HYDROLOGICAL PROCESSES Hydrol. Process. 18, 357– 371 (2004) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1380
WETTRANS: a flow-path-oriented decision-support system for the assessment of water and nitrogen exchange in riparian peatlands Michael Trepel* and Winfrid Kluge Ecology-Centre, University of Kiel, Olshausenstraße 40, 24098 Kiel, Germany
Abstract: Environmental authorities require quantitative predications of the nitrogen retention ability of riparian peatlands to aid in the selection of effective water management strategies for restoration. To support this decision-making process, a matrix model connecting flow paths and nitrogen transformation was developed with a quasi-stationary mass balance approach. The model concept is based on the assumptions that wetlands (1) receive water and nitrogen along several hydrological pathways, and (2) transformation of nitrogen occurs inside the wetland with different efficiencies, depending on the transition pattern between inflow and outflow pathways. These assumptions are formalized in a set of vectors describing the amount of water and the nitrogen concentration for each inflow pathway, a water distribution matrix between the inflowing water and several outflow pathways, a nitrogen transformation coefficient matrix of each inflow and outflow combination and, finally, two outflow vectors with the amount of water flowing out and the calculated nitrogen concentrations of each outflow pathway. The matrix model is applied to a drained valley peatland in order to quantify the effect of different water management options on an increase in nitrogen retention. Copyright 2004 John Wiley & Sons, Ltd. KEY WORDS
retention; water quality; groundwater; surface water; environmental management; wetland model; nitrogen; non-point source
INTRODUCTION Wetlands are frequently viewed as ecotones, or more specifically, as transitional zones between aquatic and terrestrial ecosystems (Naiman and Decamps, 1990; Zalewski et al., 2001). Owing to the special hydrological, hydrochemical and ecological conditions in these ecosystems, wetlands are generally considered to possess a high potential for nutrient retention, nutrient transformation, flood protection and biodiversity (e.g. Devito et al., 1989; Johnston et al., 1990; Leonardson et al., 1994; Blicher-Mathiesen and Hoffmann, 1999; Rosenblatt et al., 2001). However, wetland restoration measures often fail due to the rather incidental selection of sites and water management measures, mainly caused by neglecting the geohydrological characteristics (landscape position) of the specific wetland (Bedford, 1999; Zedler, 2000). In the federal state of Schleswig-Holstein, northern Germany, mires—peat-accumulating landscape entities—once covered 10% of the area (¾1500 km2 ). Currently, most mires are drained and the peat soils are used for agriculture and forestry (Schleuß et al., 2002). To combat these negative changes, the environmental ministry has developed a peatland restoration programme aimed mainly at restoring the high nutrient retention/transformation potential of drained peatlands in order to reduce non-point source nutrient input into aquatic ecosystems and, at the same time, to improve habitat conditions. To achieve the first goal requires a quantitative analysis of the specific hydrological and hydrochemical conditions in a wetland, together with the development of a new water management system with a higher nutrient transformation capacity than exists in * Correspondence to: Michael Trepel, Ecology-Centre, University of Kiel, Olshausenstraße 40, 24098 Kiel, Germany. E-mail:
[email protected] Copyright 2004 John Wiley & Sons, Ltd.
Received 19 November 2001 Accepted 16 January 2003
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the drained state. Decision-support systems are suitable tools for such kinds of problem; however, a decisionsupport system is not yet available for the water-management-oriented prediction of nitrogen transformation in wetlands. Existing process-based models for quantitative predictions of the effect of land-use change on nitrogen losses are data intensive and do not describe the geohydrological conditions sufficiently (Bierkens et al., 2000; Trepel et al., 2000). Quantitative models for predicting water quality improvement focus mainly on surface flow wetlands and neglect groundwater inflow and nitrogen retention in buffer zones (Dortch and Jeffrey, 1995; Bystr¨om, 1998; Spieles and Mitsch, 2000). Often, geographical information system-based models facilitate only a qualitative assessment of the suitability of sites for water quality improvement in a catchment area (Cedfeldt et al., 2000; Rosenblatt et al., 2001), or provide only rough estimates of nitrogen retention in large areas using simplified linear wetland load–wetland area relations (Jansson et al., 1998). Water authorities urgently need quantitative data on the effectiveness of different water management measures on nitrogen transformation for selection of the most effective sites and water management strategies. Therefore, our research focuses on the development of a mathematical model to assess water and nitrogen exchange and transformation in wetlands for site selection and for the quantitative comparison of different management strategies. The requirements for a suitable model for application in environmental agencies are (1) to describe water and nitrogen exchange and transformation of wetlands consistent with the geohydrological setting of the wetland, (2) to include all essential inflow and outflow pathways and (3) to be applicable with commonly available data within a short time.
THE PATH TRANSFORMATION CONCEPT The development of the model was based on a path transformation concept, where wetlands are connected with their surrounding drainage basin by hydrologically defined inflow and outflow pathways (Lloyd and Tellam, 1995; Burt, 1997; T´oth, 1999; Winter, 1999) (Figure 1). vertical in- & outflow precipitation deposition fertilisation
evapotranspiration emission harvest
flooding lateral inflow transformation
surface runoff interflow drains and pipes young, oxic groundwater old, anoxic groundwater
river riverinternal transformation
wetland retention
lateral outflow surface runoff ditch outflow pipe outflow subsurface flow
groundwater bypass Figure 1. Conceptual diagram of the path transformation concept
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Inflow pathways differ both in the amount of water and in the nutrient concentrations, depending on their origin. The proportion of different inflowing water sources depends on the landscape position of the wetland and governs the trophic stage of the wetland. The morphological and geohydrological features determine the flow pattern from the surrounding basin to the wetland (Kluge et al., 1994; Grootjans et al., 1996; Winter, 1999). The flow pattern inside the wetland is mainly controlled by (1) the thickness and physical parameters of the peat layer, (2) the occurrence of underlying aquifers, (3) the occurrence of impermeable intercalated layers such as clay, silt or gyttja (organic mud) and (4) different drainage measures. Nitrogen retention results from several biogeochemical and physical processes, including plant uptake, peat accumulation, denitrification and sedimentation (Howard-Williams, 1985; Hedin et al., 1998; Verhoeven and Meuleman, 1999). Denitrification (quantitatively, a most important process) is a microbiological process that reduces nitrate in several steps to gaseous nitrogen by using organic carbon as an electron donor. Denitrification is controlled by several direct and indirect factors operating at different spatio-temporal scales (Groffman et al., 1988; Hedin et al., 1998). As a microbiological process, denitrification is controlled directly by temperature, pH and redox state of the surrounding (micro)environment, and is limited by nitrate concentration and the availability of dissolved organic carbon. Denitrification rates are controlled by a set of indirect factors on the wetland (e.g. width, water-table depth, soil types, vegetation) and landscape scale (e.g. size of upstream basin and groundwater basin, geological setting, land-use pattern, water and nitrogen loading). Figure 2 gives an overview of some possible inflow and outflow pathways to and from wetlands. Inflow pathways include wetland precipitation, surface runoff, pipe and tile drainage from neighbouring agricultural fields, interflow, young oxic groundwater, young anoxic groundwater, old anoxic groundwater from a deeper aquifer and river water inflow. Groundwater without a direct contact point to the wetland can flow directly into the river as groundwater bypass discharge. The outflow pathways in the wetland differ in their hydrochemical
Figure 2. Overview of possible hydrological inflow pathways from the upland to the wetland and from outflow pathways from the wetland to the river. Inflow pathways are: 1 wetland precipitation, 2 overland flow (surface runoff), 3 tile drainage, 4 interflow, 5 young groundwater, 6 old groundwater, 7 deep groundwater, 8 groundwater bypass and 9 river water inflow. Outflow pathways are: 1 evapotranspiration, 2 overland (saturation) flow, 3 ditch outflow, 4 wetland drainage, 5 overbank flow (flooding), 6 subsurface flow and 7 river throughflow
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conditions and, therefore, show different transformation potentials. Important outflow pathways include wetland evapotranspiration, saturation overland flow, ditch outflow, wetland drainage with pipe and tile drains, overbank flow as a result of flooding, subsurface discharge and river flow. Water management measures affect both the inflow and the outflow patterns. For example, drains and ditches at the transition zone between the wetland and the surrounding land accelerate the transmission of inflowing surface runoff, interflow and groundwater inflow to the river and reduce the hydraulic detention time between the lateral drainage basin and the wetland. River deepening and straightening reduce the longitudinal water and nutrient exchange between the river and the wetland. Faster water flow through wetlands reduces the time required for nutrient transformation within the wetland system (Zalewski et al., 2001). Mathematical formulation of the path transformation concept The path transformation concept is formalized with a matrix approach, which allows the calculation of several inflow and outflow pathways. The area balanced with the matrix model includes a river segment with the corresponding riparian peatland, a lateral groundwater basin and an upstream river basin. Temporally, a quasi-stationary approach is used for the water budget; in the nitrogen cycle, the peatland is allowed to act as an additional nutrient source due to mineralization, wet and dry deposition and fertilizer application. The matrix model WETTRANS consists of two input vectors (a, b), two transformation matrices (C, D) and three result vectors (e, f, g): a1 b1 a2 b2 a3 b aD bD 3 ... ... ... ... am bm c11 c12 . . . c1n d11 d12 . . . d1n c21 c22 . . . c1n d21 d22 . . . d1n d32 . . . d1n c31 c32 . . . c1n d C D D D 31 ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... cm1 cm2 . . . cmn dm1 dm2 . . . dmn e1 f1 g1 e2 f2 g2 e3 f3 g eD f D g D 3 ... ... ... ... ... ... en fn gn The number of inflow pathways is i D 1 . . . m and the number of outflow pathways is j D 1 . . . n. The input from the catchment is described by the vectors a and b: ž vector a: path-specific water discharge ai D Qini 1 . . . m [m3 year1 ]; ž vector b: path-specific nutrient concentrations bi D Cini 1 . . . m [mg l1 ]. The water and nutrient load distribution and nutrient transformation in the wetland are described by the matrices C and D: ž matrix C: coefficients of water partitioning matrix cij i D 1 . . . m, j D 1 . . . n, where the sum of the partitioning coefficients for each inflow pathway is unity; Copyright 2004 John Wiley & Sons, Ltd.
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ž matrix D: coefficients of nutrient transformation matrix dij i D 1 . . . m, j D 1 . . . n, where each outflow pathway has a set of path-specific nutrient transformation coefficients. The model results are given in the vectors e, f and g: ž vector e: path-specific water outflow ej D Qoutj j D 1 . . . n [m3 year1 ], ž vector f: outflow path-specific nutrient concentrations fj D Coutj j D 1 . . . n [mg l1 ], ž vector g: path-specific nutrient output gj D Poutj j D 1 . . . n [mg year1 ]. The path-specific water outflow and the associated nutrient output are calculated using Equation (1) written in the short form as a Schur-product: g D C ° Da ° b 1 or, written as a sum equation, the path-specific nutrient output is calculated using Poutj D
n
ai bi cij dij
2
iD1
The nutrient concentration for each outflow pathway is given by Poutj Qoutj
Coutj D where Qoutj is calculated according to Qoutj D
n
3
ai cij
4
iD1
Nutrient transformation for the specific inflow to outflow pattern of a wetland is calculated as system transformation efficiency ST (%) Equation (5) written in matrix language ST D 100 ð
a ° b C ° Da ° b a ° b
or written as a path-specific sum as
ST D 100 ð
m
Qini Cini
iD1
n
Qouti Coutj
jD1 m
5
6
Qini Cini
iD1
ACQUISTION OF INPUT PARAMETERS Parameter acquisition is a crucial question during the development of a decision-support system for complex problems such as the prediction of effects of a new water management strategy on potential nitrogen transformation in wetlands. Thus, parameter acquisition must be balanced between a system adequate description of flow patterns and processes on the one hand and the availability of data and knowledge required for a solution of the problem on the other hand. The WETTRANS model has focused on a hydrologically consistent description of inflow and outflow pathways in a simple mathematical form. The approach, therefore, describes only mean annual performance in terms of quasi-stationary conditions. Because Copyright 2004 John Wiley & Sons, Ltd.
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of the difficulty in simulating the temporal variability of hydraulic detention times in a wetland, as well as short-term weather-induced changes of nutrient concentrations, the model is set up to calculate mean annual nutrient concentrations. Mean annual values derived from long-term data are effective values integrating short-term dynamics. If the model is applied to a shorter time scale, then the input data and parameters have to be adapted in a time-scale consistent manner. Inflowing water fluxes The path transformation model WETTRANS distinguishes between seven transverse inflow pathways and one longitudinal inflow pathway (Table I). The inflowing water fluxes are quantified with areadependent equations given in Table I. The required climate data (mean annual precipitation and mean annual evapotranspiration) are obtained from the nearest meteorological station. Hydrologically important spatial information, such as wetland area, upstream basin area and groundwater basin area, is determined from digital hydrological maps and digital elevation models. In order to quantify the different transverse inflow water fluxes, the groundwater basin is subdivided into three zones: (1) slopes ranging from the wetland margin to the first hilltop; (2) a zone of infiltration of young oxic groundwater ranging from the first hilltop to 500 m width measured from the wetland margin; and (3) a zone of old anoxic groundwater ranging from 500 m width measured from the wetland margin to the groundwater basin divide. These areal data are calculated from a digital elevation model. Water inflow from slopes is divided into surface runoff occurring after heavy rainfall events, pipe/tile drainage from drained slopes and interflow from non-drained slopes. Other studies have shown that surface runoff contributes little to river discharge in the glacial moraine landscape of northern Germany (Jelinek, 2000; P¨opperl et al., 2001). Therefore, a path-specific coefficient of ktSR D 0Ð15 and ktTD /ktIF D 0Ð85 for partitioning surface runoff (ktSR ) and interflow (ktIF ) and tile drainage (ktTD ) is sufficient (Table I). Nitrogen concentrations of inflow pathways Hydrological inflow pathways entering wetlands differ in their nutrient concentrations due to the specific pathway formations and different residence times of the water within the hydrological system. Table II gives an overview of the statistical variation of total nitrogen (Nt ) and nitrate nitrogen (NO3 -N) concentrations in waters from different sources in Schleswig-Holstein. Data were collected from regional water-quality monitoring programmes for precipitation, groundwater, river water and seepage water. The WETTRANS model calculates nitrogen transformation on the basis of mean annual total nitrogen concentrations for the Table I. Description of transverse (t) and longitudinal (l) inflow pathways and their calculation for use in the path transformation model WETTRANS (Q is discharge; A is area, P is precipitation, ETAL is evapotranspiration from land surface; k is path-specific coefficient) Area definition (m2 )
Calculationa
k
Wetland (W) Slope (S) Drained slope (Sd) Undrained slope (Su) Lateral groundwater basin between first hilltop and 500 m basin width (YGox) Lateral groundwater basin between 500 m basin width and basin boundary (YGan) Inflow from deeper aquifer Upstream basin area (UP)
QWP D AW P QSR D AS P-ETAL ktSR QTD D ASd P-ETAL ktTD QIF D ASu P-ETAL ktIF QYGox D AYGox P-ETAL ktYGox
1 0Ð15 0Ð85 0Ð85 1
QYGan D AYGan P-ETAL ktYGan
1
Inflow pathway t t t t t
Wetland precipitation (WP) Surface runoff (SR) Tile drainage (TD) Interflow (IF) Young oxic groundwater (YGox) t Young anoxic groundwater (YGan) t Old anoxic groundwater (OG) 1 River inflow (RI) a
QOG D after regional studies QRI D AUP P-ETAL
—
ktSR C ktTD D 1; ktSR C ktIF D 1; AS D ASu C ASd .
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Table II. Total nitrogen (Nt ) and nitrate nitrogen (NO3 -N) concentrations of different water sources in Schleswig-Holstein, Germany. Data are based on an evaluation of available regional water monitoring programmes. The column ‘range’ contains information about the most common statistical distribution of total nitrogen concentrations for the water sources Path RI WP SR TD IF YGox YGan OG
Inflow pathway
Nt (mg l1 )
Range Nt (mg l1 )
NO3 -N (mg l1 )
River water Precipitation Surface runoff Tile drainage Interflow Young oxic groundwater Young anoxic groundwater Old anoxic groundwater
5Ð0 1Ð5 5Ð0 20Ð0 10Ð0 10Ð0 1Ð0 0Ð5
2.0–7.0 1.0–2.0 2.0–15.0 5.0–30.0 5.0–30.0 1.5–20.0 0.0–10.0 0.0–1.0
3Ð6 0Ð7 4Ð3 18Ð0 10Ð0 9Ð4 0Ð5 0Ð0
different flow pathways, regardless of the fact that denitrification is limited by nitrate availability. However, nitrate is the most dominant nitrogen form in the flow pathways with the highest total nitrogen concentration (Table II). Using total nitrogen concentration has the advantage of allowing an entire nitrogen mass balance to be calculated. The transverse pathways of surface runoff, pipe/tile drainage, and young groundwater in particular have a very high variability in their total nitrogen concentrations. The total nitrogen and nitrate concentrations in groundwater decrease with increasing residence time and well depth. Higher concentrations are found in waters with short residence times, such as surface runoff, pipe/tile drainage, interflow and young oxic groundwater (P¨opperl et al., 2001; Schleuß et al., 2002). The broad range of values for these pathways is caused by a high spatial variability in soil properties and land-use intensity, and by temporal fluctuations that depend on fertilizer output, vegetation development and weather conditions. Outflow pathways and transformation coefficients The path transformation model WETTRANS distinguishes between six hydrological outflow pathways from the wetland to the river (Table III). The water-partitioning matrix C describes the distribution from the inflow to the outflow pathways. The sum of the water-partitioning coefficients for each inflow pathway in each row equals unity. The values for the water-partitioning matrix C can be obtained by runoff measurement, tracer experiments, two-dimensional modelling of groundwater flow patterns or field mapping of outflow pathways in a wetland. The path transformation concept assumes, for each outflow pathway, a path-specific nitrogen transformation potential resulting from different hydrochemical regimes and residence times. In Table III, each outflow Table III. Hydrochemical characterization of outflow pathways in riparian peatlands for the definition of path-specific nitrogen denitrification potentials given as linear nitrogen transformation coefficients. A low transformation coefficient represents a high denitrification/transformation potential Outflow pathway
Subsurface outflow Overland flow River flooding Ditch outflow Drain outflow River throughflow Wetland evapotranspiration
SSO OVF RF DIO DRO RT WE
Copyright 2004 John Wiley & Sons, Ltd.
Hydraulic detention time
Oxygen state
Carbon availability
Slow Slow-medium Slow-medium Medium Fast Fast —
Anoxic Oxic–anoxic Oxic–anoxic Oxic–anoxic Oxic Oxic —
High Medium Medium Medium Low Low No
Total nitrogen transformation coefficients 0Ð2 0Ð4 0Ð4 0Ð6 0Ð8 0Ð9 1Ð0
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pathway is characterized by the hydraulic detention time, oxygen status and carbon availability in order to define path-specific transformation coefficients based on experimental data and expert knowledge. The transformation coefficients are deduced for wetlands with (1) an organic matter content of at least 30% in the upper soil layers to foster denitrification by sufficient carbon availability and (2) a width of the wetland zone of at least 50 m to ensure long enough residence times for lateral overland flow and subsurface flow. These conditions are supported by results from carbon addition experiments on denitrification rates, which indicate that in minerotrophic peat soils the carbon availability is sufficient to support denitrification (Davidsson et al., 2002). Blicher-Mathiesen and Hoffmann (1999) have measured high denitrification rates at the groundwater–peatland interface caused by rapid changes in redox conditions, which is similar to investigations at the soil–stream interface (Hedin et al., 1998). The nitrogen transformation coefficients decrease with increasing outflow path-specific residence times. Thus, ditches have a higher transformation potential than pipe/tile drains (Kronvang et al., 1995, 1999). Generally, the nitrogen transformation potential of the outflow pathways decreases in the order subsurface outflow > overland flow and river water flooding > ditch outflow > pipe/tile drain outflow > river flow > wetland evapotranspiration. Wetland evapotranspiration is included as a quantitatively important outflow pathway in the water budget; the nitrogen transformation potential for this pathway is negligible. Overland (saturation) flow on top of the peat layer as a result of lateral water inflow and river water flooding have nearly the same hydrochemical conditions and are, therefore, treated with the same transformation coefficients; the pathways differ in their nitrogen load, and thus in their contribution to total nitrogen retention.
APPLICATION OF THE MATRIX MODEL The matrix model WETTRANS is introduced by way of example in a riparian wetland, namely the Eider valley mire in northern Germany. The study area The Eider valley peatland is a 150 ha minerotrophic mire characterized by river water inflow from a 120 km2 upstream basin area and by groundwater inflow from a 15 km2 lateral basin surrounding the peatland. The valley mire is approximately 7 km long and the wetland width varies between 100 and 600 m. A thick impermeable clay and gyttja layer at the bottom of the peatland causes groundwater emergence only at the wetland margins (see Figure 4). The upper peat layer, a medium humified sedge and brown moss peat, indicates the development of a percolation mire with continuos subsurface flow. Water management activities during the past centuries, e.g. such as the mowing of macrophyte vegetation on the river, river deepening and straightening, and the construction and maintenance of a dense network of drains and ditches, has transformed the mire into an effectively drained peatland (Trepel, 2000). Parameterization of the WETTRANS model for the Eider valley The calculation of the inflowing water fluxes to the riparian wetland is based on areal information gathered from digital hydrological maps calculated from a digital elevation model, as well as digital land-use and soil maps (Trepel, 2000) (Table IV). The water flux calculations with 800 mm year1 precipitation and an evapotranspiration from terrestrial ecosystems ETAL D 470 mm year1 result in a mean annual groundwater recharge from terrestrial areas of 330 mm year1 and are based on measurements from a nearby climate station (Kluge et al., 1994; P¨opperl et al., 2001). Inflow of old groundwater from an underlying second aquifer was quantified with a geohydrological modelling study with the software package MODFLOW (Van der Aa et al., 2001). Copyright 2004 John Wiley & Sons, Ltd.
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Table IV. Inflowing water and nitrogen fluxes for different inflow pathways into the Eider valley peatland. Water fluxes are calculated according to Table 1 Inflow pathway
Wetland precipitation Surface runoff Tile drainage Interflow Young oxic groundwater Young anoxic groundwater Old anoxic groundwater River water inflow
(WP) (SR) (TD) (IF) (YGox) (YGan) (OG) (RI)
Area (ha)
Water flux (Mm3 year1 )
Nt (mg l1 )
Nitrogen load (t year1 )
150Ð0 150Ð0 37Ð5 112Ð5 750Ð0 600Ð0 — 12000Ð0
1Ð20 0Ð07 0Ð11 0Ð32 2Ð48 1Ð98 0Ð25 39Ð60
1Ð5 5Ð0 20Ð0 10Ð0 10Ð0 1Ð0 0Ð5 5Ð0
1Ð80 0Ð37 2Ð10 3Ð16 24Ð75 1Ð98 0Ð13 198Ð00
From Table IV, the vectors a (water influx, Mm3 year1 ) and b (nitrogen concentration, Mg l1 ) can now be written for the Eider valley case study: 1Ð20 1Ð5 0Ð07 5Ð0 0Ð11 20Ð0 0Ð32 10Ð0 aD bD 2Ð48 10Ð0 1Ð98 1Ð0 0Ð25 0Ð5 39Ð6 5Ð0 The water-partitioning matrix C distributes the inflow water to the outflow pathway. The partitioning values are based on the general geohydrological situation in the Eider valley peatland and the present water management strategies (Figure 3) (Trepel, 2000; Van der Aa et al., 2001). Presently, the Eider valley peatland is mainly drained by a dense network of small ditches and only partly by drainage tiles. The drains and ditches start generally at the peatland margins in the mineral soil and collect most of the lateral inflow. Only a small amount of water flows via subsurface flow through the peatland to the river. Precipitation is mostly consumed by evapotranspiration. Mean annual evapotranspiration from peatlands in northern Germany is ETAW D 560 mm year1 , resulting in a lower wetland water recharge of 240 mm year1 (P¨opperl et al., 2001; Schleuß et al., 2002). The net precipitation surplus from the peatland flows mainly via ditches and drains to the river. Only part of the water flows as surface and subsurface flow to the river. Flooding occurs on less than 21 days per year and affects only parts of the peatland. The outflow path-specific nitrogen transformation coefficients given in Table III were applied in the nitrogen transformation matrix D (Table V). The mobilization of nitrogen from the unsaturated zone is expressed in the WETTRANS model with negative transformation coefficients. A transformation coefficient of 0Ð3 increases nitrogen loss by 3Ð6 kg N ha1 year1 and a value of 0Ð5 by 6Ð0 kg ha1 year1 , which is consistent with nitrogen leaching rates from unfertilized wet meadows on peat soils (Trepel et al., 2000). The path-specific nitrogen load and system retention can now be calculated with the vectors a and b and the matrices C and D according to Equation (1). The results of the WETTRANS model are the three vectors given in Table V. Under present hydrological conditions, the Eider valley peatland receives a mean annual nitrogen input of ¾230 t year1 , where 85% is due to longitudinal nitrogen input via river water inflow. Transverse nitrogen input (with ¾30 t year1 ) accounts for only 14%. The Eider valley presently transforms ¾40Ð6 t year1 of nitrogen; the system transformation efficiency ST is 17Ð5%. About 190 t year1 of nitrogen leave the wetland as river outflow. Copyright 2004 John Wiley & Sons, Ltd.
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Inflow pathways
Water Partitioning Matrix C (-)
Nitrogen Transformation Matrix D (-)
WP
0.7
0.03
0.06
0.12
0.09
.
1.0
0.4
0.2
-0.3
-0.5
.
SR
.
.
0.1
0.7
0.2
.
.
.
0.2
0.6
0.8
.
TD
.
.
0.1
0.7
0.2
.
.
.
0.2
0.6
0.8
.
IF
.
.
0.1
0.7
0.2
.
.
.
0.2
0.6
0.8
.
YGox
.
.
0.1
0.7
0.2
.
.
.
0.2
0.6
0.8
.
YGan
.
.
0.1
0.7
0.2
.
.
.
0.2
0.6
0.8
.
OG
.
.
0.1
0.7
0.2
.
.
.
0.2
0.6
0.8
.
RI
.
0.05
.
0.05
.
0.9
.
0.4
.
0.6
.
0.9
SSO
DIO
DRO
WE
OVF
SSO
DIO
DRO
WE
OVF
RT
Outflow pathways
RT
Outflow pathways
Figure 3. Water-partitioning matrix C and nitrogen transformation matrix D for the present situation in the Eider valley peatland. Inflow pathways are: WP D wetland precipitation, SR D surface runoff, TD D tile drainage, IF D interflow, YGox D young oxic groundwater, YGan D young anoxic groundwater, OG D old anoxic groundwater and RI D river inflow. Outflow pathways are WE D wetland evapotranspiration, OVF D overland flow and river flooding, SSO D subsurface outflow, DIO D ditch outflow, DRO D drain outflow and RT D river throughflow
Table V. Results of the WETTRANS model for the Eider valley under present hydrological conditions. (For explanation of outflow pathway abbreviations see Figure 3)
e (Mm3 year1 ) f (mg l1 ) g (t year1 )
WE
OVF
SSO
DIO
DRO
RT
0Ð8 0Ð0 0Ð0
2Ð0 2Ð1 4Ð1
0Ð6 1Ð2 0Ð7
5Ð8 3Ð5 20Ð5
1Ð2 5Ð0 6Ð0
35Ð6 4Ð5 160Ð4
Mean
45Ð2 4Ð2 191Ð7
The path transformation approach gives a detailed insight into the water and nitrogen distribution from the inflow pathways to the outflow pathways (Figure 4). In the water budget, river inflow is quantitatively the most important source, followed by young and old groundwater. Under present water management conditions, most of the river inflow is not influenced from the riparian system and leaves the system nearly untreated as river flow. Quantitatively important outflow pathways are ditch outflows and tile drainage outflows. The nitrogen flow pattern is also dominated by river nitrogen load. However, young oxic groundwater inflow has a disproportionately high nitrogen load when compared with the water budget. In order to examine the plausibility of the calculated mass balances, the results are compared with mean annual data from a downstream water quality and gauging station. A mean measured annual discharge for the period 1975 to 1997 of 46Ð1 Mm3 year1 agrees well with the calculated value of 45Ð2 Mm3 year1 (Table V). However, annual discharge varies between a minimum of 20 Mm3 year1 in 1996 and a maximum of 78Ð6 Mm3 year1 in 1981. The calculated total nitrogen concentration of 4Ð2 mg l1 at the system outflow is of the same order of magnitude as the measured mean annual total nitrogen concentration in the period 1988 to 1996, which is 3Ð9 mg l1 with a standard deviation of the mean annual concentrations of š0Ð3 mg l1 (LANU SH, 2001). Scenario calculations for different water management options The WETTRANS model was developed to predict the effect of water management strategies on water and nitrogen exchange in wetlands in a quantitative manner to guide environmental agencies to the most effective management measure. Copyright 2004 John Wiley & Sons, Ltd.
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Surface runoff
0.1 ⇒
Tile drainage
0.1 ⇒
Interflow
0.3 ⇒
Young, oxic groundwater
2.5 ⇒
Young, anoxic groundwater
2.0 ⇒
Old groundwater
0.3 ⇒
0.4 ⇒ 0.1 ⇒ 3.2 ⇒ 24.8 ⇒ 2.0 ⇒ 0.1 ⇒
39.6 ⇒
198.0 ⇒
⇓ 1.2
⇓ 35.6
⇓ ⇓ 0.0 4.1
⇓ ⇓ ⇓ ⇓ 0.7 20.5 6.0 160.4
Drain outflow
River throughflow
Evapotranspiration
Subsurface flow
Water outflow (Mio. m3 yr-1)
River throughflow
⇓ 5.8
Drain outflow
⇓ 0.6
Ditch outflow
⇓ 2.0
Overland flow
⇓ 0.8
Ditch outflow
x10
Subsurface flow
x10
Overland flow
River inflow
Nitrogen distribution 1.8 ⇒
Nitrogen inflow (t N yr-1)
1.2 ⇒
Evapotranspiration
Water inflow (Mio. m3 yr-1)
Water distribution Wetland Precipitation
Nitrogen outflow (t N yr-1)
Figure 4. Water and nitrogen distributions from inflow pathways to outflow pathways under present hydrological conditions in the Eider valley peatland calculated with the matrix model WETTRANS. Nitrogen transformation occurs at active nodes between inflow and outflow pathways. The circle diameter is proportional to the nitrogen outflow
Efficient water management for increasing nitrogen transformation should start with the pathways having the highest loading. In the example, these are the transverse young oxic groundwater inflow and the longitudinal river water inflow (Figure 4). Two management options can be identified: In lateral buffer management, the drains and part of the ditches at the hillslope margins are removed from the peatland (Figure 5). This measure will recreate primarily overland flow on the surface of the peat and partly subsurface flow in the upper peat layer. Restoration of subsurface flow paths can probably be achieved only over a longer period, due to irreversible changes in soil physical properties (e.g. decreased hydraulic conductivity, bulk density and humification degree due to subsidence and peat mineralization during the drainage period). A second management option is longitudinal river management with the objective of increasing the flood duration in the peatland. This measure can be achieved by ceasing to mow macrophyte vegetation on the river during the summer months and by ceasing the removal of wooden detritus from the river. Both measures will increase hydraulic resistance in the river and increase the water level. A third management option is to combine both management strategies. To quantify the effect of these management options, the water-partitioning matrix C has to be adapted to the suggested outflow pattern (Figure 6). In respect of the uncertainty of the input parameters, the results from the scenario calculations are expressed only as semi-quantitative data in the form of percentage values (Table VI). The visualization of the scenario results exemplifies the changes of the nitrogen distribution pattern (Figure 7). In the lateral buffer scenario, total system transformation efficiency is, with nearly 20%, only 2% higher compared with the present situation. Drain and ditch removal result in an increase of transverse transformation efficiency from 38% to 52% (Table VI). The river management scenario assumes an increase of flooding in the valley mire. Therefore, the river water outflow and nitrogen load decrease. In contrast, the nitrogen outflow via ditch outflow, overland flow and, to a minor extent, subsurface outflow increase (Figure 7). Owing to the higher transformation Copyright 2004 John Wiley & Sons, Ltd.
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Lateral buffer management
Present flow pattern
River Eider drains
peat clay/gyttja
River management
flooding
upstream area
Figure 5. Water management options for the Eider valley peatland based on the present flow pattern, which is determined by the topography and geology of the watershed, an impermeable gyttja–clay layer at the bottom of the peatland and several drainage measures
potentials of these outflow pathways compared with the river throughflow, the total system transformation efficiency increases by 7% compared with the present situation. The longitudinal transformation efficiency increases from 14% to 23%. In a combined scenario, the transversal transformation efficiency increases by 15% and the longitudinal transformation efficiency by 9%. The results from the water management scenarios in comparison with the present situation show that lateral buffer management is a very efficient strategy to reduce lateral nitrogen flow before it reaches aquatic Table VI. Nitrogen transformation efficiency for present hydrological conditions and three management options in the Eider valley peatland. Transformation efficiency is calculated according to Equation (5) for transverse and longitudinal flow paths and the total system System transformation efficiency (%)
Transverse Longitudinal Total system
Copyright 2004 John Wiley & Sons, Ltd.
Present
Buffer
Flood
Combined
37Ð5 14Ð0 17Ð5
52Ð4 14Ð0 19Ð7
37Ð5 23Ð0 25Ð1
52Ð4 23Ð0 27Ð3
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Inflow pathways
Lateral Buffer Management
River management
WP
0.7
0.09
0.06
0.15
.
.
0.7
0.03
0.06
0.12
0.09
.
SR
.
0.3
0.2
0.5
.
.
.
.
0.1
0.7
0.2
.
TD
.
0.3
0.2
0.5
.
.
.
.
0.1
0.7
0.2
.
IF
.
0.3
0.2
0.5
.
.
.
.
0.1
0.7
0.2
.
YGox
.
0.3
0.2
0.5
.
.
.
.
0.1
0.7
0.2
.
YGan
.
0.3
0.2
0.5
.
.
.
.
0.1
0.7
0.2
.
OG
.
0.3
0.2
0.5
.
.
.
.
0.1
0.7
0.2
.
RI
.
0.05
.
0.05
.
0.9
.
0.1
0.05
0.15
.
0.7
WE
OVF
SSO
DIO
DRO
WE
OVF
SSO
DIO
DRO
RT
RT
Outflow pathways
Outflow pathways
Nitrogen inflow
Figure 6. Water-partitioning matrix C used for scenario calculations in the Eider valley peatland. Inflow pathways are WP D wetland precipitation, SR D surface runoff, TD D tile drainage, IF D interflow, YGox D young oxic groundwater, YGan D young anoxic groundwater, OG D old anoxic groundwater and RI D river inflow. Outflow pathways are WE D wetland evapotranspiration, OVF D overland flow, SSO D subsurface outflow, DIO D ditch outflow, DRO D drain outflow and RT D river throughflow
WP
⇒
SR
⇒
TD
⇒
IF
⇒
YGo
⇒
Buffer
Flood
Combined
YGan ⇒ OG
⇒
RI
⇒
x10
x10
⇓
⇓
⇓
⇓
⇓
WE OVF SSO DIO DRO
⇓ RT
⇓
⇓
⇓
⇓
⇓
WE OVF SSO DIO DRO
⇓ RT
x10
⇓
⇓
⇓
⇓
⇓
WE OVF SSO DIO DRO
⇓
RT
Nitrogen outflow Figure 7. Nitrogen distribution from inflow pathways to outflow pathways for three water management scenarios. The circle diameter is proportional to the nitrogen outflow. The present conditions are given in Figure 4. Inflow pathways are: WP D wetland precipitation, SR D surface runoff, TD D tile drainage, IF D interflow, YGox D young oxic groundwater, YGan D young, anoxic groundwater, OG D old, anoxic groundwater and RI D river inflow. Outflow pathways are: WE D wetland evapotranspiration, OVF D overland flow, SSO D subsurface outflow, DIO D ditch outflow, DRO D drain outflow and RT D river throughflow
ecosystems. The effects of this measure will become more difficult to detect with increasing upstream area and would require a long-term water-quality monitoring programme. The restoration of flooding conditions should probably have measurable effects on the water quality conditions downstream due to higher changes in the total system transformation efficiency. Copyright 2004 John Wiley & Sons, Ltd.
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DISCUSSION It is an ambitious task to develop a model for water and nitrogen exchange in wetlands as a decisionsupport system for wetland managers. Flow pattern, nitrogen concentrations and transformation processes have a high spatial and temporal variability within and between wetlands (e.g. Devito et al., 1989, 1996; Hedin et al., 1998; Drexler et al., 1999; Hill et al., 2000). Process-based models for nitrogen dynamics and nitrogen retention in surface-flow wetlands try to simulate all relevant processes with a high temporal resolution (Spieles and Mitsch, 2000). High demands on input data more or less restrict their use in scientific applications, because these data are not available during the first stages of restoration planning. Processbased models for nitrogen transformation and retention in groundwater-dominated wetlands are currently at their initial phase (Gold and Kellogg, 1997; Hoffmann et al., 2000). However, environmental authorities demand quantitative data for the effectiveness of a certain measure in order to select the most suitable sites and water management measures for wetland restoration. Empirical solutions were developed by ecological economists (Bystr¨om, 1998; Jansson et al., 1998) to support environmental authorities on a large scale in their effort to develop wetland restoration programmes, but empirical solutions are of limited use in the prediction of management effects on nitrogen retention in a certain wetland because they neglect the specific geohydrological conditions that control the water and nitrogen exchange in a wetland. Screening models, such as PREWET by Dortch and Jeffrey (1995), present a sophisticated solution for surface-flow wetlands. However, many wetlands receive considerable amounts of nitrogen via transverse flow paths. The matrix model WETTRANS fills the gap between complex, process-based models and simple estimations often used by practitioners in the field. The matrix model (1) combines a transverse and longitudinal perspective on water and nitrogen exchange, (2) simplifies the complex hydrological conditions of wetlands in a geohydrologically sound way, and (3) describes the effect of human-induced water management activities on the overall outflow flow pattern to the river. Nitrogen transformation is calculated with linear nitrogen transformation coefficients that depend on hydraulic detention time, oxygen status and carbon availability of the specific outflow pathway. The linear use of transformation coefficients integrates process knowledge in the decision-support system only in a simplified form at present. To improve the WETTRANS model to predict the effect of water management activities on nitrogen transformation rates, additional work on the development of non-linear and flow-length-specific transformation coefficients is needed. However, even with the above shortcomings, the model represents a valuable approach for the assessment of water and nitrogen exchange in riparian wetlands, both for wetland research and for management purposes. ACKNOWLEDGEMENTS
This research was supported by funds from the Landesamt f¨ur Natur and Umwelt des Landes Schleswig Holstein, Abteilung Gew¨asser. Three anonymous reviewers provided thorough and constructive comments. Paulette Clowes corrected carefully the usage of the English language.
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