An Improved Understanding of Ecohydrological and Geochemical Functioning of a Mountainous Site Using Multiple Methods and Multiple Tracers

Abstract

High elevation mountainous catchments are often important sources of water for downstream arid and semi-arid basins. However, a better understanding of the ecohydrological and geochemical functioning of these mountainous systems is presently limited due to sparse observations at these sites which experience extreme climatic and topographic gradients. Therefore, high density observations are needed to improve our understanding of water sources and the temporal and spatial behavior of flow paths in mountainous ecosystems. This study utilizes multi-year observations of hydrologic fluxes, storages, conservative tracers, chemical compositions and water residence times from a mountainous site located within the Santa Catalina Mountains Critical Zone Observatory (SCM-CZO), Tucson, Arizona, to first develop and evaluate competing conceptual models of seasonal streamflow generation. A conceptual model involving four endmembers (precipitation, soil water, shallow and deep groundwater) provided the best match to observations. Subsequently, the long-term isotopic and hydrometric observations for various potential source waters were used to identify sources of water that support mountainous ecosystems during both wet and dry seasons. It was found that the sources of water for both streamflow and vegetation water demand was the same, i.e., there was no evidence supporting the ecohydrological water source separation hypothesis for the selected field site. Finally, improved practical methods for estimating catchment-scale response functions such as transit time distribution (TTD) and evapotranspiration time distribution (ETTD) are presented. The proposed methods are resistant to gaps in tracer time series. It was found that a gamma type TTD best matches the observations when using either very short (e.g., δ18O) or relatively older groundwater age tracers (e.g., 3H). Furthermore, the estimated ETTD type for the field site showed a composite ETTD type with a piston flow type for low periods (<0.1 year) and a gamma type with long-tails for higher periods. Therefore, this work improves understanding of hydrologic structure and function of the CZ in a mountainous catchment in a sub-humid setting.