LINX II, River Basin Modeling

Eco-metrics, Inc. is developing a landscape model of nitrogen dynamics to expand the results of the site-specific studies to the landscape scale.  Throughout headwater streams (1st to 3rd order) in the stream network, NO₃ uptake and denitrification will be modeled as a function of stream discharge, NO₃ concentration, and adjacent land-use type using relationships derived from our field ¹⁵N-tracer experiments.  The model will assume that NO₃ uptake and retention in the remaining streams (4th order and greater) does not occur.  If our hypothesis holds true (i.e., 1st to 3rd order streams account for the majority of N retention in stream networks), we would expect this model to overpredict NO₃ concentrations in 4th to 6th order streams, but that the overprediction will be small relative to the predicted retention in the smaller streams.  However, we may find substantial overprediction of NO₃ concentrations in 4th to 6th order streams at some of our study sites and only modest or little overprediction in others.  This would suggest that NO₃ processing in large streams varies across biomes.  If this is the case, we will use regression analyses to test whether stream channel complexity is an important determinant of NO₃ retention/processing in large streams.  Indices of channel complexity in 4th order and larger streams will be compiled using measures such as sinuosity, variation in scour-zone width, variation in floodplain width, number of side channels, etc. derived from USGS digital ortho quads.  Strong correlations between channel complexity indices and residuals from regressions of modeled vs. measured NO₃ concentrations would indicate that human activities that alter channel complexity within the stream corridor contribute to the low N retention observed in large streams.