Agricultural and Forest Meteorology 152 (2012) 212–222 Contents lists available at SciVerse ScienceDirect Agricultural and Forest Meteorology jou rn al h om epa g e: www.elsevier.com/locate/agrformet On the temporal upscaling of evapotranspiration from instantaneous remote sensing measurements to 8-day mean daily-sums Youngryel Ryu a,s,∗ , Dennis D. Baldocchi a , T. Andrew Black b , Matteo Detto c , Beverly E. Law d , Ray Leuning e , Akira Miyata f , Markus Reichstein g , Rodrigo Vargas h , Christof Ammann i , Jason Beringer j , Lawrence B. Flanagan k , Lianhong Gu l , Lindsay B. Hutley m , Joon Kim n , Harry McCaughey o , Eddy J. Moors p , Serge Rambal q , Timo Vesala r a Dep. of Environmental Science, Policy and Management, University of California, Berkeley, CA, USA b Faculty of Land and Food Systems, University of British Columbia, British Columbia, Canada c Smithsonian Tropical Research Institute, Panama d Dep. of Forest Ecosystems and Society, Oregon State University, Corvallis, OR, USA e CSIRO Marine and Atmospheric Research, Canberra, Australia f National Institute for Agro-Environmental Sciences, Tsukuba, Japan g Max-Planck Institute for Biogeochemistry, Jena, Germany h Departamento de Biología de la Conservación, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), BC, Mexico i Agroscope ART, Federal Research Station, Zürich, Switzerland j School of Geography and Environmental Science, Monash University, Victoria, Australia k Dep. of Biological Sciences, University of Lethbridge, Lethbridge, Alberta, Canada l Environmental Sciences Division, Oak Ridge National Laboratory, TN, USA m Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, Australia n Dep. of Landscape Architecture and Rural System Engineering, Seoul National University, Seoul, South Korea o Dep. of Geography, Queen’s University, Kingston, Ontario, Canada p ESS-CC, Alterra – Wageningen UR, Wageningen, The Netherlands q DREAM, CEFE-CNRS, Montpellier, France r University of Helsinki, Dept. of Physics, Helsinki, Finland s Dep. of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA a r t i c l e i n f o Article history: Received 3 December 2010 Received in revised form 23 August 2011 Accepted 12 September 2011 Keywords: Evapotranspiration Eddy covariance Temporal upscaling FLUXNET MODIS a b s t r a c t The regular monitoring of evapotranspiration from satellites has been limited because of discontinu- ous temporal coverage, resulting in snapshots at a particular point in space and time. We developed a temporal upscaling scheme using satellite-derived instantaneous estimates of evapotranspiration to produce a daily-sum evapotranspiration averaged over an 8-day interval. We tested this scheme against measured evapotranspiration data from 34 eddy covariance flux towers covering seven plant functional types from boreal to tropical climatic zones. We found that the ratio of a half-hourly-sum of potential solar radiation (extraterrestrial solar irradiance on a plane parallel to the Earth’s surface) between 10:00 hh and 14:00 hh to a daily-sum of potential solar radiation provides a robust scaling factor to convert a half-hourly measured evapotranspiration to an estimate of a daily-sum; the estimated and measured daily sum evapotranspiration showed strong linear relation (r 2 = 0.92) and small bias (-2.7%). By com- parison, assuming a constant evaporative fraction (the ratio of evapotranspiration to available energy) during the daytime, although commonly used for temporal upscaling, caused 13% underestimation of evapotranspiration on an annual scale. The proposed temporal upscaling scheme requires only latitude, longitude and time as input. Thus it will be useful for developing continuous evapotranspiration estimates in space and time, which will improve continuous monitoring of hydrological cycle from local to global scales. © 2011 Elsevier B.V. All rights reserved. ∗ Corresponding author at: Department of Landscape Architecture and Rural Sys- tems Engineering, Seoul National University, Seoul 151-921, South Korea. Tel.: +82 2 880 4871; fax: +82 2 873 5113. E-mail address: yryu@snu.ac.kr (Y. Ryu). 1. Introduction Evapotranspiration (E) is a major component of the terrestrial hydrological cycle (ca. 60% of land precipitation) (Trenberth et al., 2007). It controls land-atmosphere feedbacks via modulating land 0168-1923/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2011.09.010