PREDICTABILITY OF GEOCHEMICAL BUFFERING AND RUNOFF ACIDIFICATION IN SPATIALLY HETEROGENEOUS CATCHMENTS

Catchments are more spatially variable than catchment models assume th em to be. Does spatial heterogeneity undermine the validity of spatial ly aggregated catchment geochemical models? Here we test whether catch ment spatial heterogeneity invalidates a recently proposed technique t hat uses the bulk chemistry of runoff to predict how key water quality variables (e.g., titratable alkalinity and concentrations of base cat ions, hydrogen ions, and inorganic aluminum) will respond to changes i n stream water in acid anion concentrations. We show that these predic tions are valid for hypothetical heterogeneous catchments, created by mathematically ''mixing'' measured runoff chemistry data from chemical ly diverse streams. The chemical information in the combined runoff yi elds reliable acidification predictions, without any direct informatio n concerning either the runoff chemistries of the individual source st reams or the geochemical properties of their catchments. These results show that catchment acid buffering can be predicted directly from run off chemistry in spatially heterogeneous catchments, even if the indiv idual source regions cannot be identified and their geochemical charac teristics are unknown.