Proceedings of the 22 nd IAHR-APD Congress 2020, Sapporo, Japan 1 TRACING CROP WATER DEMAND IN THE LOWER PING RIVER BASIN, THAILAND USING CLOUD–BASED IRRISAT APPLICATION KHIN MUYAR KYAW Department of Civil and Environmental Engineering, Faculty of Engineering, Mahidol University, Nakhon Pathom, Thailand; E–mail: khinmuyar.kya@student.mahidol.ac.th AREEYA RITTIMA* Department of Civil and Environmental Engineering, Faculty of Engineering, Mahidol University, Nakhon Pathom, Thailand; E–mail: areeya.rit@mahidol.ac.th, *Corresponding Author YUTTHANA PHANKAMOLSIL Environmental Engineering and Disaster Management Program, Diversion of Engineering, Mahidol University, Kanchanaburi, Thailand; E–mail: yutthana.pha@mahidol.ac.th ALLAN SRIRATANA TABUCANON Faculty of Environment and Resource Studies, Mahidol University, Thailand; E–mail: allansriratana.tab@mahidol.ac.th WUDHICHART SAWANGPHOL Faculty of Information and Communication Technology, Mahidol University; E–mail: wudhichart.saw@mahidol.edu JIDAPA KRAISANGKA Faculty of Information and Communication Technology, Mahidol University; E–mail: jidapa.kra@mahidol.edu YUTTHANA TALALUXMANA Department of Water Resources Engineering, Faculty of Engineering, Kasetsart University, Bangkok, Thailand; E–mail: fengynt@ku.ac.th VARAWOOT VUDHIVANICH Department of Irrigation Engineering, Faculty of Engineering at Kamphaengsaen, Kasetsart University, Nakhon Pathom, Thailand; E–mail: fengvwv@ku.ac.th ABSTRACT Estimating and tracing actual crop water demands during the growing season are necessarily essential as it can be the basic and useful information for achievement of irrigation water management and reservoir operation practices. The crop water demand (ETcrop) is commonly determined by multiplying the crop coefficient (Kc) and reference crop evapotranspiration (ETo) calculating from meteorological data at the nearby stations. In this study, the planting area and growth stage of crops in the Tortongdang, Wangbua, Wangyang and Nongkhwan Operation and Maintenance Projects, which are located in the Lower Ping River Basin, were tracked and crop coefficient was evaluated using weather based irrigation scheduling (IrriSAT) application for estimation of crop water requirement. IrriSAT provides the satellite images processing of Normalized Difference Vegetation Index (NDVI) of planting area of crop and uses empirical relationship to find Kc. Kc values of season and off–season crops planting in four irrigation area during 2000–2019 were traced by IrriSAT and the results were compared with average Kc–RID values which were calculated as a function of Kc from field observation for the different types of crop and accumulated area of crops monitored by the remote sensing. The results showed the similar patterns of average Kc over the growth stages of crops. However, the Kc values calculated by IrriSAT for four irrigation area were slightly deviated from Kc–RID values in some growing periods. Therefore, calibrating Kc values calculated by IrriSAT was then conducted in this study to estimate actual crop water use and benchmark with the filed observation data. This study also revealed that applying cloud–based IrriSAT could be greatly supportive for irrigators and reservoir operators to enhance the efficiency of irrigation water management. Keywords: Crop coefficient, Reference crop evapotranspiration, Crop Water Demand, IrriSAT, ETo calculator 1. INTRODUCTION Thailand’s economic development has been basically driven by agricultural sector. Improving the agricultural productivity plays a vital role to raise the livelihood and living standard of the local people in agricultural sector. Crop productivity depends not only on the farm inputs but also irrigation facilities. Therefore, estimating and