『Abstract
Drainage tiles buried beneath many naturally poorly drained agricultural
fields in the Midwestern U.S. are believed to “short circuit”
pools of NO3--laden soil water
and shallow groundwater directly into streams that eventually
discharge to the Mississippi River. Although much is known about
the mechanisms controlling this regionally pervasive practice
of artificial drainage at the field-plot scale, an integrative
assessment of the effect of drainage density (i.r., the number
of tile drains per unit are) on the transport of nutrients and
solutes in streams at the catchment scale is lacking. In this
study, we quantified the flux and hydrological pathways of agricultural
NO3- and road-salt Cl-
from catchments lying within the Wabash River Basin, a major source
of NO3- to the Mississippi River.
The paired catchments differ primarily in drainage density (70％
vs. 31％, by catchment area), with essentially all other agricultural
management, land use, and soil drainage characteristics remaining
equal. Our study revealed two significant hydrological responses
to increased drainage density: (1) more near-surface storm even
water (dilute in both NO3- and
Cl-) was transported early in the storm and (2) higher
transport of Cl--laden pre-event soil water relative
to shallow groundwater elevated in NO3-
occurred later in the storm. These patterns are consistent with
a proposed conceptual model in which increased drainage density
results in (1) greater transport of soil water to streams and
(2) a delayed rise in the water table. With respect to nutrient
management implications, these results indicate that increased
drainage density impacts subsurface pools of Cl- and
NO3- differently, a finding that
we propose is linked to soil/ground water dynamics in artificially
drained agricultural catchments.
Keywords: Nitrogen; Stable isotopes; Hydrograph separation; Storm;
Tile drainage; Agriculture』
1. Introduction
2. Study area
3. Methods
3.1. Sampling and analysis
3.2. Hydrograph separation
3.3. Solute proxy for surface water transport
4. Results
4.1. Precipitation events and streamflow response
4.2. δ18O values of precipitation and stream water
4.3. Solute patterns in stream water
5. Discussion
5.1. Hydrograph separation
5.2. Hydrological transport pathways
5.3. Catchment exports of NO3-
and Cl-
6. Conclusions
Acknowledgements
References