Assessing seasonal variations in water sources of streamflow in a temperate mesoscale catchment with granitic bedrocks using hydrochemistry and stable isotopes

Noseongcheon catchment is a mesoscale agricultural basin with crystalline bedrocks with seasonally variable precipitation and groundwater use. This study aimed to assess the influence of headwater streams on streamflow across the stream network in the study area under the temperate monsoonal climate conditions. Using hydrochemical parameters and stable isotopes in stream water and groundwater in the dry and rainy seasons, the water sources contributing to the stream network were identified and the seasonal changes in the streamflow system were evaluated. Seasonal variations of hydrochemical parameters and water stable isotopes indicated that the contributions of major water sources to streamflow in the catchment changed according to the seasonal precipitation pattern. Groundwater affected by agricultural activities discharges into the stream at the piedmont area from mountain to plain, irrespective of season. However, the contribution of uncontaminated mountain bedrock discharge (MBD) showed a marked difference between dry season and rainy season. The contribution of MBD ranged from 20% to 75% in the dry season, while it increased up to 90% in the rainy season. These results indicate that the responses of mountainous area to precipitation in the rainy season is a major factor affecting the hydrological functioning of the whole stream network. •We assess seasonal variations in water sources of streamflow in a mesoscale basin.•Groundwater inflow significantly increased with downstream distance in dry season.•Headwater streams exerted higher influence on the entire catchment in rainy season.•Streamflow can be highly susceptible to changes in seasonal precipitation patterns.