SIMULATIONS OF CRITICAL ZONE PROCESSES FOR GRANITIC TERRAINS IN SUBALPINE AND TROPICAL CLIMATES

The Water, Energy, and Biogeochemical Model (WEBMOD) was used to simulate daily variations of chemical weathering, isotopic fractionation, and hydrology for two research watersheds in the U.S. Geological Survey’s Water, Energy, and Biogeochemical Budget (WEBB) program. Andrews Creek drains 1.74-km² of granite at the continental divide in Rocky Mountain National Park in Colorado. The Río Icacos drains 3.26-km² of granodiorites in the tropical rain forest in northeast Puerto Rico. For both watersheds, approximately one third of the precipitation returns to the atmosphere as evapotranspiration. The annual average ionic strength of rain and snow falling on Andrews Creek is approximately 27 microequivalents per liter (µeq/L). Streamflow at the outlet averages 83 µeq/L, sustantially more dilute that the salty rainwater that feeds the Río Icacos, which averages 106 µeq/L. The annual total of almost five meters of rain, along with the geology, heat, and root-zone respiration in the rain forest increases the ionic strength in the Río Icacos to 360 µeq/L, resulting in one of the highest weathering rates in the world.

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.