Assessment of the empirical methods for the development of the synthetic unit hydrograph: a case study of a semi-arid river basin Mohamedmaroof P. Shaikh * , Sanjaykumar M. Yadav and Vivek L. Manekar Civil Engineering Department, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat, India *Corresponding author. E-mail: shaikhmaroof034@gmail.com MPS, 0000-0002-5566-538X ABSTRACT This study aims to assess various empirical synthetic unit hydrograph (SUH) methods and find the best method. Ideally, each river should have a definite rain gauge station (RGS) to get sufficient rainfall data that is available for carrying out meaningful analysis. The provisions of Indian Standard (IS) 4987:1994 determined the optimum number of RGS. In the absence of RGS, the SUH is recommended. SUHs have been developed using various methods such as Snyder’s, Taylor and Schwarz, Soil Con- servation Service, Mitchell’s and Central Water Commission (CWC). In the present study, the Rel River Basin (RRB) is considered as the study area which has two existing RGS. IS 4987:1994 suggested that four RGS are required for more reliable rainfall data. Various efficiency criteria such as Correlation Coefficient, Pearson Coefficient, Nash Sutcliffe Efficiency, Index of Agreement, Normalized Root Mean Square Error, Mean Absolute Error, Root Mean Square Error and Kling-Gupta Efficiency have been used to compare SUH methods. The ranking of SUH methods was reported based on the compound factor (CF) through effi- ciency criteria. The 1.125 CF was observed as the minimum for the CWC method and recommended for determining peak discharge and timing for the study area. Key words: compound factor, multiple criteria decisions making, optimum number of rain gauge stations, synthetic unit hydro- graph, ungauged basin HIGHLIGHTS • An optimum number of RGS has been examined at the initial level of the study. • Various empirical methods of SUH have been evaluated. • Numerous efficiency criteria have been used to compare different SUH methods. • The compound factor (as an MCDM technique) has been used to find the best SUH method, which is a new concept in SUH. INTRODUCTION Rainfall is one of the most essential and complex hydrological modelling parameters (Mishra & Singh 2010). After Bangladesh, India is an another important country frequently affected by floods (Gupta et al. 2003). 12.5% of India’s geographical area is prone to flooding (Patel & Dholakia 2010). Due to this large area of sub- mergence in India, flood analysis has become an essential feature (Kumara & Mehta 2020; Mehta et al. 2020). Rainfall measurement for any basin is the primary requirement for flood analysis (Marchi et al. 2010). RGS must be distributed equally in the basin to achieve reliable results (Berne et al. 2004). The installed RGS in the basin should be adequate to measure the rainfall. If the RGS is higher, the economy will be affected, and if the RGS is lower, the results will be unreliable (Buytaert et al. 2006). In the ungauged and semi-gauged basins, flood estimates and predictions are among the significant issues resulting from heavy rainfall over the basin. There are many approaches in existence to the assessment of floods (Shaikh et al. 2018). Few approaches are based on statistics, while others based on the characteristics of the basin are designed to meet this objective. The Unit Hydrograph (UH) concept has been implemented in the runoff estimation for the last four decades. Sherman (1932) was the originator of the UH theory. UH is defined as a surface runoff hydrograph in a given basin due to effective rainfall for the unit duration. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY-NC-ND 4.0), which permits copying and redistribution for non-commercial purposes with no derivatives, provided the original work is properly cited (http:// creativecommons.org/licenses/by-nc-nd/4.0/). © 2021 The Authors Water Practice & Technology Vol 00 No 0, 1 doi: 10.2166/wpt.2021.117 corrected Proof Downloaded from http://iwaponline.com/wpt/article-pdf/doi/10.2166/wpt.2021.117/977586/wpt2021117.pdf by guest on 17 December 2021