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Agricultural 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 234 A. Grimvall et al. 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 123 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. 236 A. Grimvall et al. 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 238 A. Grimvall et al. 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. 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