Monsoonal affected dynamics of nitrate and dissolved organic carbon in a mountainous catchment under intensive land-use
In recent decades, complex mountainous landscapes have been increasingly under pressure by deforestation and intensified highland agriculture. This trend not only compromises the quality of water in the highlands, but also impacts the availability of water resources downstream. Hence, an effective and sustainable management of these mountainous regions is essential and needed to mitigate this risk. Developing sustainable management principles to ensure the freshwater supply, however, requires a profound understanding of decisive factors and processes that control the water quality in mountainous landscapes. Amongst other substances, nitrate and dissolved organic carbon (DOC) play a critical role in the ecosystem health of water bodies. The major focus of this dissertation is on determining the nitrate and DOC mobilization processes and dynamics in the Haean Catchment, a mountainous watershed located in South Korea, which is agriculturally productive and strongly influenced by the prevailing East-Asian monsoon. The primary objective was to identify decisive factors that control the nitrate and DOC mobilization processes and dynamics in such catchments. To these ends, we conducted stream water quality and discharge measurements during a range of conditions, from monsoonal rainfall events to dry, baseflow conditions. Assessments were completed along the topographic elevation gradient of the catchment, from an upland deciduous forest, over areas intensively used for agriculture, down to the catchment outlet. In order to gain a better understanding of nutrient fate within the Haean Catchment, we investigated river-aquifer exchange fluxes. In addition to hydraulic gradient monitoring, we used heat as tracer. To resolve the river-aquifer exchange fluxes, we set up a two-dimensional flow and heat transport model using the numerical code HydroGeoSphere (HGS). Potential effects of river-aquifer exchange dynamics on local water quality were investigated by collecting both, river and groundwater samples. Furthermore, we determined the impact of the ridge and furrow cultivation that is commonly applied in South Korean dryland agriculture, on nitrate leaching and evaluated fertilizer best management practices (FBMPs) using a three-dimensional flow and solute transport model (HGS). Our results show that DOC and nitrate sources as well as their mobilization differ between the forest and agricultural river sites. In the forest, elevated in-stream DOC concentrations during rainfalls 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 stream occurred predominantly along interflow transport pathways. At the agricultural sites, in-stream DOC concentrations were considerably higher and supplied from adjacent rice paddies. The highest in-stream nitrate concentrations occurred in the lower agricultural part of the catchment. Through hydraulic gradient monitoring, we identified in this part of the catchment a distinct connection between the river and aquifer, and nitrate-rich groundwater inputs to the river elevated the in-stream nitrate concentration. In the mid-elevation portion of the catchment, however, a limited connectivity between the river and aquifer was identified and river water quality was consequently unaffected in these areas. The results of our HGS modeling study show a high temporal and spatial variability in river-aquifer exchange fluxes with frequent flow reversals during the monsoon season. Our results also suggest that these flow reversals may enhance the natural attenuation of nitrate in the shallow groundwater below the stream. The simulation results on evaluating FBMPs demonstrate that applying a combination of several FBMPs such as an adapted placement and timing is recommended to minimize the risk of groundwater nitrate contamination. Overall, this dissertation demonstrates that the hydrologic pathways resulting from the monsoon climate drive the in-stream DOC dynamics in the forested catchment, whereas sources and mobilization of DOC in downstream agricultural areas are mainly controlled by the specific land-use type. In particular, the widely used rice paddy “plot-to-plot” irrigation system in Korean highlands was shown to control in-stream DOC concentrations. Nitrate dynamics in streams and aquifers in the agricultural areas of the catchment reflect the combined effects of land-use type and monsoonal hydrology. Particularly, the highly variable river-aquifer exchange fluxes with frequent flow reversals, which may enhance the nitrate attenuation, are driven by monsoonal extreme precipitation events. Since it has been forecasted that global warming will increase the frequency and the intensity of extreme precipitation events also in other regions of the world, our results will become increasingly important in future water quality assessments and research.
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