Recent Results
HYDROCHEMICAL MODELING
We have developed and tested a model to
assess the hydrologic and biogeochemical responses of
seasonally snow-covered alpine areas
to changes in inputs of water, chemicals and
energy.
This alpine hydrochemical model (AHM) is capable of
incorporating a detailed understanding of watershed processes
in order to simulate events critical to biota such as the
ionic pulse associated with spring snowmelt, which is
only a few days long and may involve only
a portion of the catchment.
The model computes integrated water and chemical balances
for multiple terrestrial, stream,
and lake subunits within a watershed, each of which can have
a unique and variable snow-covered area.
Two years of data from the Emerald Lake watershed, a 120 ha
basin in the southern
Sierra Nevada, were used for fitting and testing, by comparing
observations with modeled daily output.
To the extent possible, model parameters were set based
on independent physical or chemical measurements, leaving only a few
fitted parameters.
In its current application, model capabilities include: 1)
tracking of chemical inputs from precipitation, dry deposition, snowmelt,
mineral weathering, flows external to the
watershed, and user-defined sources and sinks; 2) tracking
surface and subsurface water and chemical movements through
vegetation canopy, snowpack, soil litter,
multiple soil layers, streamflow, and lakes;
3) calculating chemical speciation, including
precipitates, exchange complexes, and acid-neutralizing capacity; 4)
simulating nitrogen reactions; 5) using a snowmelt optimization procedure
to aid in
matching observed watershed outflows, and 6) modeling
riparian areas.
Using one year of stream data for parameter estimation and a second for
evaluation, the agreement between model and data was
judged to be quite good.
This modeling effort has highlighted the importance of hydrologic
flow-paths in determining chemical output from the basin.
SNOWMELT MODELING
In order to route water from the melting
snowpack through the proper hydrochemical component
of the AHM, we calculate the
spatial distribution of snowmelt using a numerical model.
Methods for spatially distributed physical modeling of snowmelt have
been developed based on one-dimensional finite difference physically based
simulations
of snowpack thermal, mass, and momentum transport performed for
various conditions found in the basin.
Estimation of snow-water equivalence using regression trees and
distributed snowmelt modeling using iterative clustering are carried out
using a geographical information system; these
methods are applied to Emerald Lake Basin.
While the bulk hydrology (i.e., the basin outflow
hydrograph) is fairly accurately reproduced, the
modeling of snow-cover patterns do not always agree with observations.
Modeled snow-melt and basin discharge substantially agree,
yet modeled and observed snow-covered area agree only over ~65% of the basin after
five weeks of melt, highlighting the importance of using snow-cover patterns to
assess model performance for spatially distributed snow-melt models.