Journal of Geophysical Research: Biogeosciences

Land use control of stream nitrate concentrations in mountainous coastal California watersheds

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Abstract

[1] Concentration versus runoff relationships can reveal how land use and watershed hydrology interactively regulate solute inputs to streams and downstream aquatic ecosystems. In six mountainous southern California coastal watersheds, consistent nitrate-runoff patterns within three broad land use classes exist: dilution in agricultural watersheds, invariance in urban watersheds, and enrichment in an undeveloped watershed. Locally weighted scatterplot smoothing (LOWESS) revealed these patterns in nitrate-runoff relationships. A hyperbolic equation reproduced these relationships, also identifying the most common nitrate concentration (i.e., nitrate mode) observed in stream water during periods of low runoff (i.e., base flow) and high runoff (i.e., stormflow). LOWESS and the hyperbolic equation were also used to reveal and reproduce electrical conductance-runoff relationships, which contrary to nitrate-runoff relationships, demonstrated uniform behavior (significant dilution; p < 0.0001) in all watersheds regardless of land use. Nitrate-electrical conductance plots revealed seasonal shifts in stormflow nitrate modes, indicating nitrate flushing behavior at the beginning of the winter wet season for all watersheds except the two with the highest agricultural land usage. Despite variation in land uses between watersheds, we found a consistent reduction in the variability of stormflow nitrate (∼12%) and electrical conductance (∼21%) modes relative to base flow modes indicating a common water and nitrate source during periods of high runoff. We propose the undeveloped, mountainous upland regions of the watersheds as this source, and suggest that this region plays an important role in determining watershed stream nitrate concentrations and nitrate flux to the Santa Barbara Channel (Pacific Ocean).

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