『Abstract
Many studies have investigated nitrate and dissolved organic
carbon (DOC) export mechanisms during storms in forested mountainous
to hilly catchments. However, the number of studies across land
uses in artificially drained landscapes of the Midwest remains
relatively small despite the importance of nitrate and carbon
losses to streams on water quality in this region. This study
investigates water, nitrate, and DOC delivery (timing) to streams
during storms, and the mechanisms (flowpaths) affecting nitrate
and DOC flushing trajectories during three spring storms in an
agricultural watershed and a mixed land use watershed in glaciated
landscape of the Midwest. Hydrograph separation techniques using
oxygen-18 isotopes of water and the analysis of major cation flushing
trajectories were used to determine the flowpath associated with
the export of nitrate and DOC in each watershed. Higher anthropogenic
N inputs in the agricultural watershed are associated with higher
stream nitrate concentration during storms, while DOC concentration
in streams across land uses is mainly influenced by storm characteristics/discharge.
In both watersheds, DOC concentration quickly increases and decreases
with discharge. In the mixed land use watershed, the peak in nitrate
is consistently delayed relative to the peak in DOC; however,
nitrate peaks vary in relation to discharge in the agricultural
watershed. The comparison of the nitrate/DOC concentration patterns
to the concentration patterns of major cations during the storms
studied suggests that DOC is likely exported via a combination
of overland flow and preferential flow through soil macropores
in both watersheds. Data suggest nitrate is exported with groundwater,
either as tile-drain flow or subsurface flow to the streams. Results
also indicate that the connectivity between nitrate/DOC reservoirs
and the stream is an important control mechanism on the timing
of delivery of nitrate and DOC to the stream in the watersheds
studied. Finally, results indicate that even in agricultural watersheds
where tile-drain flow is an important contributor to stream flow
during storms, there is not necessarily a consistent export mechanism
for all solutes. These results underscore the need for further
studies investigating the relative importance of matrix flow,
overland flow and macropore flow at the watershed scale in order
to fully understand event scale processes controlling the transport
of solutes to streams in glaciated landscapes of the Midwest under
various land use conditions.
Keywords: Nitrate; Dissolved organic carbon; Storm; Flowpath;
Midwest』
Introduction
Materials and methods
Study area
Experimental approach
Water sample analysis
Hydrograph separation
Results
Storm characteristics and stream hydrology
Water quality at baseflow
Nitrate, DOC and cation dynamics during storms
Discussion
Variability in water, nitrate and DOC delivery to streams
during storms as related to precipitation characteristics and
differences in land use
What are the mechanisms controlling nitrate and DOC delivery
to streams during storms?
Conclusions
Acknowledgments
References