Intra-event concentration-discharge relationships affected by hydrological connectivity in a karst catchment

Concentration-discharge (C-Q) relationship in streamflow provides insights into hydrological transport at the catchment scale. Changes in hydrological connectivity during runoff events often dominate flood and solute export in the karst catchment. However, only few studies have explored intra-event C-Q relationships and how they are affected by hydrological connectivity in karst catchment. In this study, we explored the intra-event C-Q relationships in underground channel flows by integrating solute concentration, discharge and modelled flow age in a 1.25 km(2) karst catchment in southwest China. We apply piecewise functions to characterize the C-Q relationships on rising and falling limbs of hydrograph. Geogenic solutes exhibited dilution C-Q patterns during the runoff event, which could be fitted by two power-law models with different coefficients on rising and falling limbs. Affected by the strong hydrological connectivity between surface and subsurface, hillslope and depression, a steeper C-Q slope on the rising limb indicated an exhaustible, proximal source, that is, groundwater. In contrast, the C-Q patterns of the soil enriched solutes changed from enrichment to dilution during the runoff event. The enrichment pattern occurring at the early of rising limb was caused by strong hydrological connectivity between the surface and subsurface, which indicated a distal and plentiful sources of soil water. Whilst the strong hydrological connectivity between hillslope and depression caused a dilution pattern at the latter of rising limb. On the falling limb of hydrograph, a dilution pattern implied that the small fractures could be another source zone of soil-enriched solutes in addition to soil layer in the depression. The C-Q behaviours of soil-enriched solutes can be fitted by a combination of a parabola model and a power law model for rising and falling limbs, respectively. The current study highlights the variations in intra-event C-Q relationships of different solutes affected by hydrological connectivity in the karst catchment. Which is crucial for assession of hydrochemical processes and fertilization management in this area.