Relationship between catchment scale and the spatial variability of stream discharge and chemistry in a catchment with multiple geologies

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This study investigated whether the representative elementary area (REA) concept can be adopted in catchments with multiple geologies. We observed stream discharge at 65 points and water chemistry at 157 points in a 55 km2 catchment that included multiple geologies. At observation points with uniform geology, stream chemistry became constant beyond about 1 km2 in granodiorite and volcanic rocks. At observation points with multiple geologies, spatial variability remained large beyond a few square kilometers. SiO2 and Mg2+ concentrations became constant above 10 km2, but Ca2+ and electrical conductivity did not become constant until 55 km2. Our calculations revealed that for areas of 1–17 km2, almost all of the observed variables were explained by mixing based on geological percentages. However, at greater than 17 km2, the observed values were higher than the calculated values. Therefore, in regions with multiple geologies, the range of the REA with single-parameter geology was confirmed. In our catchments, the REA concept was applicable to areas of 1–17 km2, but areas larger than 17 km2 was outside the range.