Stream chemistry modeling of two watersheds in the Front Range, Colorado

We investigated the hydrologic, geochemical, and biogeochemical controls on stream chemical composition on the Green Lakes Valley and Andrews Creek wa tersheds using the alpine hydrochemical model (AHM). Both sites had compara ble data sets from 1994 and 1996, including high-resolution spatial data an d high-frequency time series of hydrology, geochemistry, and meteorology. T he model of each watershed consisted of three terrestrial subunits (soil, t alus, and rock), with the routing between the subunits determined by spatia l land cover data. Using 1994 data for model calibration and 1996 data for evaluation AHM captured the dominant processes and successfully simulated d aily stream chemical composition on both watersheds. These results confirm our procedure of using spatial and site-specific field and laboratory data to generate an initial catchment model and then calibrating the model to ca lculate effective parameters for unmeasured processes. A net source of nitr ogen was identified in the Andrews Creek watershed during the spring snowme lt period, whereas nitrogen was immobilized in the Green Lakes Valley. This difference was most likely due to the larger and more dominant area of tal us in the Andrews Creek watershed. Our results also indicate that routing o f snowmelt through either soil or talus material is sufficient for retentio n of H+ and release of base cations but that N retention is more important on areas mapped as soil. Owing to the larger ionic pulse and larger fractio n of surface runoff the Green Lakes Valley was more sensitive to a doubling of wet deposition chemistry than the Andrews Creek watershed.