The effect of reforestation on stream flow in Upper Nan river basin using Soil and Water Assessment Tool (SWAT) model

Forests are an important natural resource, vital to all life. Forests are an important source of food, clothing, and medicines, as well as a place to live. Water released from forests drains into the soil causing groundwater to emerge as stream flow throughout the year. In Thailand, most forests have been encroached by logging, paper production, and housing construction. Population growth and the need for farming area for crop and livestock production have also caused forest encroachment. Technical tools are needed to support the decision makers and planners if they are to achieve objectives of water conservation, and development. These technical tools are needed for assistance in the engineering, socio-economic, and environmental planning. The Soil and Water Assessment Tool (SWAT) was used in the hydrological modeling in this study of the complex and dynamic problems of The Upper Nan river basin. This was a case study to evaluate the impact of changing conditions in the river basin affected by the stream flow due to reforestation. The watershed area was delineated into 5 sub-watersheds based on surface topography provided by the Digital Elevation Model (DEM) and the parameters of each of these watersheds were calculated. The land use data was processed and reclassified to match with the SWAT model land use code. Ten different categories of land use in the study area were used for SWAT processing. Types of land use consist of: mixed forest (33. 7%), disturbed forest (30. 2%), evergreen forest(17. 7%), paddy field(7. 1%), orchard(3. 7%), range brush(2. 7 %), field crop(1. 7%), planted forest (1. 7%), urban (1. 4%) and water resources (0. 4%) . Twenty-two types of soil were found in the basin. The initial curve number values were assigned based on the land use type and soil hydrologic group for the average antecedent moisture condition of the curve number method. The potential evapotranspiration was computed using the Penman-Monteith method. The simulation was performed using three reforestation scenarios to assess stream flow:(1) improved disturbed forest, (2) field crops and range grass, and (3) both disturbed forest and field crops. The results of reforestation from scenarios 1 and 3 can increase stream flow in the drought season and can also reduce the flow in the wet season in the main stream and its tributaries. For scenario 2 Reforestation had no significant effect on the main stream