The contributions and composition of baseflow sources across an extended recession period were quantified for six subwatersheds of varying size in a structurally complex watershed in coastal California using endmember mixing analysis and related to catchment characteristics (e.g., topography, geology, land use, and soil characteristics). Both shallow subsurface and deep groundwater reservoirs were important contributors for streamflow during low flow periods, and the composition of baseflow sources across subwatersheds was directly related to geologic indices. A binary classification of underlying bedrock permeability (e.g., low vs. high) best explained the changes in shallow subsurface water and deeper groundwater inputs through the seasonal recession. Dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and specific UV absorbance at 254 nm (SUVA254) were used to provide additional insight into endmember characteristics and their contributions to baseflow. Stream water DIC concentrations were broadly controlled by mixing of groundwater and shallow subsurface water endmembers with relatively constant DIC concentrations, while stream water DOC concentrations reflected both spatial and temporal changes in shallow subsurface water DOC. Results from this study show (1) the importance of considering baseflow as a dynamic mixture of water from multiple sources, (2) the effect of geology on source composition at the subwatershed scale during low flow conditions, and (3) the impact of shifting baseflow sources on stream water dissolved carbon concentrations and the utility of using dissolved carbon concentrations to obtain additional insight into temporal variability in baseflow sources.
High‐elevation mountain catchments are often subject to large climatic and topographic gradients. Therefore, high‐density hydrogeochemical observations are needed to understand water sources to streamflow and the temporal and spatial behaviour of flow paths. These sources and flow paths vary seasonally, which dictates short‐term storage and the flux of water in the critical zone (CZ) and affect long‐term CZ evolution. This study utilizes multiyear observations of chemical compositions and water residence times from the Santa Catalina Mountains Critical Zone Observatory, Tucson, Arizona to develop and evaluate competing conceptual models of seasonal streamflow generation. These models were tested using endmember mixing analysis, baseflow recession analysis, and tritium model “ages” of various catchment water sources. A conceptual model involving four endmembers (precipitation, soil water, shallow, and deep groundwater) provided the best match to observations. On average, precipitation contributes 39–69% (55 ± 16%), soil water contributes 25–56% (41 ± 16%), shallow groundwater contributes 1–5% (3 ± 2%), and deep groundwater contributes ~0–3% (1 ± 1%) towards annual streamflow. The mixing space comprised two principal planes formed by (a) precipitation‐soil water‐deep groundwater (dry and summer monsoon season samples) and (b) precipitation‐soil water‐shallow groundwater (winter season samples). Groundwater contribution was most important during the wet winter season. During periods of high dynamic groundwater storage and increased hydrologic connectivity (i.e., spring snowmelt), stream water was more geochemically heterogeneous, that is, geochemical heterogeneity of stream water is storage‐dependent. Endmember mixing analysis and3H model age results indicate that only 1.4 ± 0.3% of the long‐term annual precipitation becomes deep CZ groundwater flux that influences long‐term deep CZ development through both intercatchment and intracatchment deep groundwater flows.
more » « less- PAR ID:
- 10453452
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Hydrological Processes
- Volume:
- 33
- Issue:
- 4
- ISSN:
- 0885-6087
- Page Range / eLocation ID:
- p. 476-494
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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