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Scenarios and Model Describing Fate and Transport of Pesticides in Surface Water for Danish Conditions

5 General Conditions for the Simulations

5.1 Simulation Period
5.2 Time Steps of the Simulations
5.3 Initial Conditions

5.1 Simulation Period

It was initially expected to simulate a 10-year period. However, due to problems with the size of the pre-calculated files with results of the water simulations, the simulation period was reduced to four years.

The selected years are 1994, 1995, 1992 and 1997 for Lillebæk and 1998, 1999, 1990 and 1997 for Odder Bæk.

For Odder Bæk, 1998 has an average spring, a wet autumn, 1999 has a wet spring and an average autum. The result is a long wet winter, favouring leaching. 1990 is an average year all through. 1997 is a dry year, which is required to bring the total water balance back to what it is for the 10-year period as such. In all, for the four years, the precipitation minus the actual evaporation is about 19 mm above the average value for the 10-year period.

Table 5.1 Accumulated values of rainfall minus actual evaporation for Odder Bæk for the years 1990 to 2000. The values are accumulated over the specified period and ranked. `= 1990, * = 1997, # = 1998 and ¤= 1999. The boxed values in the middle of the table are the average value for the total period.
Tabel 5.1 Akkumulerede værdier af nedbør minus aktuel fordampning for Odder Bæk for årene 1990-2000. Værdierne er akkumulerede over de specificerede perioder og sorteret efter størrelse. `= 1990, *=1997, # = 1998 og ¤= 1999. De indrammede værdier i midten af tabellen er gennemsnittet for hele perioden.

Table 5.1 shows how the selected years compare to other years. It was attempted to ensure that two values were above average and two were below average for each analysed period. Although this was not fulfilled for all periods, the resulting combination was the best that could be obtained.

The same sort of logic has been tried implemented for Lillebæk Table 5.2. 1994 is a very wet year, particularly with a wet spring but also with a wet period in September. The winter 1994- 1995 is a continuous period with high percolation. 1992 is close to average, while 1997 is a dry year. The distribution of dry and wet years is such that for each of the analysed periods, two are above and two are below average, except for the yearly totals, where two is below, one is average and one is above the average value. The average value for the four years is 16 mm above the average for 1990-2000.

Table 5.2 Accumulated values of rainfall for Lillebæk for the years 1990 to 2000. The values are accumulated over the specified periods and ranked. ' = 1992, ¤ = 1994, # = 1995 and * = 1997. The boxed values in the middle of the table are the average value for the total period.
Tabel 5.2 Akkumulerede værdier af nedbør for Lillebæk for årene 1990-2000. Værdierne er akkumulerede over de angivne perioder og sorteret efter størrelse. ' = 1992, ¤ = 1994, # = 1995 og * = 1997. De indrammede værdier midt i tabellen er de gennemsnitlige værdier for hele perioden.

Table 5.2 shows how the selected years for Lillebæk compare to other years. Also the recharge to groundwater and the drain flow was analysed. This is shown in Table 5.3.

Table 5.3 Ranking of yearly flows from drain to stream and recharge to groundwater. The asterisks indicate the selected years.
Tabel 5.3 Sorterede værdier af årlige afstrømninger fra dræn til å og nedsivning til grundvand. Stjernerne indikerer de udvalgte år.

One particular event in the September 1994 – included in the Lillebaek-simulations as 1994 and 1998 requires particular attention. On the 16th and 17th September, 56.6 mm and 18.5 mm of rain falls in the scenario. These values are found in other rainfall records from Funen as well, and are as such realistic. However, such values do not occur very often. For the 24-hour rainfall the value equals a 10 year-occurrence and for the combined 48-hour-rainfall, it equals a one in 20 year-occurrence. These values are derived according to the recommendations given by Spildevandskomiteen and the appropriate excel-sheets can be found on http://www.er.dtu.dk/projects/svk26/.

The same four years are used as a warming up period for the solute transport calculations, resulting in an 8-year simulation of solute transport. The evaluation may be done on the last four years.

NB- due to the fact that the warming up period in Lillebæk is one day too short, the dates for the last four years are wrong by one day. It has been ensured that all files are compatible, but it means that when spraying takes place on eg. 1.5, it appears in the result files on 30.4 for the last four years.

5.2 Time Steps of the Simulations

Time steps vary over the different parts of the model. During the water simulation, the models adjust their internal time steps to the conditions, and during rainfall events, the models may calculate in fractions of seconds. However, due to the approach chosen, where the water files are produced and stored, and the solute transport is implemented based on the stored flows, there is a certain averaging taking place in the model. The MIKE 11 water movement is stored every 60 minutes, and the level of detail in this part of the model therefore lives up to the original expectations. Wind drift and deposition will therefore be simulated as “instant” concentrations.

With respect to the storing steps in MIKE SHE, there was a considerable conflict between the length of the simulation period and the preferred storing frequency. The following steps were taken to overcome the problem:

• The reading and writing routines of MIKE SHE were rewritten to zip the information while writing,
• Drainage flows are now stored in MIKE SHE at the same frequency as overland flow, and not, as before as groundwater,
• The simulation period has been reduced from 10 years to 4 years to minimize the period required with detailed storing in MIKE SHE.
• Investigations were made to find a way to avoid the approx. 4 Gigabytes limit for one file in the Windows system. By formatting the disks as NTFS, larger files can be generated. However, it was not, in the time of the project, possible to make the solute transport programme accept the large files. There may be ways around this problem with specialised compilers, but the time did not allow further investigations.
• External files of 160 Gigabytes with FireWire connection were bought to the project with the purpose of storing MIKE SHE water movement files and handing them over to the Danish EPA.

The final storage frequency of water movement information in for Odder Bæk stream and pond is as follows:

• 2 hours on the overland flow and drainage components,
• 24 hours in the unsaturated zone, and
• 24 hours in the saturated zone

The calculation time steps are maximum 2 hours for the overland and unsaturated zone components, and maximum 6 hours for the saturated zone. The internal MIKE 11 timestep is 3 minute. However, no more than 0.5 mm rain is allowed within a timestep.

For the Lillebæk scenario, the storing frequency is 4 hours for the overland flow and drainage component, 16 hours for the unsaturated zone and 48 hours for the saturated zone. The calculation time step are maximum one hour for the overland flow and saturated zone-components and 6 hours for the saturated zone. The timestep for MIKE 11 is 3 minutes. Maximum 1 mm of rain is allowed in a timestep.

The storing frequency is considered too low to obtain optimal results from the simulations. To improve the quality of data it is necessary either

• to break the limit of approximately 4.2 Gigabytes in one file,
• to change the output file system of MIKE SHE so the result files are split up, or
• to run the water and solute simulation together to avoid the intermediate files. This last solution will, however, require considerable code changes and will make the simulations very slow.

The first solution may be technically possible, while the second definitely is. It does, however, require some re-coding.

The internal time step of the solute transport calculation is regulated to ensure model stability. The results of the solute transport calculation in MIKE 11 are stored with variable frequency. During the first two hours after spraying, values are stored every 10 minutes. For the next 30 days, data are stored hourly. The rest of the period, data are stored every third day. These files are used for the output programme. MIKE 11 can store the results with much larger frequency, but the result presentation programme cannot read the data.

Solute transport is stored every 30 days in MIKE SHE, where it is only used for generation of monthly and yearly solute balances.

5.3 Initial Conditions

The initial conditions for the model stem from the original model. No solute is present in the system at the beginning of the simulation. However, with the four years warming-up period, it is expected that the system has warmed up sufficiently, and that pesticide concentrations in the stream can be used for analysis after this time.

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Version 1.0 Maj 2004, © Danish Environmental Protection Agency