## 1 Introduction
The performance of the SWAT model to simulate the streamflow and water balance was reliable when calibrated with streamflow [24]. [PERSON], [PERSON] [25] found that statistical downscaling (SD) is the most predominant uncertainty source in projections of extremely high flow and has a considerable contribution to the uncertainty in monthly streamflow over the Xijiang River Basin. [PERSON], [PERSON] [26] determined that changes in the discharge time series were identified by applying the Mann-Kendall test and Pettitt test for abrupt changes in the Pearl River Basin. They maintained that the annual water discharge had been significantly influenced by the precipitation variability and that the introduction of reservoirs at the Upper Xinjiang River Basin had little influence on water discharge. Unlike previous studies, [PERSON], [PERSON] [27] compared a physically-based distributed model, Liuxihe (LXH), and a lumped conceptual model, Xinanjiang (XAJ), to assess the impact of climate change on streamflow using one global climate model BCC-CSM-1.1 under two future scenarios, representative concentration pathway (RCP)-4.5 and RCP-8.5, over the Xijiang River Basin. They found that the distributed model generated a high magnitude and correctly simulated higher peaks than the lumped model. Hydrological response to climate change in the Tarim Basin, China, under the mean conditions of four RCPs, showed a decrease of 21.5-40% in annual mean runoff [28]. Many studies have shown interest in analyzing the chain of uncertainties--from the general circulation models (GCMs) over all four emission scenarios (RCPs) to hydrological models--in order to improve the reality and robustness of future projections [28; 29]. The Xijiang River Basin is the main tributary of the Pearl River Basin, which lies in the sub-tropical region of South China. The Pearl River Basin is the third largest river basin of China, serving the demands of more than 100 million people. Since 1990, the Xijiang River Basin observed frequent flooding disasters due to heavy storm events [30]. The Xijiang River Basin has not undergone rapid land-cover changes historically; however, government reforms in the early 1980s, which aimed to enhance the economic growth and improve the living standards in the area, yielded an urbanization rate of 300% and reflected notable impacts over the area's hydro-climatology [31; 32]. [PERSON], [PERSON] [33] presented evidence on the rapid urbanization effect. An increase in temperature of 0.05 \({}^{\circ}\)C per decade was recorded because of urbanization in mainland China. Many researchers have analyzed the impact of climate change on water resources in the Xijiang River Basin. However, to the best of our knowledge, no study has reported a comprehensive evaluation of the impacts of climate change on streamflow and water yield using several global climate models (GCMs) under all four future scenarios. [PERSON], [PERSON] [34] summarized that the climate plays a key role in changing basin hydrology and streamflow in the Xijiang River Basin. This study aimed to assess the effects of future climate change on streamflow and water yield over the Upper Xijiang River Basin. The Soil and Water Assessment Tool (SWAT) was used to simulate streamflow using global climate models (GCMs) under the future scenarios of all four representative concentration pathways (RCPs). The uncertainty in runoff projections related to GCMs and land use land cover (LULC) scenarios were also investigated. Finally, the projections in streamflow and water yield were revealed. This study was intended to serve as a contribution to the literature and to assist decision-makers, policy formulators, and so on.