[AHW33-03] Contrasting patterns in the decrease of spatial variability with increasing catchment area between stream discharge and water chemistry
Keywords:Spatial variability , Stream discharge , Stream water chemistry, Catchment area
1) How does the rate of decrease in spatial variability with increasing catchment area differ among catchments and for stream discharge and water chemistry?
2) What type of spatial structure causes different rates of decrease in spatial variability with increasing catchment area?
We observed stream discharge and water chemistry in multiple points within three forested headwater catchments and investigated differences in this decrease of variability with increasing catchment area among catchments, and among specific discharge (Qs) and water chemistry parameters. We quantified the slope of the decrease in the variability with increasing catchment area as the rate of decrease in the standard deviation and coefficient of variation (δSD and δCV, respectively), both of which are −0.5 for the simple mixing of random variables (random mixing). Then, we defined two indices to evaluate the spatial structure of the parameters. One was “relative difference”, which referred to the ratio (in percentage) of the parameters' mean values between low- and high-order streams. The other was “D200”, which indicated the dissimilarity of parameter values within sub-catchments.
All δSD and δCV values of Qs were less than −0.5, while those of most water chemistry values were greater than −0.5, indicating that with increased catchment area the spatial variability of Qs decreased more steeply than for random mixing, while for water chemistry they decreased less steeply. δSD and δCV had positive linear relationships with the relative difference and negative linear relationships with D200. It suggested that differences in δSD or δCV for Qs and water chemistry can be explained by the different spatial structures, where dissimilar values of Qs and similar values of water chemistry, respectively, are located close together in space. Differences in δSD and δCV according to Qs and water chemistry should significantly affect the determination of representative elementary area (REA), and therefore need to be considered when predicting REA from spatial variability of low-order streams.