Factors influencing water transit times in snowmelt-dominated, headwater catchments of the western U.S.

Abstract:

In catchments, water transit times (TTs) refer to the elapsed time between entry of water at the ground surface and exit of water at the catchment outlet. Transit times are an important characteristic of catchments in that they reflect the time available for interaction between water, soil, and biota within the system. Thus, they exert a strong influence on hydrologic resilience to drought and climate change, and on the sensitivity of aquatic ecosystems to atmospheric pollutants. Transit times may vary spatially due to variations in basin characteristics, such as slope, size, and amount and type of soil and vegetation; however, the relative influence of these factors on TTs is poorly known. In this study, we estimate mean transit times (MTTs) for 11 snowmelt-dominated, headwater catchments in the western U.S. using the convolution integral approach, which relies on differences in the magnitude of seasonal variability in δ¹⁸O in precipitation and stream water to estimate MTTs. Seasonal variability in δ¹⁸O was calculated based on analyses of precipitation and stream water samples collected at weekly to monthly intervals. Results indicate that MTTs ranged from 0.6 to 2.1 years, and were positively influenced by percent of the catchment covered by forest (r² = 0.56; p = 0.008), and negatively influenced by barren terrain (e.g., bedrock; r² = 0.48; p = 0.019). MTTs showed a weak negative relation to mean basin slope (r² = 0.31; p = 0.076) and no relation to basin size or elevation. These results illustrate the importance of soil as a key factor influencing MTTs, with basin slope acting as a secondary influence. Heavily forested basins tend to have deep, well-developed soils with substantial water storage capacity; these soils help maintain baseflow during drought conditions, providing hydrologic resilience to the system, and they are an important location for geochemical and biological processes that neutralize acidity and assimilate atmospherically deposited nitrogen. In contrast, basins with abundant exposed bedrock and steep slopes, and that lack substantial forest, tend to have little soil or water storage capacity; these basins will be less resilient to drought, and atmospherically deposited pollutants will pass quickly through the terrestrial landscape and into streams and lakes where they may impact aquatic biota.