Streamwater hydrochemistry in headwaters: mixing, inter catchment groundwater transfer, and instream processes

To test the understandings about the hydrochemical processes from one small catchment to other catchments, the streamwater chemistries in a 2-order catchment (5.99ha) and four 0- or 1-order subcatchments (0.086-1.75ha) were investigated with the mixing model approach. The catchment is underlain by granite. The total area of the four subcatchments corresponds to 49% of the area of the 2-order catchment. As the result, the streamwater chemistries in these catchments were explained in principle by the one mixing model. Based on the water budget, the evidence of inter catchment transfer of bedrock groundwater, which transferred beyond the topographically delineated watershed and was important for the water and chemical flux, between the four subcatchments was found. Comparing the water budget between the 2-order catchment and the four subcatchments, the total annual discharge from the subcatchments corresponded to 35% of that from the 2-order catchment. The concentrations of conservative tracer, Cl-, and the geochemical substances, SiO2 and Na+, did not changed through the instream processes in the 2-order catchment. The annual fluxes of these solutes from the subcatchments corresponded to 36, 33, and 37% of those from the 2-order catchment, respectively, and equivalent to the percentage of the discharge. On the other hand, the concentrations of biologically active substances, NO3-, Mg2+, and Ca2+, were decreased through the instream processes in the 2-order catchment. However, the concentrations of these solutes increased again at the outlet of the 2-order catchment, and annual fluxes of these solutes from the subcatchments only corresponded to 25, 26, and 14% of those from the 2-order catchment, respectively. There is no perennial spring other than the four subcatchments, and the water budget has been considered to balance at the 2-order catchment. Thus, as well as from the pathways detected in the subcatchments, the water and chemical flux from other pathway(s), which may from the slope side along the mainstream or from deeper bedrock layer, contributed in the 2-order catchment. To consider the hydrological scaling, it is important to clarify the role of these pathways.