Caleb Fassett1, Olga Rostapshova2
1 Williams College
{Caleb.I.Fassett@williams.edu}
2 Swarthmore University
{olga@sccs.swarthmore.edu}
The RiverWeb Water Quality Simulator is being used in high school environmental science
classes to promote structured, team-based exploration of the dynamic and quantitative
relationships between land use, precipitation, and a variety of water quality
indicators, both chemical and biological, within an archetypal watershed. The most
easily examined properties of a stream in the field include its physical properties,
such as average depth, width, and velocity. After determining discharge, we calculate
these physical properties using regression-based power laws on a daily basis. The
actual form of the power laws varies a great deal regionally and even locally in
nature, so for our archetypal watershed, we feel justified in picking a precise form
for demonstration purposes. Other physical characteristics such as water temperature
are currently calculated using empirical models. The accuracy of this has not been
ascertained, and further research will likely be needed to determine whether we are
calculating water temperature reasonably. This is a reasonably difficult problem, since
in real life local conditions such as local slopes and local velocities can have
important effects. As part of nationwide initiatives in the 1970s and 1980s to reduce
non-point source water pollution, a great deal of study went in to determining how
land-use and nutrient production was correlated (see, for example NURP). Although these
studies have demonstrated the complexity of the impacts that humans have on natural
watersheds, they did produce a great deal of “average” and “typical” effects of
changing land use on water qualities. These “event mean concentrations” (EMC’s) for a
given nutrient and land use are the source of the nutrient chemistry in our model. This
data does not allow for a process-by-process assessment of nutrient chemistry, but
instead represents an average of how the combination of natural processes in a normal
stream affects average chemistry and allow determination of nutrient load. For each
land use, different pairs of EMC values for runoff and groundwater were used, because
nutrients propagate differently in groundwater and runoff. Sometimes adequate EMC data
were not available for a land use or nutrient. In these cases, we made estimates based
on what we considered to be the dominant processes in the natural system. This section
of the model will likely be refined with further research.