Introduction

Sustainable water management in basins that supply drinking water requirements has been provided by applying protection management strategies about water quantity and quality. Anthropogenic activities such as agricultural, industrial, settlements have been tried to be under control by domestic legislations implemented by countries in their drinking water basins since these activities have threatened water quality and quantity. In Turkey, sustainable water management in drinking water basins has been ensured by the protective regulations surrounding the Regulation on Protection of Drinking and Utility Water Basins (PDUWB 2017). The legislation includes measures about land-use practices such as agricultural activities to take under control point and non-point pollution sources in basins. Although point pollution loads are easy to keep under control, it is not easy to keep the non-point source (NPS) loads under control for water resources decision-makers because non-point source pollution loads contain many uncertainties (Chen et al. 2017; Wang et al. 2018). The primary source of water quality degradation is NPS pollution rather than point source pollution (Mi et al. 2015). However, the water management policies prepared based on the legislation don’t cover land use/cover and climate change impacts on flow rate and NPS pollutants. Land use/cover and climate changes are the most important key factors that have effects on hydrological processes and NPS pollution loads in water resources (Wagner et al. 2017; Gashaw et al. 2018; Tamm et al. 2018; Zhan et al. 2020). Implementation of successful water resource planning and management is possible by understanding the hydrological response of watersheds to physical (land use) and climatic (precipitation and air temperature) changes (Vorosmarty et al. 2000; Anand et al. 2018). Although the “paired catchment” experimental method (Bosch and Hewlett 1982) and the time series analysis (Li et al. 2012) have been mainly used to assess the impacts of land-use change on water resources, hydrological models are the most popular method since it provides an integrated approach for the water resources management by studying the relationships between climate change, land-use/cover change, and the water cycle. The Soil and Water Assessment Tool (SWAT), a semi-distributed and physically-based watershed-scale hydrological model developed by Arnold et al. (1998), has been widely used among these models since SWAT calculates long-term impacts of land use, land management practices and buildup of pollutants with a continuous time model (Neitsch et al. 2005). SWAT has been applied to show the potential impact of land management scenarios and land-use/cover and climate change impacts on the hydrology to decide better management scenarios (Bouraoui et al. 2005; Mango et al. 2011; Shen et al. 2015; Abbaspour et al. 2015; Hajihosseini et al. 2019; Aboelnour et al. 2020).
This study aims to show the effects of climate and land-use/cover changes on hydrological processes and NPS pollutants to generate more accurate water management plans on land use measures in drinking water basins and help future water management planning. SWAT was carried out to predict climate and land-use impacts on flow rate and non-point pollutions (NPS). Two different climate change scenarios, RCP 4.5 and RCP 8.5, and land-use scenarios, conversion of shrubland to forest and conversion of agricultural areas to the forest, were used, and statistical analyses were utilized to assess the results. These impacts were shown on the Namazgah dam basin in Turkey to give an example of the application in the drinking water basin. The basin supplies drinking water to Kocaeli province in Turkey.