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Scenarios and Model Describing Fate and Transport of Pesticides in Surface Water for Danish Conditions 6 Mass Balances of Scenarios6.1 Water balances 6.1 Water balancesThe water balances are fixed for each scenario, as the water files have been prepared in advance. The summarised water balances are included in the tables below. The monthly water balances for the full runs are shown in Appendix E. Due to the fact that the actual time steps used in the model are not completely identical, there are small differences between the precipitation sums in the balances. It is seen that surface runoff is negligible for all scenarios. There are differences between the scenarios due to the crop choice, but they are small. Click here to see Table 6.1 – 6.8. Table 6.1 Average monthly waterbalance for Odder Bæk pond, when the catchment is cropped with spring barley. Table 6.2 Average monthly waterbalance for Odder Bæk pond, when the catchment is cropped with winter barley. Table 6.3 Average monthly waterbalance for Odder Bæk, when the catchment is cropped with spring barley. Table 6.4 Average monthly waterbalance for Odder Bæk, when the catchment is cropped with winter wheat. Table 6.5 Average monthly waterbalance for Lillebæk pond, when the catchment is cropped with spring barley. Table 6.6 Average monthly waterbalance for Lillebæk pond, when the catchment is cropped with winter wheat. Table 6.7 Average monthly waterbalance for Lillebæk , when the catchment is cropped with spring barley. Table 6.8 Average monthly waterbalance for Lillebæk , when the catchment is cropped with winter wheat. 6.2 Solute balance examplesSimulations were carried out for Substance 2 defined for the FOCUS scenario in order to produce a set of examples. Mass balances were produced in order to show how the substance spreads through the different scenarios. In addition, plots from the different runs are presented. When comparing with the FOCUS surface water scenarios it should be noted that a very high percentage of the upstream catchment is sprayed in these scenarios, while in the FOCUS surface water scenarios, it is only 20 % of the area. Secondly, for the Lillebæk scenario the peak occurring around the 16th of September 1994 and 1998 is a one in 10-years or one in 20 years-event based on the 24 and 48 hour rainfall alone. Several active substances simulated with the model produce very high peaks on this particular date. Figure 6.1 to Figure 6.4 show the concentrations along the stream or in the pond. The upper left plot shows the concentrations as a function of time and position along the stream. The plot in the upper right corner shows the highest concentration per event within a 24 hours period along the length of stream. In the lower left is shown a number of time series for selected points along the stream. The highest concentration obtained in the Odder baek scenario is 1.815µg/l, descending to about 0.5 µg/l after 12 hours. Substance 2 does not seem to have been calculated for the FOCUS D3-stream-scenario, which is most comparable. However, for the D3-ditch, concentrations reach 5.27 µg/l. The Odder pond-scenario reaches 1.6 µg/l at the maximum. The pond overflows from time to time, and due to this, it is to some extent comparable to a stream section. For Lillebæk, the FOCUS D4-scenario is the most comparable. The prediction of maximum stream concentrations for substance 2 appears to be 3.92 µg/l. It should be taken into account that only 20 % of the upstream catchment is sprayed, meaning that the values from PestSurf is likely to be about 4.5 times higher due to the larger area sprayed. However, the highest concentration recorded is 6.6 µg/l in the downstream end of the catchment and 5.4 µg/l in the top end. The highest event observed occurs on the date with a 20 year-rainfall-event. In the upstream end, it is about a factor 1.5 higher than other events. For the downstream end, other events of the same magnitude are observed. Compared with the FOCUS-scenarios, the model thus simulates relatively lower concentrations, taking into considerations the assumptions. This is to be expected because the model takes into account a larger degree of variation between fields and drainage conditions. The Lillebæk pond scenario reaches 3.3 µg/l at the maximum. The pond seldom overflows, and pesticide in the pond tends to stay until it breaks down. For the shown simulations, breakdown in the groundwater is assumed for Odder pond only. This scenario is not drained, and this assumption leads to groundwater within the upper meter of the soil, for at least part of the year. It is therefore necessary to assume a breakdown in the upper part of the groundwater for this scenario. It is, however, obvious, that the water bodies over time receive a considerable amount of pesticide through the groundwater. This is a function of the shortcomings of the model described in Styczen et al. (2004a) and of the assumption that no breakdown takes place below 1 m from the surface. Click here to see Table 6.9 – 6.12. Table 6.9 Yearly mass balance for substance 2 in the Odder stream scenario in absolute values and percent Table 6.10 Yearly mass balance for Substance 2 in the Odder pond scenario scenario in absolute values and percent Table 6.11 Yearly mass balance for Substance 2 in the Lillebæk stream scenario scenario in absolute values and percent. Table 6.12 Yearly mass balance for Substance 2 in the Lillebæk pond scenario scenario in absolute values and percent. Click here to see Figure 6.1 – 6.4. Figure 6.1 Concentrations of substance 2 simuleret in the Odder Bæk stream scenario. Figure 6.2 Concentrations of substance 2 simuleret in the Odder Bæk pond scenario Figure 6.3 Concentrations of substance 2 simuleret in the Lillebæk stream scenario a) the upstream part b) the downstream part. Figure 6.4 Concentrations of substance 2 simuleret in the Lillebæk pond scenario
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