultural Remote Sensing, Ministry of Agriculture/Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, China 2 1. Introduction The significance of estimating land surface evapotranspiration (ET, water in mm, equivalent to latent heat flux and often used interchangeably with latent heat flux) has been widely recognized in the fields of hydrology, meteorology, agriculture, and global change. Remote-sensing technology is regarded as the only viable means to map regional or global ET in a globally consistent and economically feasible manner. However, direct ET estimates from models using remote-sensing data are instantaneous values provided at satellite over- pass times (Kalma, McVicar, and McCabe 2008;Lietal.2009), which do not help researchers and practitioners who require ET datasets at daily and longer timescales. Several techniques with varying degrees of complexity and accuracy have been developed to convert remotely sensed instantaneous ET values to daily values over the past three decades (Brutsaert and Sugita 1992;Deloguetal.2012;Ryuetal.2012; Van Niel et al. 2012;Tang,Li,andSun2013; Cammalleri, Anderson, and Kustas 2014), primarily including the constant evaporative fraction (EF) method, the constant reference evaporative fraction method, the constant decoupling factor (Ω) method, and the constant flux ratio method (e.g. the ratio of latent heat flux to global solar radiation/extraterrestrial solar radiation/surface net radiation). The constant Ω method, recently proposed by Tang and Li (2017a, 2017b)througha simplified deviation of the Penman–Monteith equation, is a new, physically based technique that has proven to provide good estimates of daily ET from instantaneous values. This method assumes that the relative constancy of Ω in a diurnal cycle or, in a more straightforward way, that the instantaneous Ω derived at satellite overpass times equal the Ω at a daily scale. Because this new method requires instantaneous and daily meteorological measurements of air temperature, wind speed, air vapour pressure, and air pressure as inputs, it can capture the effect of temporally variable meteorological factors in the diurnal pattern of surface ET. This method is more physically based than the commonly applied constant EF method and can be applied operationally with little model complexity and less computation time. This method can also adjust the relative difference between the instantaneous EF at the time of the satellite overpass and the EF during other timeframes in a theoretically reasonable manner, and thus, it significantly reduces the underestimation of the daily ET, estimated using the constant EF method. Tang and Li (2017a, 2017b) have demonstrated the robustness of the new ET conversion method and its superiority over the conventional constant EF method for con- verting instantaneous ET to daily values. The constant reference evaporative fraction (EFr,the ratio of actual to reference grass/alfalfa ET) method, proposed by Trezza (2002), is another technique that can also incorporate the effect of horizontal advection and variable environ- mental factors on the ET in a diurnal cycle. Similar to the constant Ω method, the constant EFr method assumes that the instantaneous EFr equals daily EFr. The daily ET is estimated by multiplying the daily EFr bythedailyreferenceET.TheconstantEFr method has been widely applied for converting remotely sensed instantaneous ET to daily values, and its effectiveness has been demonstrated by a number of studies (Colaizzi et al. 2006;Chávezetal.2008; Allen, Tasumi, and Trezza 2007;Tang,Li,andSun2013;Tangetal.2015a;Tangetal.2017c). Previous studieshavealsoreportedthatthismethodprovides better daily ET estimates than the constant EF method (Tang, Li, and Sun 2013;Chávezetal.2008; Colaizzi et al. 2006). The daily ET can be estimated either by (1) summing multiple half-hourly or hourly ET estimates that are derived through an application of the two ET conversion methods to half-hourly or hourly meteorological variables (hereinafter referred to as aggregating the ET outputs) or by (2) 3 directly applying the two ET conversion methods to daily meteorological variables (herein- after referred to as aggregating the meteorological