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

Soil and Water Conservation Techniques in Tropical and Subtropical Asia: A Review

Soil and water loss is a severe environmental problem in tropical and subtropical Asia (TSA). This review systematically summarizes the techniques that have been widely applied in the TSA region and compares the conservation efficiency of these techniques based on the runoff and sediment reduction ratios (ηr and ηs). The results show that the current techniques can be divided into biological, engineering and agricultural practice measures, and in most cases, their efficiencies in reducing sediment loss (ηs = 14.0–99.5%, 61.3–100.0% and 0.6–95.4%, respectively) were higher than in reducing runoff loss (ηr = 2.8–9.38%, 0.28–83.3% and 1.62–70.2%, respectively). Monocultures of single tree species (e.g., Pinus massoniana) sometimes showed very limited conservation effects. Vetiver and alfalfa were more effective at reducing soil loss than other hedgerow species. Contour tillage, ridge farming, and reduced tillage generally showed high efficiencies in reducing soil loss compared with other agricultural practice measures. The combination of engineering and biological techniques could more effectively reduce soil and water loss compared with the application of these techniques along. Future works should be conducted to build unified technical standards and reasonable comprehensive evaluation systems, to combine these techniques with environmental engineering technologies, and to develop new amendment materials