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Effects ou Ground and Surf ice Waters: Research at the Edge of Science and Society
(Proceedings o f a symposium held at Wageningen. October 2000). IAHS Pnbi. no. 2 7 3 . 2002.
233
Changes in agricultural practices and regional
export of nitrogen from land to sea
A. G R I M V A L L , À. F O R S M A N
Linkôping University, SE-58IS3
e-mail: [email protected]
Linkôping,
Sweden
B. K R O N V A N G , D.-I. M Û L L E R - W O H L F E I L
National Environmental
Research Institute, DK-8600 Silkeborg,
Denmark
F. W E N D L A N D & R. K U N K E L
Research Centre Jiilich, D-52425 Jiilich,
Germany
Abstract We developed a model system to facilitate interpretation of temporal
trends in riverine loads of nitrogen (N) and to predict the impact of measures
taken to reduce N fluxes from land to sea. The role of subsurface retention is
highlighted and special attention is paid to river basins where groundwater
residence times are long and leaching from arable soils represents a major
source of riverine N. If measured water quality data are available as a reference, the subsurface transport and retention of N are estimated by combining
models for N leaching from the root zone with models for residence times and
denitrification in groundwater. Unmonitored areas are handled by statistical
extrapolation of data from monitored catchments. Case studies in Denmark
and Germany demonstrate that the model system is operational on a catchment
or regional scale and shows potential for studies on a European scale.
Key words catchments; denitrification; Denmark; Germany; groundwater; metamodels;
nitrogen leaching; subsurface retention
INTRODUCTION
It is generally accepted that extensive changes in land use can have a rapid and distinct
effect on riverine loads of nitrogen (N). For example, upward N trends were noted in
both Western and Eastern European rivers when the consumption of N fertilizers and
emissions of N oxides accelerated in the middle of the twentieth century (Grimvali et
al., 2000). More recently, it was noted that the dramatic drop in fertilizer use that
occurred in Eastern Europe around 1990 had a marked effect on the N loads carried by
the Danube (Olâh & Olâh, 1996). In contrast, there was no clear water quality response
in the Daugava River when N application was reduced to a small fraction of the level
that had previously prevailed in the Baltic republics (Lôfgren et al., 1999). Likewise, it
was recently noted in Denmark that altered agricultural practices did not result in any
remarkable change in water quality. This raises the question of why the response in
water quality varies so greatly between regions.
Here we describe the results of a project aimed at integrating models to facilitate
both the interpretation of the observations cited above and to predict the large-scale
and long-term impact of measures taken to reduce N fluxes from land to sea. The role
of subsurface retention in the regional fluxes of N is highlighted, and we pay special
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attention to N retention in river basins where groundwater residence times are long and
leaching from arable soils represents a major source of riverine N.
Empirical evidence of subsurface retention
Existing knowledge about subsurface N retention has been derived from a great variety
of observations made on different spatial and temporal scales. The large-scale aspects
of this part of the N cycle are well illustrated by the above-mentioned findings in the
Baltic republics. Mass balance calculations in small catchments have further illustrated
that the output of N can be much smaller than the input and also enabled identification
of the environmental compartments and mechanisms responsible for N retention. Table 1
summarizes extensive measurements of agricultural runoff in Denmark, and highlights
the role of groundwater by demonstrating that the delivery of N to surface water can be
much smaller than input of this element to groundwater. Other studies have provided
solid evidence that accumulation/depletion of N in soil can play an important role in
the regional turnover of N. For example, plot experiments in arable and forested areas
have demonstrated that a substantial fraction of N fertilizer can be retained in soil
organic matter long after application (Shen et al., 1989; Tamm et al., 1995).
Table 1 Annual leaching of nitrogen from the root zone and delivery of this element to surface water in
small agricultural catchments in Denmark; average values for 1989/90 to 1995/96 (Grant et al, 1997).
1
Nitrogen flux (kg ha" N)
Sandy soils
Loamy soils
Leaching from the root zone
Delivery to surface water
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12
72
27
A model system for regional analysis of subsurface retention of nitrogen
General features of the model The regional scale of the issues addressed has
important implications for the input data that can be used. We emphasize that all model
inputs must be readily available for a vast majority of the areas of Europe where N
leaching is a problem. Moreover, we recognize the need for a special procedure that
can be applied in unmonitored catchments.
When measured data on runoff and riverine loads of N are available, we propose
an approach involving hydrogeochemical modelling of N transport along different
runoff pathways. First, N surpluses at the soil surface are derived from official
statistics on land use and agriculture. Thereafter, the subsurface transport and retention
of N are estimated by combining models for N leaching from the root zone with
models for residence times and denitrification in groundwater. Measured riverine loads
serve as references against which the model calculations are validated.
If no measurements of runoff or N loads are
approach involving extrapolation of N fluxes
catchments. Finally, it should be stressed that the
frame of years to decades, thus model features that
dynamics of N deliveries are omitted.
available, we propose a statistical
from monitored to unmonitored
N fluxes are considered in a time
are relevant only for the short-term
Changes in agricultural practices and regional export of nitrogen from land to sea
235
CD
a.
(/I
OCl
Cl
to c
> CO
Runoff generation
SOIL
b Accumulation/depletion ^
of soil organic nitrogen j
Denitrification
I
j
Nitrate leaching from the root zone
RUNOFF
PATHWAYS
Nitrate delivery to surface
water via direct runoff
Nitrate load of recharged groundwater
AQUIFER
Residence time
Denitrification
Release of
N and N 0
2
2
Nitrate delivery to surface
water via groundwater runoff
Fig. 1 Pathways, environmental compartments, and processes considered in the
regional analysis of subsurface retention of nitrogen.
The different runoff pathways define a physical structure for the hydrogeochemical model of subsurface N retention. In some areas, a substantial fraction of the
precipitation percolates into soil, and it is not unusual that groundwater residence times
are of the order of years to decades, or even centuries. Marshes and areas of shallow
moraine can be regarded as groundwater transit areas, where the groundwater flows
below the impermeable layer, and groundwater recharge is negligible. Yet other areas
are strongly influenced by artificial drainage systems, and the water that is transported
in such systems usually reaches a local surface water recipient within a few days or
weeks. Figure 1 shows a schematic diagram of both the runoff pathways and the
biological and chemical processes that we consider particularly important for the
subsurface transport and N retention in a region.
Nitrogen surpluses at the soil surface The N surpluses that are taken as a starting
point for the hydrogeochemical modelling of subsurface N retention can usually be
calculated in a straightforward manner, as long as only average values for relatively
large areas are requested. Information about atmospheric deposition of N is available
as contour or grid maps. Data regarding application of manure and mineral fertilizers
and N removal through harvesting are generally available for some kind of admini­
strative units. Furthermore, rough estimates of N fixation by different crops and fluxes
of this element at the surface of non-agricultural land are easy to procure. However,
calculating spatially distributed N surpluses is a more demanding task. Therefore, we
developed a procedure in which a geographical infonnation system (GIS) is employed
to estimate spatially distributed N surpluses. The main feature of this procedure is that
high-resolution raw data are used to generate realistic spatial distributions of other
variables for which only low-resolution data are available.
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Leaching from the root zone The procedure used to estimate N leaching from the
root zone was derived from the SOILN model (Johnsson et al., 1987), which describes
the vertical transport of heat, water and N through a soil column. Due to conservation
of mass, we have the identity:
L=A+F-H-D-C
where L is the amount of N leached from the root zone, A is the atmospheric
deposition, F is the input of N via fertilizers and N-fixing crops, H is the N harvest, D
is the loss of N through denitrification, and C is the change in the N pool in the soil. In
the regional analysis of N losses from the root zone, we used SOILN-based estimates
of D and C, whereas empirical data were employed to calculate A, F and H. More
precisely, D and C were modelled by regression equations derived from extensive
analyses of the SOILN response to different inputs.
Considering that climate, soil type, crops grown, and application of fertilizers can
vary greatly in a study area, it may appear to be an insurmountable task to derive
leaching equations for all relevant combinations of such factors. However, two findings
facilitate estimation of regional N leaching. First, we noted that the meteorological
conditions mainly influence the timing of the leaching processes, whereas the total
amount of N leached over a period of several years is relatively insensitive to the
climate. Accordingly, a single meteorological station can represent a fairly large area.
Secondly, we found that, for each crop and soil type, the SOILN model is almost linear
in one important respect: both the change in the N pool in the soil and the amount of
nitrate that is denitrified vary almost linearly with the N harvest. Hence regional totals
of these two variables can be derived from regional N harvests for each combination of
crop and soil type.
Separation of runoff components Groundwater runoff was separated from the
total runoff by using flow graphs to estimate runoff ratios; we assumed that such ratios
are site specific but constant over time (Wendland & Kunkel, 1999). Denitrification in
the fast flow is assumed to be negligible, whereas this process can be of major
importance in the aquifers responsible for the slow flow.
Groundwater residence times Groundwater residence times were calculated by
employing the W E K U residence time model (Kunkel & Wendland, 1997), a twodimensional analytical procedure that was developed to estimate the length of time
needed for water to pass through groundwater-bearing rock and reach a river, a lake, or
the sea. W E K U operates on grid data, and the flow lines in the aquifer are assumed to
run parallel to the groundwater table. The basic information required to calculate
residence times is provided by a contour map of the groundwater table along with
spatially distributed data on the hydromechanical properties of the aquifers through
which the water flows.
On a spatial scale, the W E K U model was developed for medium-sized to large
river basins, and residence times are usually measured in years to decades and centuries.
The natural heterogeneity of the studied aquifers is taken into account by employing a
stochastic approach to describe the uncertainty regarding important model parameters,
such as hydraulic conductivity. Thus the primary output of the W E K U model is a
distribution of residence times for each grid cell in the study area. Several summary
measures of groundwater residence times can then be derived from such distributions.
Changes in agricultural practices and regional export of nitrogen from land to sea
237
Denitrification in groundwater In our study, the W E K U model was equipped
with a denitrification module. In this expanded model, measurements of the chemical
composition of groundwater are used to divide the entire study area into one area
where denitrification cannot occur and another where it does take place. In the latter
area, denitrification is regarded as a first-order process in which the nitrate in
groundwater has a certain half-life.
Unmonitored catchments The model system described above cannot be
employed if runoff data are lacking. In such cases, we propose that empirical data on
monitored catchments in the region under consideration be analysed statistically to
derive numerical relationships between N deliveries and readily available catchment
characteristics, such as topography and land use. These relationships can then be used
to estimate N outputs from unmonitored catchments
CASE STUDIES
Two case studies were carried out to ensure that the model system is operational and to
validate the model against available empirical data. Both study areas, the Ùcker basin
(4722 k m ) in northern Germany and the Gjem basin (109 Ian") on the Jutland
peninsula in Denmark, represent hydrogeological conditions that are prevalent in the
North European Lowland; the upper aquifers consist of unconsolidated rocks of glacial
origin, groundwater runoff is significant, and favourable conditions for denitrification
in groundwater are prevalent. Agriculture is the dominating land use at both sites.
However, the varying availability of agricultural statistics made the two case studies a
test of the flexibility of the model system. In the Gjern basin, we used detailed information about crops, harvests and fertilization on the block level, whereas the Ùcker
study was based on agricultural statistics of low spatial resolution.
-
Validation of calculated N deliveries to surface water against measured riverine
loads showed that our model system was able to mimic the observed spatial pattern in
riverine loads in the Gjem basin. Moreover, CFC dating of some groundwater samples
indicated that the estimated groundwater retention times were realistic. The validation
of the ticker study was restricted to the total riverine N load. However, also in this
case there was a good agreement between calculated and measured data.
DISCUSSION A N D C O N C L U S I O N S
We have created a model system for subsurface N retention that is operational on a catchment or regional scale. All model components can be ran with data that are readily available for a vast majority of the European catchments where N leaching is a problem. W e
developed a special procedure to estimate N loads from unmonitored catchments. Furthermore, handling of catchment or regional data entails a reasonable computational burden.
The model system is very flexible. From a technical point of view, all model
components can be run with inputs of low spatial resolution. However, the quality of the
model output is, of course, a function of the quality of the raw data. In particular, we
would like to emphasize the need for: (a) spatially distributed information regarding the
hydromechanical properties of the groundwater-bearing rocks, and (b) measurements
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of groundwater quality that can elucidate the potential for denitrification. Furthermore,
the quality of the leaching estimates depends largely on the availability of spatially
coupled data on crops, fertilizer application, harvests, and soil type. Finally, it should be
noted that it is the general procedure, not the specific formulae, that can be transferred
from one study area to another. The interpretation of geological conditions will always
require expert knowledge, and the handling of data from a new region may call for
extensive statistical analyses of SOILN inputs and outputs. Likewise, the procedure used
to extrapolate N loads from monitored to unmonitored areas must be adapted to sitespecific conditions.
The validation of the model calculations in the Gjern and Ucker basins produced
very satisfactory results, although it must be admitted that the validation was mainly
restricted to the final output of the model. A thorough analysis of site-specific data is
needed to determine whether the model system can also explain the temporal trends in
large-scale nutrient loads that were mentioned in the introduction. Notwithstanding, it
is already possible to draw some general conclusions.
There is no contradiction between our calculations of subsurface N retention and
the general post-war increase in riverine loads. Direct runoff is the primary pathway in
large parts of Europe, and changes in the composition of that runoff component may
explain why a rapid and distinct response in water quality has also been noted in
catchments where groundwater runoff plays an important role. The non-appearance of
water quality trends in the Baltic republics and Denmark is logical, provided that the
major changes in land use and agricultural practices have occurred in areas character­
ized by long residence times and possibly also conditions that favour denitrification in
groundwater. However, it would be an oversimplification to rely solely on explan­
ations involving N retention in groundwater. Particularly in the Baltic republics, there
is an obvious need for a closer analysis of both groundwater residence times and
changes in the N pool in soil.
Acknowledgements The authors are grateful for financial support from the European
Commission, contract ENV4-CT97-0435, and the Swedish Water Management Research
Program.
REFERENCES
Grant, R., Blicher-Malhicsen, G., Andersen, H. F.., Laubel, A., Jensen, P. G. & Rasmussen, P. (1997) Vandmiljoplanens
Overvc'tgningsprogram—Landovervitgningsoplande
(in Danish). Technical report no. 210, National Environmental
Research Institute, Silkeborg, Denmark.
Grimvall, A., Stâlnackc, P. & Tonderski, A. (2000) lime scales ot nutrient losses from land to sea—a European
perspective. J. Ecol. Engng 14, 363-371.
Johnsson, II., Bergstrom. L., Jansson, P. E. & Paustian, K. (1987) Simulated nitrogen dynamics and losses in a layered
agricultural soil. Agric. Ecosystems & Environ. 18, 333-356.
Kunkel, R. & Wendland, F. (1997) WEKU—a GIS-supported stochastic model of groundwater residence times in the
upper aquifers for the supraregional groundwater management. Environ. Geol. 30, 1-9.
Lôfgren, S., Gustafson, A., Steineck, S. & Stâlnacke, P. (1997) Agricultural development and nutrient Hows in the Baltic States
and Sweden after 1988. Ambio 28, 320-327.
Olàh, J. & Olâh, M. (1996) Improving landscape nitrogen metabolism in the Hungarian lowlands. Ambio 25, 331-335.
Shen, S. M., Hart, P. B. S.. Powlson, D. S. & Jenkinson, D. S. (1989) The nitrogen cycle in the Broadbalk Wheat Experiment:
' N labelled fertilizer residues in the soil and in the soil microbial biomass. So/7. Biol. Biocbem. 21, 529-533.
Tainm, C. O., Aronsson, A. & Popovic, B. (1995) Nitrogen saturation in a long-term forest experiment with annual
additions of nitrogen. Wat. Air Soil Potlut. 85, 1683-1688.
Wendland, F. & Kunkel. R. (1999) Das Nitrulabbauvermogen im Grundwasser des Elbeeinzugs-gebietes. Buchreihe
Umwelt, Band 13. Forschungszentrum Jiilich, Jiilich, Germany.