Understanding uncertainties when inferring mean transit times of water trough tracer-based lumped-parameter models in Andean tropical montane cloud forest catchments

摘要

Weekly samples from surface waters, springs, soil water and rainfall werecollected in a 76.9 km2 mountain rain forest catchment and itstributaries in southern Ecuador. Time series of the stable water isotopesδ18O and δ2H were used to calculate mean transittimes (MTTs) and the transit time distribution functions (TTDs) solving theconvolution method for seven lumped-parameter models. For each model setup,the generalized likelihood uncertainty estimation (GLUE) methodology wasapplied to find the best predictions, behavioral solutions and parameteridentifiability. For the study basin, TTDs based on model types such as thelinear–piston flow for soil waters and the exponential–piston flow forsurface waters and springs performed better than more versatile equationssuch as the gamma and the two parallel linear reservoirs. Notwithstandingboth approaches yielded a better goodness of fit for most sites, but withconsiderable larger uncertainty shown by GLUE. Among the tested models,corresponding results were obtained for soil waters with short MTTs (rangingfrom 2 to 9 weeks). For waters with longer MTTs differences were found,suggesting that for those cases the MTT should be based at least on anintercomparison of several models. Under dominant baseflow conditions longMTTs for stream water ≥ 2 yr were detected, a phenomenon also observedfor shallow springs. Short MTTs for water in the top soil layer indicate arapid exchange of surface waters with deeper soil horizons. Differences intravel times between soils suggest that there is evidence of a land useeffect on flow generation.