 Corresponding author: Fathia Jarray Copyright © 2023 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Assessment of runoff variability under various factors using SWAT model in a Tunisian semi-arid watershed Fathia Jarray 1, * , Taoufik Hermassi 1 , Mohamed Mechergui 2 and Mohamed Ali Ben Abdallah 1 1 Laboratory of Rural Engineering, National Research Institute of Rural Engineering, Water and Forests (INRGREF), Rue Hédi El Karray El Menzah IV-BP N 0 10, Ariana 2080, Tunisia. 2 Department of Rural, Water and Forest Engineering, National Institute of Agronomy of Tunisia (INAT), 43, Avenue Charles Nicolle, Tunis 1082, Tunisia. GSC Advanced Research and Reviews, 2023, 17(02), 075–086 Publication history: Received on 02 October 2023; revised on 12 November 2023; accepted on 15 November 2023 Article DOI: https://doi.org/10.30574/gscarr.2023.17.2.0430 Abstract This research aims to establish a correlation between prolonged shifts in precipitation patterns and runoff and to delineate spatial surface runoff distribution within the Wadi Rmel watershed using the Soil and Water Assessment Tool (SWAT) model. The calibration and validation processes reveal good agreement between measured and simulated flows with a monthly time step. Significantly, the Nash-Sutcliffe Efficiency (NSE) values of 0.61 and 0.83, accompanied by the coefficient of determination (R 2 ) values of 0.66 and 0.85 for calibration and validation, respectively. It clearly indicates a strong agreement between the observed and simulated streamflow data. Subsequently, an exploration of the spatiotemporal variability of runoff underscores the synchronous relationship between runoff and monthly and annual precipitation fluctuations between 2000 and 2020. The values of R 2 were 0.7 and 0.79 for monthly and annual studies, respectively, confirming a strong correlation between the two cited variables. Additionally, anthropogenic impacts, particularly Soil and Water Conservation (SWC) measures, were investigated. A high R² coefficients (0.91 for linear relationship) were obtained and showed the high relation between SWC and runoff. In fact, the combination of SWC techniques, vegetation cover, topography and precipitation led to a reduction of 44% of surface runoff, during 2015- 2020. Spatially, higher runoff occurred in central and eastern parts due to agriculture and hilly terrain, while forested areas and gentle slopes exhibited lower runoff. In conclusion, this study underscores the model's utility in characterizing arid catchment surface runoff. Its capacity to simulate and assess runoff enriches water resource management comprehension in such contexts. Keywords: Streamflow; Surface runoff; Rainfall; SWAT; Arid condition 1. Introduction Water Scarcity is a looming threat to life in developing countries in the 21st century. Water is the essence of life, and its scarcity [1], caused by the rising population and its mismanagement, represents a significant challenge to human survival and development. While the world faces numerous environmental concerns, water scarcity emerges as a pressing issue, particularly in developing countries. On the other hand, this problem can also be largely explained by hydrometeorological phenomena, such as the irregular distribution of precipitation and soil water content [2]. Therefore, it is important to understand the hydrological reactions of the catchment. Understanding the behaviour and responses of the watersheds requires the use of hydrological models [3]. These models were created and employed as mathematical representations of hydrological processes, aiming to enhance comprehension of how natural and anthropogenic disturbances affect hydrological features [4]. The study of streamflow modelling holds great importance for various aspects of water resource management and understanding hydrological processes [5]. Moreover, it is important to investigate the relationship between rainfall and runoff under various climate conditions [6]. Worldwide