Spatial patterns of stream solute concentrations in a steep mountainous catchment with a homogeneous landscape

The spatial patterns of stream solute concentrations in a catchment reflect its hydrology and biogeochemistry. The spatial relationship between drainage areas and solute concentrations may be asymptotic or convergent (large variation in small streams that becomes dampened as they merge into larger streams), probably depending on landscape features and/or solute type. Thus, we investigated the concentration spatial patterns of dissolved silica and major ions (Na+, K+, Mg2+, Ca2+, NO3−, SO42−, and Cl−) in steep, humid headwaters of a 4.27 km2 catchment on Tanakami Mountain, central Japan. Because the catchment had a nearly homogenous landscape of incised valley and granitic bedrock covered in forest, we could eliminate any potential effects of variable landscape features. We found convergent relationships between drainage area and solute concentrations for all solutes. The concentrations of all solutes were similar among sampling locations with an area of more than 0.1–1.5 km2 independent of variable biogeochemical property of measured solutes. When considering only the longitudinal profile, solute concentrations showed a mostly asymptotic pattern. Taken together, these results indicate that longitudinal profiles with asymptotic patterns form convergent solute spatial patterns and suggest that variation in zero‐order hollow discharges and the mixing of water from a variety of hillslopes control downstream chemistry.