『Summary
In-stream nitrogen concentrations of a small agricultural
lowland study catchment in Saxony-Anhalt, Germany, show high seasonal
dynamics. Field data lead to the hypothesis that these dynamics
are controlled by groundwater level fluctuations and the associated
falling dry and rewetting of intermittent channels and not by
seasonal variations of soil nitrogen leaching processes. To test
this hypothesis a hypothetical model experiment was carried out
using MODFLOW and MT3DMS. We simulated steady state and transient
flow and nitrate transport in a two-dimensional model transect,
driven by average and monthly lysimeter data of recharge and nitrate
leaching. Falling dry and rewetting of open channels was accounted
for. The transient groundwater flow model can capture the supposed
mechanisms quite well. For conservative transport, the resulting
concentration changes are in the order of 25 mg/l NO3-N,
for the reactive transport simulation, the concentration ranges
was still in the order of 12 mg/l NO3-N.
It can therefore be concluded, that the dynamics of groundwater
flow considerably control the dynamics of in-stream nitrate concentrations.
The results of the steady state simulation based on averaged inflows
are not equivalent to the averaged outflows given by the transient
simulation, reflected in different loads. In such a dynamic system
the steady state flow approach is not an acceptable representation
of the system. Observed data and simulations showed similar relations
of in-stream nitrate concentrations to groundwater level and discharge.
Minor differences were explained by the simplified two-dimensional
modelling concept in contrast to a spatially distributed, three-dimensional
real-world catchment system. In a real-world system further processes
superpose the assumed hydrological controls, such as in-stream
or hyporheic zone denitrification, contribution of tile drains
and observed fertilization practice.
Keywords: Nitrate; Diffuse pollution; Groundwater-surface water
interaction; Groundwater dynamics; Modelling』
Introduction
Description of Schaugraben study catchment
Model set up
Flow and transport processes
General model design
Simulation results
Steady state simulations
Groundwater concentrations
In-stream concentrations and loads
Effect of geological setup
Transient state simulations
Hydrological regime
Groundwater concentrations
Seepage concentrations
In-stream concentrations and loads
Effect of geological setup
Relation of in-stream concentrations to discharge and groundwater
level
Comparison of steady state and transient simulations
Discussion of simulation results
Review of model results
Comparison of steady state and transient simulations
Relation of simulated behaviour to observed behaviour in the
study area
General uncertainties of the modelling approach
Relation to contemporary research
Outlook on future research
Conclusions
Acknowledgement
References