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The requirements under the present River Basin Management Plans (RBMP) for the EU Water Framework Directive (WFD) are to reduce the total nitrogen (TN) loadings to the Hjarbæk Estuary in Denmark from the present (2015-2019) annual loading of 1900 tonnes N to 662 tonnes N in 2027. The catchment area to the estuary represents a total of 1177 km2 and the catchment is drained by four major streams. The Danish national monitoring programme has established gauging stations covering 969 km2 of the catchment area the remaining 208 km2 being ungauged areas. Modelled data on N-leaching from the root zone on agricultural fields and surface water monitoring data on N-export losses are available from the 1980’ies and onward.A detailed mapping of nitrogen (N) attenuation in the catchment have been conducted at a scale of ca. 15 km2 (ID15 sub-catchments) including mapping of both N-retention in groundwater and surface waters as well as N-delays in groundwater in Karst sub-catchments. The mapping shows large differences in N-retention in groundwater within the ID15 sub-catchment (80 %) and the same large variation is seen for N-retention in surface waters (80 %). An analysis of delays in the transport for N from fields to surface waters have shown that especially one of the four monitored catchments (Simested stream) experiences a long delay in N repsonses (> 10 years). A new portfolio of N mitigation measures to be adopted at source (e.g. catch crops, early seeding, set a side, afforestation) or during transport from field to surface water (several types of constructed wetlands, riparian buffers and restored wetlands) has been scientifically approved and made available for farmers by the Danish EPA and Agricultural Agency.The huge N-reduction needed in the Hjarbæk coastal catchment (65%) will require management efforts where farmers and authorities utilize both source oriented and transport oriented mitigation measures. These solutions should be implemented in a targeted manner guided by the local N-retention maps, as well as using all available monitoring data to pinpoint high-risk areas for N-leaching from fields and N-exports from the four sub-catchments as well as the ungauged areas. In this presentation we will showcase examples on how both targeted and collective mitigation measures can optimally be dosed in the Hjarbæk coastal catchment to reach the targets set in the RBMP 3.

The links between nitrate-nitrogen (N) leaching from agricultural fields and N measured in streams can in general terms be divided in two pathways: groundwater and a more surface-near transport (e.g. tile drains). The former with a typical slower hydrological response than the latter. Therefore, catchments with quick hydrologic pathways respond also quickly on programme of measures for N. On the other hand, catchments having bot high N-attenuation or longer N time lags makes it complicated for managers and policy makers as the response of the implemented programme of measures might both be dampened and delayed. As River Basin Management Plans (RBMPs) under the Water Framework Directive (WFD) runs in 6 years periods – such time lags might end up as an overdosing of measures. Therefore, attenuation and time lags needs to be mapped as they have major effects on the expected effects of RBMPs and its legacy for water quality. The aim of this study is to improve our understanding of N lags mapped based on 30 years of data from 160 Danish stream monitoring stations.A national wide screening for trends in annual flow-weighted total nitrogen (TN) concentrations at 163 river monitoring stations shows in most cases a downward trend (average: 30% ± 17%) during the last 30 years 1990-2019). The N-surplus has been reduced (farm gate: -44%; field: -45%) during the same period. Before 1990, the N-surplus in agriculture was increasing and started at first levelling off in the mid 1980ies. Diffuse N-sources and mostly agriculture contributed the most to TN in streams (93% ±8%) during the period 1990-2019). The reduction in the diffuse N loadings are paralleling the development of the N surplus for most Danish streams. However, in certain parts of Denmark several river monitoring stations shows a much different response, which in some cases is no response at all. Such a pattern can only be explained by N-flows in the catchments to be delayed in groundwater aquifers. Using long term data for national N-surplus a simple lag-time analysis shows that the time lags for N are long for 21 catchments (up to 20 years), medium long for N in 62 catchments and with nearly no delay for N in 80 catchments (Fig. 1). Moreover, all the stream stations experiencing long time lags are situated in the chalk and partly karstic landscapes of Denmark from the Danien period. The catchments having long delays for N shows in most cases also a very low attenuation of N in groundwater as measured N-concentrations are substantially higher than found in the streams having nearly no time lags. Therefore, we conclude that incorporation of biogeochemical and hydrologic time lag principles into water quality regulations will be necessary for providing managers and regulators with realistic expectations when implementing new policies for N.Figure 1: Map of Denmark showing catchments with short (< 20% older than 10 years), medium (20-40 % older than 10 years) and long (> 40% older than 10 years) for nitrogen in groundwater.

Intensive monitoring data from the Danish National Monitoring programme (NOVANA) from 24 smaller catchments (mean: 14 km2) was used in a two layer cross-validation to establish a model for the annual diffuse phosphorus (P) flow-weighted concentration in Danish streams. A total of 196 monitoring years with data from automatic sampling (ISCO) of water from the 24 streams were used as a training dataset. Data in the training dataset covers the period 1994-2002.Moreover, another dataset consisting of 108 agricultural mini-catchments with discrete water samples covering the period 1990-2017 was used as a control in the eight different georegions of Denmark. A total of four different models was established three models based on the intensive dataset and one model based on the larger dataset with discrete water sampling.The best model established included eight explanatory parameters and explained 53 % of the variation in the annual flow-weighted total P concentrations in the training dataset. A validation of the four different models established showed that the best model has to be bias-corrected in some of the georegions. The result of the validation shows that the models generally overestimate the total P concentrations. An overestimation of around 10-20% was to be expected as intensive automatic water sampling in streams has shown that the flow-weighted concentration of total P obtained from discrete sampling (monthly or fortnightly) is normally underestimated.The validations of the three models based on intensive dataset showed an R-square between 0.08 and 0.12. The model based on the larger data with discrete samples had an R-square (0.29).

Nitrogen (N) loads and concentrations have been successfully reduced in most Danish streams during the last 30 years. Thereby also reducing the impact of the main driver of marine eutrophication in Danish coastal waters. However, the trend in N-loads and concentrations vary substantially among the monitored streams. The understanding of this variation are of great importance and interest for the evaluation of measures implemented to combat N eutrophication and for forecasting of effects of further measures.River hydrographs can be split into base flow and quick flow components and the N concentrations in these two components can, thereafter, be calculated. The N concentration in the two components varies over time showing both longer term and seasonal variation. The quick flow component typically having a high variation reflecting present days leaching of N from fields and this strata has been significantly reduced during the last 3 decades due to a more sustainable farming practices.During base flow conditions, stream water typically holds less nitrogen due to N removal in groundwater. Reductions in agricultural nitrogen leaching over the past three decades has reduced concentrations in the quick flow component and reduced the load to ground water aquifers. As groundwater aquifers are often large with a capacity of several years of recharge, the response in base flow N-concentrations is expected to be slow compared to the response in quick flow. The low response of the N-concentrations in base flow have implications on the rate of change of the river concentrations and consequently riverine N-loads to coastal waters. In some cases, the base flow N-concentration might still be influenced by the larger N-leaching of the past (1960-1990).We have analyzed a national data set for developments in N-concentrations during base flow and quick flow. The data set covers the in country range in catchment size, land use and geology. The data set spans 29 years covering the period 1990 – 2018. In addition, measurements from a few streams monitored for a longer period have been included in the analyses