Monsoonal-type climate or land-use management: Understanding their role in the mobilization of nitrate and DOC in a mountainous catchment

• DOC dynamics in 1st order forested streams controlled by monsoonal climate.
• DOC dynamics in higher order agricultural streams driven by rice paddy water inputs.
• Monsoonal climate controls river-aquifer exchange dynamics.
• Frequent hydraulic gradient reversals are specific to the monsoonal climate.

SummaryThe linkage between hydrologic dynamics and the delivery of nitrate and DOC (dissolved organic carbon) to streams was studied in the Haean catchment, a mixed land-use mountainous catchment in South Korea. Three monsoonal precipitation events were analyzed, which varied in total rainfall amount (39–70 mm) and intensities (mean: 1.6–5.6 mm h−1), by high-resolution (2–4 h interval) stream water-quality sampling along the topographic elevation gradient of the catchment, from an upland deciduous forest stream, over areas intensively used for agriculture (dryland farming and rice paddies) down to the catchment outlet. The dynamics of river-aquifer exchange were investigated at two piezometer transects at mid and lower elevations. DOC and nitrate sources and their transport pathways to the receiving surface waters differed between the forested and the agricultural stream site. In the forest stream, elevated DOC concentrations (max: 3.5 mgC l−1) during precipitation events were due to hydrologic flushing of soluble organic matter in upper soil horizons, with a strong dependency on pre-storm wetness conditions. Nitrate contributions to the forested stream occurred along shallow subsurface transport pathways. At the agricultural sites stream DOC concentrations were considerably higher (max: 23.5 mgC l−1) supplied from adjacent rice paddies. The highest in-stream nitrate concentrations (max: 4.1 mgN l−1) occurred at river reaches located in the lower agricultural part of the catchment, affected by groundwater inputs. Groundwater nitrate concentrations were high (max: 7.4 mgN l−1) owing to chemical fertilizer leaching from dryland fields forced by monsoonal rainfalls.Overall, this study demonstrates that the hydrologic dynamics resulting from the monsoonal climate drive the in-stream DOC dynamics in the forested 1st-order catchment whereas sources and mobilization of DOC in downstream agricultural areas are mainly controlled by the prevailing land-use type and irrigation management. Nitrate dynamics in higher order agricultural streams and their connected aquifers reflect combined effects of land-use type and monsoonal hydrology.