Application of a modified conceptual rainfall–runoff model to simulation of groundwater level in an undefined watershed Nian Hong, Takehide Hama, Yuichi Suenaga, Sayed Waliullah Aqili, Xiaowu Huang, Qiaoyan Wei, Yasunori Kawagoshi ⁎ Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan HIGHLIGHTS • We establish a groundwater level tank model (GLTM) to simulate groundwa- ter levels. • The GLTM is based on a conceptual rainfall–runoff model. • The GLTM simulates groundwater levels in an undefined watershed. • The GLTM is developed using only rain- fall and groundwater level. • The applied GLTM provided accurate results in Kumamoto, Japan. GRAPHICAL ABSTRACT abstract article info Article history: Received 14 May 2015 Received in revised form 28 August 2015 Accepted 5 September 2015 Available online xxxx Editor: D. Barcelo Keywords: Groundwater level Rainfall Tank model Kumamoto City Groundwater level simulation models can help ensure the proper management and use of urban and rural water supply. In this paper, we propose a groundwater level tank model (GLTM) based on a conceptual rainfall–runoff model (tank model) to simulate fluctuations in groundwater level. The variables used in the simulations consist of daily rainfall and daily groundwater level, which were recorded between April 2011 and March 2015 at two representative observation wells in Kumamoto City, Japan. We determined the best-fit model parameters by root-mean-square error through use of the Shuffled Complex Evolution-University of Arizona algorithm on a simulated data set. Calibration and validation results were evaluated by their coefficients of determination, Nash–Sutcliffe efficiency coefficients, and root-mean-square error values. The GLTM provided accurate results in both the calibration and validation of fluctuations in groundwater level. The split sample test results indicate a good reliability. These results indicate that this model can provide a simple approach to the accurate simulation of groundwater levels. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Groundwater is an important natural resource that is essential for agricultural, municipal, and industrial uses. During the last few decades, groundwater has become an important source of freshwater through- out the world. It is estimated that groundwater accounts for about 50% Science of the Total Environment 541 (2016) 383–390 ⁎ Corresponding author. E-mail address: qzt00564@nifty.ne.jp (Y. Kawagoshi). http://dx.doi.org/10.1016/j.scitotenv.2015.09.026 0048-9697/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv