Project at a Glance

Title: Development of watershed models for Emerald Lake watershed in Sequoia National Park and for other lakes of the Sierra Nevada

Principal Investigator / Author(s): Sorooshian, Soroosh

Contractor: Department of Hydrology and Water Resources, University of Arizona

Contract Number: A732-035

Research Program Area: Ecosystem & Multimedia Effects

Topic Areas: Acid Deposition, Ecosystem Impacts


Lakes in alpine watersheds in the Sierra Nevada are susceptible to damage by acid deposition due to their generally low buffering capacity and long response time for neutralization to take place. Changes from both repeated, long-term acidic fluxes and intense, short-term depositions/runoff events are expected to cause shifts in the chemical composition and, therefore, in the biotic communities in alpine lakes and streams. In order to better understand the implications of acid deposition in watershed systems in the Sierra Nevada, the California Air Resources Board (CARB) initiated an intensive integrated watershed study at Emerald Lake in Sequoia National Park. The comprehensive nature of the data obtained from these studies provided an opportunity to develop a quantitative description of how watershed characteristics and inputs to the watershed influence within-watershed fluxes, chemical composition of streams and lakes, and, therefore, biotic processes. In August 1987, the University of Arizona's research team started work on the development of physical/chemical models of the Emerald Lake watershed as a pan of the Sierra Watershed Modeling Project (SWMP).

Two different but closely-related modeling approaches were followed. In the first, the emphasis was placed on the development of systems-theoretic. models. In the second approach, development of a compartmental model was undertaken. The systems-theoretic effort results in simple time-series models that allow the consideration of the stochastic properties of model errors.

One-step-ahead time-series models relating Lake Outflow as a function of snowmelt estimates were identified and showed good prediction ability. The model for the period of increasing melt contained ten snowmelt terms, indicating longer retention (up to ten days) of water in the watershed. On the other hand, the model for the period of decreasing melt had only three snowmelt terms, indicating short retention (less than three days) and, therefore, faster movement of water through the watershed. The resulting models, however, need to be verified further with independent sets of data from other water years. The results of stepwise regression analysis for the snowmelt period point to flow as being the most influential factor (among temperature and cumulative flow) in deciding the value of acid-neutralizing capacity (ANC). Although the sample of size 17 is too small for strong conclusions, substituting flow as a proxy for cations and anions would amount to a significant reduction in data collection efforts. On the other hand, the compartmental model [the University of Arizona Alpine Hydrochemical Model (AHM)] is a comprehensive and detailed description of the various interacting physical and chemical processes occurring on the watershed. It is capable of modeling stream hydrology and chemistry at daily time steps. Limited testing of ARM was performed using the 1986 Water Year data.

The results indicate that AHM is capable of providing reasonable estimates of hydrologic and chemical outflows from the Emerald Lake basin. Similarities between this and other basins in the Sierra Nevada suggest that it can be used for evaluations in other basins. The initial testing has also identified a few potential limitations that should be addressed before the model's full capabilities can be used. For example, more flow-routing information from the watershed is needed, and data for mineral weathering, litter decay, soil mineralization, dry deposition, and other processes and fluxes need further analysis and selective augmentation. A physically based model of chemical release from the snowpack is also needed. Future efforts should also address coupling the AHM to lake and vegetation models for assessment of deposition and climate changes in alpine watersheds.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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