SIMULATIONS OF CRITICAL ZONE PROCESSES FOR GRANITIC TERRAINS IN SUBALPINE AND TROPICAL CLIMATES
Model construction began with the identification of sets of geochemical reactants for both watersheds, and literature-derived expressions were used to estimate rates of weathering of primary minerals as a function of temperature, pH, and dissolved oxygen. Secondary minerals were allowed to precipitate to equilibrium. Surface area to volume ratios for the minerals were calibrated to match average concentrations. Further calibration of hydrologic and geochemical parameters resulted in models that faithfully reproduce observed daily variations in discharge, pH, alkalinity, major ion concentrations, and stable isotopes for the two watersheds.
Results demonstrate that weathering rates, and corresponding precipitation of secondary minerals, in granitic terrain responds primarily to flow, temperature, and soil respiration. In Andrews Creek, the simulated weathering rates of feldspar in the unsaturated zone in the summer were four times the rates simulated in winter. The Río Icacos responded to storms with less relative variation, but absolute rates were much greater, exporting 800 milliequivalents of alkalinity per square meter per year (meq/m2/yr) compared to 30 meq/m2/yr exported from Andrews Creek.