Hans Thodsen

Diversity and Distribution of Riparian Plant Communities in Relation to Stream Size and Eutrophication

The present study was conducted in 47 different riparian areas distributed throughout Denmark to investigate diversity and distributional patterns of plant communities along a lowland stream size gradient (first to fifth order). The investigated areas were representative for Danish riparian areas not in use for agricultural production. We investigated plant community richness along a stream size gradient and the influence of eutrophication on the abundance of different plant communities. Vegetation analyses were performed in transects placed perpendicular to the stream channel, with a total of 1798 plots analyzed. Overall, we found a positive relationship between stream mean depth as a measure of stream size and the number of plant community types identified in the riparian areas. We also found that the abundance of the identified communities was positively correlated with their nutrient preference and negatively correlated with their moisture preference. The abundance of alkaline fens and Molinia meadows (protected community types) in riparian areas decreased with increasing size of the stream, whereas the abundance of humid meadows and wet herb fringes increased with increasing size of the stream. Based on our findings, we recommend that wide buffer zones be established along streams with protected habitat types in the associated riparian areas to reduce the direct impact from agriculture. Furthermore, we recommend that wide buffer zones be established along middle-sized and large streams because several community types may develop.

Multiobjective Calibration for Comparing Channel Sediment Routing Models in the Soil and Water Assessment Tool

Sediment transport is important for ecology and water quality in receiving waters. Physically based channel erosion methods were implemented in the Soil and Water Assessment Tool version 2009 (SWAT2009) to improve sediment concentration (SS) results. In the study, the default simplified Bagnold sediment routing method (EQN-0) and the physically based simplified Bagnold sediment routing method (EQN-1) were compared with Pareto fronts from multiobjective calibration. Two SWAT models using EQN-0 and EQN-1 were set up for a small agricultural Danish catchment and calibrated with multiobjective calibration on daily and weekly flow-weighted SS (WF SS). Results showed that the Pareto front of EQN-1 was slightly closer to the optimal point than EQN-0 in the objective space. Trade-off between WF SS and daily flow in EQN-1 was smaller than in EQN-0. The EQN-0 method generated more surface erosion to improve WF SS. The EQN-1 method generated more base flow to improve WF SS. The EQN-1 method was less dependent on surface erosion and simulated peak and low values of WF SS better than EQN-0. Therefore, EQN-1 is more suitable than EQN-0 in modeling SS in small lowland catchments.

The combined effects of fertilizer reduction on high risk areas and increased fertilization on low risk areas, investigated using the SWAT model for a Danish catchment

Some agricultural areas lose considerably more than the average amounts of nutrients to waterways (high risk areas, HRAs) and others considerably less than the average (low risk areas, LRAs). These areas are of great interest when river catchment managers seek to both reduce nutrient loads to lakes and marine areas and to allow intensive agriculture. If HRAs were farmed with decreased inputs of fertilizers the environmental benefit would be larger here than from any other areas, and if LRAs were farmed with increased fertilizer use it could be done here causing less environmental damage than at any other areas. If both these changes were applied within the same catchment they might counter balance each other and give the possibility of intensified farming without causing environmental deterioration. We used the semi-distributed SWAT model to identify both HRAs and LRAs in an intensely farmed lowland catchment in Denmark. These areas are classified as the 10% of the agricultural area leaching, respectively, the most and the least nitrogen. Two scenarios were run for HRAs (reduced fertilizer input by 20%) and LRAs (increased fertilizer input by 20%) separately and two were run where both HRAs and LRAs were included. The scenario results showed that the HRA scenario yielded a decrease (3.3%) in nitrate river load at the catchment scale and that the LRA scenario yielded only a small increase (0.9%). The combined scenarios showed an overall decrease in river nitrate load (2.2%).

Quantifying the combined effects of land use and climate changes on stream flow and nutrient loads: A modelling approach in the Odense Fjord catchment (Denmark)

Water pollution and water scarcity are among the main environmental challenges faced by the European Union, and multiple stressors compromise the integrity of water resources and ecosystems. Particularly in lowland areas of northern Europe, high population density, flood protection and, especially, intensive agriculture, are important drivers of water quality degradation. In addition, future climate and land use changes may interact, with uncertain consequences for water resources. Modelling approaches have become essential to address water issues and to evaluate ecosystem management. In this work, three multi-stressor future storylines combining climatic and socio-economic changes, defined at European level, have been downscaled for the Odense Fjord catchment (Denmark), giving three scenarios: High-Tech agriculture (HT), Agriculture for Nature (AN) and Market-Driven agriculture (MD). The impacts of these scenarios on water discharge and inorganic and organic nutrient loads to the streams have been simulated using the Soil and Water Assessment Tool (SWAT). The results revealed that the scenario-specific climate inputs were most important when simulating hydrology, increasing river discharge in the HT and MD scenarios (which followed the high emission 8.5 representative concentration pathway, RCP), while remaining stable in the AN scenario (RCP 4.5). Moreover, discharge was the main driver of changes in organic nutrients and inorganic phosphorus loads that consequently increased in a high emission scenario. Nevertheless, both land use (via inputs of fertilizer) and climate changes affected the nitrate transport. Different levels of fertilization yielded a decrease in the nitrate load in AN and an increase in MD. In HT, however, nitrate losses remained stable because the fertilization decrease was counteracted by a flow increase. Thus, our results suggest that N loads will ultimately depend on future land use and management in an interaction with climate changes, and this knowledge is of utmost importance for the achievement of European environmental policy goals.

Future water availability in the largest freshwater Mediterranean lake is at great risk as evidenced from simulations with the SWAT model

Inter- and intra-annual water level fluctuations and changes in water flow regime are intrinsic characteristics of Mediterranean lakes. Additionally, considering climate change projections for the water-limited Mediterranean region, increased air temperatures and decreased precipitation are anticipated, leading to dramatic declines in lake water levels as well as severe water scarcity problems. The study site, Lake Beyşehir, the largest freshwater lake in the Mediterranean basin, is - like other Mediterranean lakes - threatened by climatic changes and over-abstraction of water for irrigated crop farming. Therefore, implementation of strict water level management policies is required. In this study, an integrated modeling approach was used to predict the future water levels of Lake Beyşehir in response to potential future changes in climate and land use. Water level estimation was performed by linking the catchment model Soil and Water Assessment Tool (SWAT) with a Support Vector Regression model (ε-SVR). The projected increase in temperature and decrease in precipitation based on the climate change models led to an enhanced potential evapotranspiration and reduced total runoff. On the other hand, the effects of various land use scenarios within the catchment appeared to be comparatively insignificant. According to the ε-SVR model results, changes in hydrological processes caused a water level reduction for all scenarios. Moreover, the MPI-ESM-MR General Circulation Model outputs produced the most dramatic results by predicting that Lake Beyşehir may dry out by the 2040s with the current outflow regime. The results indicate that shallow Mediterranean lakes may face a severe risk of drying out and losing their ecosystem values in the near future if the current intensity of water abstraction is not reduced. In addition, the results also demonstrate that outflow management and sustainable use of water sources are vital to sustain lake ecosystems in water-limited regions.

The impact of the objective function in multi-site and multi-variable calibration of the SWAT model

Abstract Automatic calibration of complex hydro-ecological models is an increasingly important issue which involves making decisions. One of the most relevant is the choice of the objective function, but its effects have been scarcely studied in complex models. We have used the SWAT model to assess the impact of the objective function for a multi-site (4 stations) and multi-variable (OrgP, OrgN, NO 3 − , PO 4 3− ) calibration of the Odense catchment (Denmark). Six calibration schemes were tested, varying the objective function and the nutrient fractions targeted. The best performance metrics (R 2 , NSE, PBIAS) were obtained when using NSE as objective function and targeting N-fractions and P-fractions separately. The scheme was validated in another SWAT set-up in northern Denmark. Although NSE is often questioned, we found it as an adequate objective function when addressing a multi-site and multi-variable calibration. Our findings may serve as guideline for hydro-ecological modellers, being useful to achieve watershed management goals.

Modelling diffuse nitrogen loadings of ungauged and unmonitored lakes in Denmark: Application of an integrated modelling framework

Abstract According to the EU Water Framework Directive all freshwater bodies must obtain good ecological status by 2015. In Denmark this means that all lakes with a surface area above 5 ha (∼600 lakes) must be evaluated individually and mitigation measures must be enforced if the ecological status is below “good”. In consequence, the nutrient pressures from point and diffuse sources must be assessed based on a quantification of the nutrient loading of each lake. In this study we focus on the loading of nitrogen. Few of the 600 lakes are monitored for runoff and nitrogen loading. Therefore, a national 3D hydrological model covering all major parts of the country was used to estimate runoff from the lake catchments. The diffuse nitrogen input to each lake was simulated with an empirical nitrogen model. Where lakes are located upstream/downstream of each other, a calculation chain involving the nitrogen retention in lakes was created. A case study was selected using a large river basin including 23 lakes in Denmark. The cost‐effectiveness of reducing nitrogen loadings from different sub‐catchments in the case study river basin to the sea was calculated. The cost‐effectiveness associated with reducing the N‐loading from the most expensive sub‐catchment is shown to be 20 times more expensive than from the cheapest sub‐catchment.

Climate change effects on lowland stream flood regimes and riparian rich fen vegetation communities in Denmark

ABSTRACT There is growing awareness that an intensification of the hydrological cycle associated with climate change in many parts of the world will have profound implications for river ecosystem structure and functions. In the present study we link an ensemble of regional climate model projections to a hydrological model with the aim to predict climate driven changes in flooding regimes in lowland riparian areas. Our specific aims were to (1) predict effects of climate change on flood frequencies and magnitudes in riparian areas by using an ensemble of six climate models and (2) combine the obtained predictions with the distribution of rich fen communities to explore whether these are likely to be subjected to increased flooding by a climate change induced increase in river runoff. We found that all regional climate models in the ensemble showed increases in mean annual runoff and that the increase continued through the two scenario periods, i.e. 2035–2065 and 2070–2099. We found concomitant increases in flood levels and flood frequencies. Flood levels and frequencies increased at sites both where the maximum water level was governed directly by river water runoff and where it was governed by river flow roughness (weed cover). We did not find evidence that the present flooding regime was an overall key factor determining the distribution of fen vegetation. However, with the predicted changes in flooding frequencies in the investigated areas we expect to see changes in species compositional patterns within the fen areas under a future climate that may affect the conservation value of these. EDITOR Z.W. Kundzewicz ASSOCIATE EDITOR T. Okruszko

Comparison of active and passive stream restoration: effects on the physical habitats

Modification and channelization of streams and rivers have been conducted extensively throughout the world during the past century. Subsequently, much effort has been directed at re-creating the lost habitats and thereby improving living conditions for aquatic organisms. However, as restoration methods are plentiful, it is difficult to determine which one to use to get the anticipated result. The aim of this study was to compare two commonly used methods in small Danish streams to improve the physical condition: re-meandering and passive restoration through cease of maintenance. Our investigation included measurement of the physical conditions in 29 stream reaches covering four different groups: (1) re-meandered streams, (2) LDC streams (the least disturbed streams available), (3) passively restored streams (>10 years stop of maintenance) and (4) channelized and non-restored streams. The in-stream habitats were compared through analysis of the measured physical parameters and by applying a habitat model. We found that re-meandering is a more effective way of re-creating near-natural physical conditions in small streams compared to passive restoration. This is probably due to the limited energy in small streams which restricts re-shaping of the stream channel. However, based on habitat suitability modelling, the change to the physical condition did not translate into improved habitat suitability for young of the year brown trout highlighting the value of using several methods when evaluating restoration success.

Potential benefits of farm scale measures versus landscape measures for reducing nitrate loads in a Danish catchment

To comply with the EU Water Framework Directive, Denmark must further reduce the nitrate (N)-load to marine ecosystems from agricultural areas. Under the anticipated future spatially targeted regulation, the required N-load reductions will differ between catchments, and these are expected to be mitigated by a combination of land and water management measures. Here, we explored how the expected N-load reduction target of 38% for a Danish catchment (River Odense) could be achieved through a combination of farm and landscape measures. These include: (a) N-leaching reduction through changing the crop rotation and applying cover crops, (b) enhancing N-reduction through (re)establishment of wetlands, and (c) reducing N-leaching through spatially targeting of set-aside to high N-load areas. Changes in crop rotations were effective in reducing N-leaching by growing crops with a longer growing season and by allowing a higher use of cover crops. A combination of wetlands and changes in crop rotations were needed for reaching the N-load reduction target without use of set-aside. However, not all combinations of wetlands and crop rotation changes achieved the required N-load reduction, resulting in a need for targeted set-aside, implying a need for balancing measures at farm and landscape scale to maximize N load reduction while minimizing loss of productive land. The effectiveness of farm scale measures is affected by farm and soil types as well as by N-reduction in groundwater, while the possibilities for using wetlands for decreasing the N-load depends on landscape features, allowing the establishment of wetlands connected to streams and rivers.