9. Assessment of Stream Flow Impact on Physicochemical Properties of Water and Soil in Forest Hydrology Through Statistical Approach

Forests affect the hydrology of watersheds in various ways with different hydrological parameters such as evapotranspiration, infiltration, intercepting cloud moisture, surface runoff, and groundwater. Sikkim, a part of northeastern India, a hilly area, is basically a critical terrain condition, and collection of secondary and primary data might be a challenging task to assess overall impacts, particularly on hydrological services. Three micro-watersheds, categorized as undisturbed, semidisturbed, and disturbed catchments covering a distance of 12 km at Geyzing, located in West Sikkim, were selected for this research work. Hydro-meteorological instruments were installed and recorded data for 4 years in three micro-watersheds to monitor the impact of forest cover on stream discharge and water quality. A composite rectangular weir was installed at field level to accommodate measuring stream discharge during the monsoon period as well as the pre-monsoon period. Water and soil quality parameters were analyzed during the monsoon as well as the pre-monsoon period. The overall impact of forests on the hydrological regime of a watershed is twofold. First, comparatively less discharge from a forest watershed during the rainy months shows that the forest controls excess runoff downstream. Second, forests induce infiltration, which leads to more uniform flow all year from the dense forest watershed stream. Conversions to forestland would have the potential to reduce erosion and subsequent sedimentation, as well as reduce levels of nutrients and pesticides in surface runoff and groundwater. These improvements in water quality can be a function of lower amounts of runoff and leaching as well as lower concentrations of potential pollutants.