water Article Application of SWAT in Hydrological Simulation of Complex Mountainous River Basin (Part I: Model Development) Suresh Marahatta 1, *, Laxmi Prasad Devkota 2,3 and Deepak Aryal 1   Citation: Marahatta, S.; Devkota, L.P.; Aryal, D. Application of SWAT in Hydrological Simulation of Complex Mountainous River Basin (Part I: Model Development). Water 2021, 13, 1546. https://doi.org/10.3390/ w13111546 Academic Editor: Raghavan Srinivasan Received: 8 April 2021 Accepted: 24 May 2021 Published: 31 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Central Department of Hydrology and Meteorology, Tribhuvan University, Kathmandu 44600, Nepal; deepak.aryal@cdhm.tu.edu.np 2 Nepal Academy of Science and Technology (NAST), Kathmandu 44600, Nepal; lpdevkota1@gmail.com 3 Water Modeling Solutions Pvt. Ltd. (WMS), Kathmandu 44600, Nepal * Correspondence: suresh.marahatta@cdhm.tu.edu.np Abstract: The soil and water assessment tool (SWAT) hydrological model has been used extensively by the scientific community to simulate varying hydro-climatic conditions and geo-physical environ- ment. This study used SWAT to characterize the rainfall-runoff behaviour of a complex mountainous basin, the Budhigandaki River Basin (BRB), in central Nepal. The specific objectives of this research were to: (i) assess the applicability of SWAT model in data scarce and complex mountainous river basin using well-established performance indicators; and (ii) generate spatially distributed flows and evaluate the water balance at the sub-basin level. The BRB was discretised into 16 sub-basins and 344 hydrological response units (HRUs) and calibration and validation was carried out at Arughat using daily flow data of 20 years and 10 years, respectively. Moreover, this study carried out additional validation at three supplementary points at which the study team collected primary river flow data. Four statistical indicators: Nash–Sutcliffe efficiency (NSE), percent bias (PBIAS), ratio of the root mean square error to the standard deviation of measured data (RSR) and Kling Gupta efficiency (KGE) have been used for the model evaluation. Calibration and validation results rank the model performance as “very good”. This study estimated the mean annual flow at BRB outlet to be 240 m 3 /s and annual precipitation 1528 mm with distinct seasonal variability. Snowmelt contributes 20% of the total flow at the basin outlet during the pre-monsoon and 8% in the post monsoon period. The 90%, 40% and 10% exceedance flows were calculated to be 39, 126 and 453 m 3 /s respectively. This study provides additional evidence to the SWAT diaspora of its applicability to simulate the rainfall-runoff characteristics of such a complex mountainous catchment. The findings will be useful for hydrologists and planners in general to utilize the available water rationally in the times to come and particularly, to harness the hydroelectric potential of the basin. Keywords: hydrological simulation; SWAT; water balance; complex mountain; Budhigandaki 1. Introduction Complex interactions between the atmospheric system and the underlying topography determine river discharge. It is a part of rainfall that appears in a stream and represents the total response of a basin. Surface flow, subsurface flow, base flow and precipitation that directly falls on the stream constitutes the total discharge in the river [1,2]. Time series of flow data is one of the most important requirements for planning, operation and control of all water resources projects [3–5]. However, measured flow data are not available in most of the cases in such project sites [5]. It is because of the lack of sufficient flow gauging stations in most river basins. The situation is more severe in mountainous basins [6] because of the inaccessibility of most of these sites for local observations. It is the reason why water budget analyses in such basins are not as easy as in other gauged basins [7,8]. However, most of the large rivers (e.g., the Ganga, the Indus, the Sutlej, the Brahmaputra, the Mekong, the Yellow) in the world originate from the mountains and are Water 2021, 13, 1546. https://doi.org/10.3390/w13111546 https://www.mdpi.com/journal/water