Based on Great Soil Group and combination of soil properties, soil depth is defined. After having on the Great Soil Group and depth, percentage of clay, sand and silt amount are defined according to determination soil properties study of Ardas and Creutberg (1995) in Turkey. Soil texture is determined by using soil texture triangle (Ley et al. 1994). The USLE equation soil erodibility (K) factor (USLE_K) was established with respect to the soil textures. The value of USLE_K is between 0.1 and 0. A value of < 0.02 shows a soil of low erodibility; 0.02 - 0.04 shows moderate erodibility; and > 0.04 shows high erodibility. When the silt content of soil type increases, it can become more erodible regardless of whether there is a comparable reduction in the sand or clay fraction (Rosewell 1993). The soil bulk density (SOL_BD) values employed in SWAT database are defined based on soil texture groups by using Guides for Editing Soil Properties (2005). The soil available water capacity (SOL_AWC) has been defined by using soil texture groups, as described in Ley et al. (1994). The soil hydraulic conductivity (SOL_K) has been determined based on soil texture group, as suggested by Guidelines for Soil Description (Jahn et al. 2006) (Fig. 1c). After these processes, SWAT-relevant soil database of the basin was generated.

Three different data, land-use/ land cover map produced by Kocaeli Buyuksehir Municipality, crop cultivation data of Kocaeli Provincial Directorate of Agriculture, land-use/ land cover created by the Ministry of Agriculture and Forestry within the framework of the STATIP project, were used to generate the land-use/ land cover map of the study area due to having some challenges about these data such as definitions of land use/land cover, lack of crop pattern, etc. The project data map was generated by using SPOT satellite data with 5 and 2.5-meter resolution on the 1/25000 topographic map. Although the spatial resolution of STATIP project land use/landcover map is good, the definition of absolute irrigated agricultural areas, dry absolute agricultural areas, marginal irrigated agricultural areas and dry marginal agricultural areas cause confusion about agricultural products in the basin. These definitions were made based on the framework of the Regulation on the Conservation, Use and Planning of Agricultural Lands (Official Gazette 2017 no: 30265). Marginal agricultural land means private croplands and planted lands where traditional tillage agriculture cannot be carried out due to soil and topographic limitations. “Absolute” agricultural land means that the land is currently suitable for agricultural production in terms of the physical, chemical, and biological properties of the soil, non-limiting, no or very little topographic restrictions. In addition, irrigated agricultural land is defined as that the land where additional water needed by the cultivated plants during the growing phase. Although land-use/land cover map produced by Kocaeli Buyuksehir Municipality includes just hazelnut and agricultural areas, there is no specific definition about crop production in the agricultural areas. Crop cultivation data of Kocaeli Provincial Directorate of Agriculture is represented by points instead of areas based on villages in the basin as ton/year production. Thus, the land-use/cover map was produced by combining these data in GIS (Geographic Information System) (Fig. 2). Based on the produced map, there are mostly wheat and evergreen broadleaf forest areas. Hazelnuts are not in SWAT database, so these crops are defined as shrubland since hazelnut is in the shrubland flora.

NCEP/CFSR (The National Centers for Environmental Prediction/Climate Forecast System Reanalysis) data were used to obtain meteorological data of the study area. These data have advantage that they come along with in situ measurements from several ground stations. Meteorological data handled from the station near the study area is between 1979 and 2014 years in the daily time step.

There are 3768 sheep and 3006 dairy cattle in the Namazgah Dam basin based on TUIK data that was obtained from villages in the basin (Fig. 3). The amount of manure caused by poultry, cattle, and sheep were calculated using coefficients used for animal pollution loads as suggested by Uttormark et al. (1974). Animal-based fertilizer calculations were made for villages for three different animal categories: poultry, cattle, and sheep. The average coefficients for cattle were determined for beef and dairy cattle to calculate manure since cattle are not divided into precisely beef and dairy cattle in the basin.

Wheat cultivation is carried out in many different soil types. The pH of the soil is very important in terms of plant nutrition and fertilization. The best development environment for wheat plants is in soils with neutral pH (6.6-7.3). In addition, one of the most significant factors affecting wheat yield is the amount of precipitation and the distribution of precipitation during the growing period. Thus, the amount of nitrogen fertilizer per decare (da) during the development period of the wheat should be given based on these factors. Top fertilization in wheat cultivation is carried out in October or before and in the spring. 26 kg/da nitrogen, 8.5 kg/da phosphorus, and 0.96 potassium kg/da are given in October or before to make balanced fertilization in wheat and barley cultivation in the basin. Fertilization for the hazelnut cultivation is carried out as 2.7 kg/m² nitrogen, 0.8 kg/m² phosphorus and 0.09 potassium kg/m² on March. The fertilizer amount applied to each crop in the basin was determined by considering the Kocaeli Agriculture Provincial Directorate Fertilizer guideline.