Nitrate (NO 3 − ) export from agricultural land poses an ongoing threat to both inland and coastal waters. Experimental studies investigating the hydrology-NO 3 − -export mechanisms require long-term ...data to identify reliable causal relationships. In this study, utilizing a 23-year continuous dataset with a high temporal resolution (daily to twice a week), we aim to identify potential drivers for NO 3 -losses and assess the impact of nitrogen (N) soil surface budgets on NO 3 -export. A drainage plot (4.2 ha) and a ditch catchment (179 ha) were fully equipped to register hydrological parameters, including water sample collection. Mean annual NO 3 − -N concentrations (loads) for the drainage plot and the ditch catchment were 9.4 mg l −1 (20.6 kg ha −1 ) and 6.0 mg L −1 (20.9 kg ha −1 ), respectively. Annual discharge was closely positively correlated with annual NO 3 -losses, highlighting the significant influence of prevailing weather and, consequently, hydrologic conditions on NO 3 -export rates. The majority of the annual NO 3 − -load was exported during winter (56% at the drainage plot, 51% at the ditch catchment), while the rest was exported during spring (28, 29%), summer (9, 9%) and fall (7, 11%). We could not find any direct relationships between N soil surface budgets and NO 3 -losses. Putting all results together, it can be concluded that agricultural activities for many decades resulted in high soil N stocks, which determined the general high NO 3 − -N concentration levels. Nevertheless, temporal NO 3 -export dynamics during the last two decades were clearly driven by hydro-meteorological conditions, nearly independently of land management and N soil surface budgets on the fields.
•Two infiltration methods were tested in a tile-drained catchment using SWAT.•Recently introduced Hooghoudt and Kirkham tile drain equations were applied.•Curve number method performed slightly ...better than the Green and Ampt method.•Tile flow was predicted reasonably by both infiltration methods.•Hooghoudt and Kirkham tile equations are suitable for modeling tile flow using SWAT.
Artificial drainage can contribute significantly to nutrient pollution in surface waters of tile-drained catchments. A realistic estimation of nutrient sources such as surface runoff, tile flow, and groundwater flow is essential in order to be able to predict nutrient loads realistically. The Soil and Water Assessment Tool (SWAT) provides two options for separating surface from subsurface flows, the empirical curve number and the physically based Green and Ampt method. In this study, we evaluated both infiltration models for a small tile-drained agricultural catchment in northeastern Germany using observed data from 2004–2013 and applying the recently introduced Hooghoudt and Kirkham tile drain equations. Model performance statistics indicated that the curve number method performed slightly better than the Green and Ampt method. Nash–Sutcliffe efficiencies (NSE) for discharge on a daily basis were 0.50 during calibration (0.42 during validation) for the curve number and 0.45 (0.39) for the Green and Ampt method. Tile flow was predicted with NSE values of 0.35 during calibration (0.36 during validation) for the curve number and 0.33 (0.62) for the Green and Ampt method, again on a daily basis. Discharge was mainly divided into tile flow and groundwater flow under both infiltration models. The only important difference with respect to flow components was related to surface runoff with negligible surface runoff using the Green and Ampt method and about 5% surface runoff using the curve number method. Greater tile drain depth and narrower spacing resulted in increased tile flow for both infiltration models, while total discharge remained unaffected by different drainage specifications. We conclude that both infiltration models have their justifications for other SWAT application projects in tile-drained catchments with similar characteristics. Overall, our study revealed that the physically based tile drainage routines which make use of the Hooghoudt and Kirkham tile equations are a promising instrument for modeling tile flow with SWAT.
This study aimed to assess trends of hydro-meteorological variables in the Upper Omo-Ghibe river basin, Ethiopia. Data records from eleven rainfall, eight air temperature, and five streamflow ...stations between 1981 to 2008 were investigated. The trends and change points were evaluated for different periods of time depending on data availability. Mann-Kendall and Pettit tests were used to identify trends and change points at a 5% significance level. The tests were applied to mean annual, monthly and seasonal time scales. Rainfall exhibited statistically decreasing trends at a mean annual time scale, while seasonal rainfall depicted heterogeneous results in both directions. For the majority of the stations, air temperature showed statistically significant increasing trends. The magnitude of change in temperature for mean annual, wet and dry season has increased about 0.48, 0.46, and 0.61 °C per decade for Jimma station. The Pettit test revealed that the late 1980s and 1990s were the change points. There is generally a decreasing trend in streamflow. The decline in annual rainfall and rise in temperature affected the streamflow negatively. Overall, the results indicate that trend sand change point times varied considerably across the stations and catchments. The identified significant trends can help to support planning decisions for water management.
In this study, 14 years of climate, stream flow, land management, nitrate-nitrogen (NO3-N) load and concentration data were analyzed to identify potential drivers for NO3-N losses at two tile-drained ...catchments under cropland use in northeastern Germany. Mean (±standard deviation) annual NO3-N concentrations were 9.7 ± 2.9 (drainage plot) and 6.8 ± 2.4 mg l−1 (ditch catchment), while mean annual NO3-N loads amounted to 22 ± 16 and 20 ± 16 kg ha−1, respectively. Significant positive relationships between annual discharge and annual NO3-N losses underlined the importance of hydrologic conditions on NO3-N export mechanisms. No direct relationships were found between N soil surface surpluses and NO3-N losses. Any possible impact of N soil surface surpluses on NO3-N export rates was overridden by the hydro-meteorological conditions in the catchment. Positive correlations between the climatic water balance and NO3-N losses suggest that agricultural catchments with similar characteristics as ours may face-without countermeasures-increased N losses in the future as regional climate projections predict wetter winters in the coming decades. Our analysis has further shown that effects of land management strategies aiming at reducing N losses into surface waters might only become visible with a delay of years or even decades.
Tile drainage is a widespread practice in agriculturally dominated lowlands with naturally high groundwater tables. A realistic estimation of the stream flow composition including tile drainage is an ...essential precondition for identifying major flow sources of nutrients. In this study, the Soil Water Assessment Tool (SWAT) was applied to the partially tile-drained Warnow catchment in north-eastern Germany to evaluate the effect of tile drainage systems on stream flow composition on a subbasin scale. In addition, model performance was tested after excluding tile drainages from the calibrated model setup. A sensitivity analysis revealed the highest sensitivities for parameters concerning evapotranspiration, soil characteristics, and groundwater flow, with a large variability in sensitivity ranks among the subbasins. Nash-Suttcliffe-Efficiencies (NSE) varied strongly among the subbasins for the tile-drained model setup ranging from 0.22 to 0.81 for the calibration and from −0.81 to 0.66 for the validation period. The percentage of tile flow varied between 0.3 and 31.9 %, and reflected statistically significantly (p < 0.05) the spatial extent of tile-drained areas within the subbasins. Excluding tile drainages from the model setup led to a strong decrease in model quality and to a changed stream flow constitution dominated by groundwater. The results of our study indicate that the SWAT model realistically represented the actual fractions of tile flow on discharge on the subbasin scale within the Warnow catchment. Therefore, we conclude that the incorporation of tile drainage systems is essential to calculate flow components accurately.
•Analysis of constructed wetland (CW) efficiency in reducing nitrogen (N) loading.•Using high-frequency sampling to determine nitrogen fractions.•CW performance was high when temperatures were > 8 °C ...and retention times were high.•Overall, decreasing N retention performance at increasing nitrogen loading.
A surface-flow constructed wetland to retain nitrogen was created between an agricultural tile-drainage plot and the outflow-receiving river in a maritime rural area of north-eastern Germany close to the southern Baltic Sea. The nitrogen loads were monitored from October to May over a period of four years at the inlet and outlet using high-frequency water sampling and flow measuring to assess the performance of the wetland. The effects of the physical boundary conditions on the nitrogen retention processes were analyzed by measuring meteorological indicators and water temperatures in the wetland as well as by a detailed mass balance calculation. The development of vegetation and the sedimentation of carbon and nitrogen in the wetland were also recorded. The results showed that the nitrogen retention efficiency of the wetland decreased with increasing nitrogen loading, while it increased with increasing residence times and temperatures. We conclude that the hydraulic retention time should be >20 days and the water temperature should be >8 °C in order to be able to effectively retain nitrogen. However, both conditions were only met from spring to autumn. In these periods, the development of dense, structure-rich macrophyte vegetation and the sedimentation of carbon and nitrogen quickly provided good conditions for nitrogen retention. In autumn and spring, however, less than 20% of the annual nitrogen load was observed. The total nitrogen loading was reduced by only 2.9% during the four monitored years. The study results suggest that the meteorological and hydrological conditions were not conducive to achieve a significant nitrogen retention with the help of surface-flow constructed wetlands in this climatic region. The successful implementation of wetlands in the maritime rural area close to the Baltic Sea should therefore consider the inner-annual distribution of the outflows from the tile drainage plot.
► Dendrochronological investigations of beech and oak along precipitation gradient. ► Growth strongly depends on water availability especially during June and July. ► Sensitivity of tree growth and ...correlations to climate increase along gradient. ► Numbers of pointer years increase, changes more pronounced for beech. ► oak might gain competitive advantages under the projected drier climate.
For north-eastern Germany regional climate models project rising temperatures in combination with decreasing summer and increasing winter precipitation. The resulting overall drier conditions during the growing season will considerably impact forest growth there. We evaluate the consequences of increasing drought on the growth of the two locally most important broadleaf tree species common beech (
Fagus sylvatica L.) and pedunculate oak (
Quercus robur L.). Three mixed forests of beech and oak were sampled along a west-east gradient of declining precipitation. In total we used 257 ring-width samples from 133 trees to build six species and site specific chronologies. Additionally, we modelled the soil water budget for each site. We performed continuous and discontinuous (pointer year) analysis of climate-tree-growth relationships with particular emphasis on inter-annual-variations and their dependence upon climatic factors (temperature, precipitation, soil moisture) and on the stability of the obtained relationships. Results of climate-growth correlations together with pointer year analysis indicate a strong dependency of growth of both species from water availability, especially during early summer (June and July). General correlation pattern between growth and climate are similar for both species, but climate sensitivity of beech is generally higher. We identified drought as the main driver of negative growth depressions in both species. Increasing drought stress along the gradient is expressed in higher correlations to climatic variables, higher sensitivity (variance) of growth, and a higher number of negative pointer years for both species. For beech we also found a significant trend of decreasing average growth rates along the gradient. Growth superiority of beech compared to oak declines with decreasing precipitation. The relationships were generally stable throughout the 20th century. A rise of sensitivity together with a higher frequency of negative pointer years during the last decades suggests that increasing climatic variability together with rising temperatures might be influencing growth of
Fagus at the more humid sites. If we substitute space by time it seems that already small changes in precipitation regime can have considerable impact, especially on the growth of beech. Other, more drought tolerant species like oak might gain competitive advantages under the projected climatic changes.
Nitrate (NO3−) concentrations and loads are still above ecologically acceptable levels in many open waters, including coastal seas. Agricultural measures aimed at reducing the nitrogen (N) surplus in ...fields usually only have an effect in the long term. A short-term reduction of N pollution can be achieved through technical solutions such as surface-flow constructed wetlands (SFWs). We analyzed the effect of SFWs to reduce nitrate‑nitrogen (NO3−-N) loads in the tile-drained agricultural Warnow river basin (approx. 3000 km2) in northeastern Germany. The aim of the study was to transfer the results from individual SFWs to the scale of the river basin. For this purpose, an eco-hydrological model (Soil and Water Assessment Tool) was used to reproduce streamflow and NO3−-N loads on a monthly basis for the basin outlet and six subbasins. Measured water and compound fluxes depicted well with the model, so that we could conduct a scenario analysis. 97 locations for the establishment of SFWs in tile-drained areas throughout the river basin were identified and integrated into the model environment. The results indicate that the NO3−-N loads for the entire basin could be reduced from 902,200 kg yr−1 to 842,900 kg yr−1, which corresponds to a NO3−-N reduction of 6.6%. NO3−-N removal rates varied widely between the subbasins, ranging from 5 to 86 g m−2 yr−1, and were positively correlated with NO3−-N input loads. We conclude that SFWs, if installed area-wide, can help to significantly reduce NO3−-N loads in a river basin and can be an important component of a comprehensive management strategy to reduce N loads in surface waters to acceptable levels. The installation of SFWs should be prioritized in hot-spot areas where the greatest benefit can be expected.
•Impact of wetlands on nitrate loads at catchment scale was analyzed using SWAT model.•97 potential wetland locations using GIS-algorithms were identified.•Nitrate loads can be reduced by 6.6% at the catchment outlet.•Nitrate removal rates varied strongly among sub-catchments.•Installation of wetlands should be prioritized focusing on hot-spot areas.
•Rainfall and moisture patterns determine discharge generation in tile-drained areas.•Six event types accommodate 83% of 212 analyzed storm events.•Nitrate dynamic is a function of the hydrograph ...independent of spatial scale.•Nitrate concentrations differ among event types.•Nitrate management in tile-drained catchments should consider storm event specifics.
Nitrate is one of the most important sources of pollution for surface waters in tile-drained agricultural areas. In order to develop appropriate management strategies to reduce nitrate losses, it is crucial to first understand the underlying hydrological processes. In this study, we used Principle Component Analysis (PCA) and Linear Discriminant Analysis (LDA) to analyze 212 discharge events between 2004 and 2011 across three spatial scales (68 events at the collector drain, 72 at the ditch, and 72 at the brook) to identify the controlling factors for hydrograph response characteristics and their influence on nitrate concentration patterns. Our results showed that the 212 hydrological events can be classified into six different types: summer events (28%), snow-dominated events (10%), events controlled by rainfall duration (16%), rainfall totals (8%), dry antecedent conditions (10%), and events controlled by wet antecedent conditions (14%). The relatively large number of unclassified events (15%) demonstrated the difficulty in separating event types due to mutually influencing variables. NO3-N concentrations showed a remarkably consistent pattern during the discharge events regardless of event type, with minima at the beginning, increasing concentrations at the rising limb, and maxima around peak discharge. However, the level of NO3-N concentrations varied notably among the event types. The highest average NO3-N concentrations were found for events controlled by rainfall totals (NO3-N=17.1mg/l), events controlled by wet antecedent conditions (NO3-N=17.1mg/l), and snowmelt (NO3-N=15.2mg/l). Average maximum NO3-N concentrations were significantly lower during summer events (NO3-N=10.2mg/l) and events controlled by dry antecedent conditions (NO3-N=11.7mg/l). The results have furthermore shown that similar hydrological and biogeochemical processes determine the hydrograph and NO3-N response on storm events at various spatial scales. The management of tile-drained agricultural land to reduce NO3-N losses should focus explicitly on flow events and, more specifically, active management should preferably be conducted in the winter season for discharge events after snowmelt, after heavy rain storms and when the soil moisture conditions are wet.
Phosphorus (P) is mainly leached by subsurface transport pathways in tile-drained landscapes. In this study, we modified the SWAT model (SWAT-P) by incorporating dissolved reactive phosphorus (DRP) ...losses from drainage water and the deep aquifer. SWAT-P was tested in a tile-drained lowland catchment using a multi-site calibration and validation approach. SWAT reached a good statistical performance regarding discharge for all sub-catchments and a daily time step. As discharge was dominated by subsurface flow, we optimized DRP concentrations for the drainage water (SWAT-P), the shallow aquifer (SWAT, SWAT-P), and the deep aquifer (SWAT-P) and left other P-related parameters at their default settings, since they did not influence the model output. DRP losses were simulated at a monthly time step using SWAT and SWAT-P. The predictive power was weaker compared to discharge for both SWAT and SWAT-P. Nevertheless, SWAT-P performed considerably better than SWAT. Additionally, calibrated DRP concentrations were unrealistically high for SWAT, whereas calibrated DRP concentrations reflected the prevailing conditions in the region using SWAT-P. The results indicated improved prediction accuracy for DRP losses into streams by using SWAT-P, as well as a roughly realistic estimation of DRP losses from tile drainage water. Further research is necessary to account for the temporal DRP concentration dynamics in drainage water. SWAT-P is ready to use after defining DRP concentrations in tile drainage water and the deep aquifer in the SWAT-P input files. In addition, the model output was extended in SWAT-P to visualize DRP losses from drainage water and from the deep aquifer.
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