Materials and Methods

The method used in this study: (1) generating SWAT of the basin, (2) the model calibration by using the FACT (Fast Auto Calibration Tool; Ozdemir and Leloglu 2018) (3) examining the effects on climate change water resources in the basin by using the Representative Concentration Pathways, RCP 4.5, and RCP 8.5 climate change scenarios, (4) predicting the effects of climate change on streamflow rate and NPS pollution loads by using different land-use scenarios; conversion of shrubland to the forest, conversion of agricultural areas to the forest, and (5) the assessment of the results by using statistical analysis (6) determining the effects of climate and land-use changes on water resources in the basin to provide further research on the subject in the future.

Study Area

The Namazgah dam is between 40055’-41004’ north longitude and 30000’- 30025’east latitude in the Marmara region, Turkey (Fig. 1). The dam was constructed on the Namazgah river to provide water demands such as irrigation, drinking, etc., of villages and towns of Izmit and Korfez districts and tourist settlements at the Black Sea coast. The area of the basin is 100.64 km2. The land in the basin is predominantly used for agriculture activities, especially for hazelnut production and pasture in the basin. The recorded mean annual rainfall, evaporation, maximum and minimum temperature values are 805 mm/a, 744.3 mm/a, 250C and 5 0C, respectively. The mean annual flow rate of the Namazgah river is measured as 48.88 hm3. The planning amount of water to be supplied as drinking and domestic water from the Namazgah Dam of 2014, 2030, and 2045 is 8.695.399, 11.300.658, and 12.965.044 m3/year, respectively.

SWAT

The primary elements of the SWAT model are weather, hydrology, soil temperature and properties, plant growth, nutrients, pesticides, bacteria and pathogens and land management. In SWAT, sub-basins are generated by dividing the basin based on the topography. Hydrologic response units (HRUs) for each sub-basin, the smallest unit in the model, are created from equivalent combinations of land use and soil type. The calculation of hydrologic processes is made at the HRU level and happens in two steps: (1) calculating flow and sediment, nutrients, bacteria, and pesticides loads at each HRUs, and then combining area‐weighted HRU‐level loadings to the sub-basin level; and (2) routing loadings from each sub-basin through the channel/stream network (Gassman et al. 2007).