Non-point Source Pollution Modeling Using Geographic Information System (GIS) for Representing Best Management Practices (BMP) in the Gorganrood Watershed

The most important pollutants that cause water pollution are nitrogen and phosphorus from agricultural runoff called Non-Point Source Pollution (NPS). To solve this problem، management practices known as BMPs or Best Management Practices are applied. One of the common methods for Non-Point Source Pollution prediction is modeling. By modeling، efficiency of many practices can be tested before application. In this study، land use changes were studied from the years 1984 till 2010 that showed an increase in agricultural lands from 516908. 52 to 630737. 19 ha and expansion of cities from 5237. 87 to 15487. 59 ha and roads from 9666. 07 to 11430. 24 ha. Using L-THIA model (from nonpoint source pollution models) for both land use categories، the amount of pollutant and the volume of runoff were calculated that showed high growth. Then، the seventh sub-basin was recognized as a critical zone in terms of pollution among the sub-basins. In the end، land use change was considered as a BMP using Multi-Criteria Evaluation (MCE) based on which a more suitable land use map was produced. After producing the new land use map، L-THIA model was run again and the result of the model was compared to the actual land use to show the effect of this BMP. Runoff volume decreased from 367. 5 to 308. 6 M3/ha and nitrogen in runoff was reduced from 3. 26 to 1. 58 mg/L and water BOD from 3. 61 to 2. 13 mg/L. Other pollutants also showed high reduction. In the end، land use change is confirmed as an effective BMP for Non-Point Source Pollution reduction.