The aim of this study is two folds. The primary aim of the study is to apply a detailed watershed model for the quantification of climate and land use change impacts on the hydrology and NPS pollution in the case study watershed in a developing country. Secondly, it aims to develop a pathway for researchers in developing countries that takes into account the data availability, cost of data collection and model performance to design a watershed modelling project tailor made to the specific modelling objectives based on a threshold level of data requirement, model results and cost, optimized for maximum benefit. In this study Better Assessment Science Integrating point and Non-point Sources (BASINS), and Hydrological Simulation Program-FORTRAN (HSPF), are used for the characterization of watershed hydrology and diffuse pollution and quantification of the effects of land use and climate change on the hydrologic regime and NPS pollutant loads in the Koycegiz watershed, Turkey. Based on the Weight of Evidence (WoE) analysis; Best Professional Judgment (BPJ) in modeling experience and literature review are used to define the relationships between Data, Cost and Model Performance Index and objective evidences are put forth to develop a hypothesis incorporating an application scenario for BASINS/HSPF application in a developing country identifying the minimum data requirement, corresponding relative cost and acceptability of model results, respectively.