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.