Assessing the Reliability of Satellite-Derived Evapotranspiration Data Using Numerical Modifed Penman Method at Citarum Watershed Evi Anggraheni 1 *, Faris Zulkarnain 2 , Pranita Giardini 1 , Kintan Maulidina 2 , Babag Purbantoro 3 , Raisya Affah 1 , Anang Muchlis 4 , Siswanto 5 , Andry Rustanto 2 , Muhammad Dimyati 2 , Ahmad Zubair 2 , Triarko Nurlambang 2 , Ratih Dewanti 3 , Iqbal Putut Ash-Shidiq 2 , Irma Susanti 6 1 Civil and Environmental Engineering, Faculty of Engineering, Universitas Indonesia, Indonesia. 2 Department of Geography, Faculty of Mathematics and Natural Science, Universitas Indonesia, Indonesia. 3 Remote Sensing Technology and Data Center, National Research and Innovation Agency, Indonesia. 4 Citarum River Basin Agency, Ministry of Public Work and Housing, Indonesia. 5 Meteorological, Climatological, and Geophysical Agency, Indonesia. 6 Center for Applied Geography Research, Universitas Indonesia, Indonesia Submit: 2022-09-21 Received: 2023-02-03 Accepted: 2023-05-20 Key words: Evapotranspiration; Satellite MOD16A2; Water Availability; Modifed Penman, Citarum Watershed *Correspondent email: evi.anggraheni@eng.ui.ac. id Abstract. Evapotranspiration (ET) is crucial in the analysis and estimation of water availability and demand for crops. Due to limited discharge measurement stations, assessing water availability becomes a challenge in water management planning. Specifcally, Citarum watershed, the biggest watershed in West Java, serves as a primary source of raw water for Jakarta, Indonesia’s capital city. Modifed Penman method, which was originally developed by the Food and Agriculture Organization (FAO) and subsequently modifed for tropical areas, is commonly employed to analyze ET. In water balance equation, ET represents one of its key components, and solving this equation becomes necessary to determine water losses accurately. Accordingly, the standard product of the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite, MOD16A2, ofers an alternative source of ET data. To address the absence of ground station data, the reliability of satellite data becomes crucial. Terefore, the aim of this study is to compare and analyze the reliability of satellite- derived ET potential images with numerical Modifed Penman method specifcally within Citarum Watershed. Tis method is one among several other methods used to calculate ET potential based on climate data. Te MOD16A2 and Modifed Penman equation were used for the simulation and baseline data respectively. Te reliability of the two simulations was analyzed by examining the skewness percentage of each pixel and period. By observing the distribution of percent skewness, it becomes possible to evaluate the performance comparison of two simulations. Lastly, it is important to note that the sensitivity of satellite is signifcantly afected by local weather conditions. ©2022 by the authors. Licensee Indonesian Journal of Geography, Indonesia. Tis article is an open access article distributed under the terms and conditions of the Creative Commons Attribution(CC BY NC) licensehttps://creativecommons.org/licenses/by-nc/4.0/. 1. Introduction Water resource management is being challenged by the rising demand for water in various sectors such as households, food production, energy generation, and agriculture (Tsouni et al., 2008). To efectively manage water resources, it is essential to have access to high-quality information encompassing both the geographical and temporal aspects of watershed (Loucks, 2000). Accordingly, Evapotranspiration (ET) serves as an essential component of water resource data, despite the inherent difculties associated with its direct measurement and temporal variability (Tsouni et al., 2008). In general, ET process at the ground surface is calculated using method for measuring water loss in the hydrological cycle (Konukcu, 2007). ET plays an important role not only in determining water availability but also in estimating irrigation water supply (Tomas, 2000; Wu et al., 2014). Terefore, the accurate representation of ET is important for rainfall-runof modeling due to its impact on the hydrological responses (Anggraheni et al., 2018), but unfortunately, many watershed in Indonesia lack sufcient ET measurements. To address this issue, previous study was conducted where ET was calculated using a variety of numerical models, such as Penman-Monteith (Aprialdi et al., 2019; Widyanto et al., 2014), Tornthwaite method (Pramudya et al., 2019), and modifed Penman (Batchelor, 1984). Modifed Penman equation, also known as the FAO Modifed Penman Method, is widely used for calculating ET and was developed by the Food and Agriculture Organization (FAO). A previous study investigated that the FAO Modifed Penman is specifcally suitable for tropical areas (Batchelor, 1984). However, this method relies on parameters obtained from ground-based climatological stations. Over the past two decades, signifcant advancements have been made in the feld of remote sensing technology and methodologies for acquiring ISSN 2354-9114 (online), ISSN 0024-9521 (print) Indonesian Journal of Geography Vol 55, No 2 (2023): 213-220 DOI: 10.22146/ijg.77725 website: htps://jurnal.ugm.ac.id/ijg ©2023 Faculty of Geography UGM and Te Indonesian Geographers Associaton Indonesian Journal of Geography, Vol 54, No. 3 (2022) 463-470 REVIEW ARTICLE