Pesticides Research No. 104 2006 – Establishment of a basis for administrative use of PestSurf – 5 Proposed changes to PestSurf and its presentation programs

Establishment of a basis for administrative use of PestSurf

5 Proposed changes to PestSurf and its presentation programs

During the process of data analysis, errors were found in the programme with respect to the calculation of adsorption of substance to sediment in the stream. These errors were corrected before the simulations were re-run. The required changes to the model documentation are described in Annex 14.

The originally used maximum time step for data storage for the stream was 3 days. This was changed to 1 day before the models were re-run in order to obtain a better description of the drainage peaks. However the storage frequency may be optimized further.

The variation of macrophytes over time is implemented in the streams only. This is in accordance with the text in Styczen et al. (2004b), but an issue to consider for an update. The change can be made by changing the templates used by the interface, and is thus a simple issue.

The PestSurf templates were updated with the points of maximum concentration identified as part of the present project. Furthermore, some minor errors in the spreadsheets were corrected as part of the project. The templates do not contain points from the tributaries to the two streams. It is not recommended to change this as the tributaries run dry for periods of the year. Nor is it recommended to use the data upstream of the “ovrelillebaek-500 m-location” for evaluations of toxicity, as the stretch may run dry during the summer. Any specific studies of results in other points can be analysed using the MIKE VIEW programme.

As mentioned earlier in the report an error was identified regarding the addition of wind drift to the water bodies. The error has been corrected and the simulations now generate values according to the assumptions made. However, for the streams, the assumptions that the whole stream system receives drift within 30 minutes is not realistic, and the concentrations generated, particularly in the sandy loam stream, are unrealistically high. Other assumptions are, of course, possible. The correct level is difficult to identify – not a single drift event was caught in the measurement programme set up during the PestSurf project (Styczen et al, 2004a).

The drift could be added over a longer time. This would still lead to a buildup of concentrations downstream. However, spraying during two days of 12 hours would lead to concentrations that would be up to 48 times smaller than the drift calculations under the present assumption. The drift contribution could be spread over the whole time period, where spraying is relevant. A spraying window of one week and assuming 12 hours of spraying time per day would lead to a reduction by a factor of up to 168. In comparison, only two of the simulations in the sandy loam catchment have shown changes in maximum concentrations from before to after the code change of more than this value. Presently, the difference between the D3-ditch and the sandy stream is a factor 4.3. Even by changing the spraying period to 12 hours, the scenario would produce concentrations that are considerably lower that what is generated in the D3-ditch.
A second method for reduction could be to consider wind directions and reduce the area contributing according to the time when wind directions are more than +/- 30 degrees away from the prevailing wind direction (and the directly opposite wind direction, as the stream may receive drift from both sides). We would then assume that farmers spray without taking into account the wind direction. In this case, the total dose going to the river would be reduced by the calculated fraction of time. This would lead to a reduction of less than a factor 2.
The present model (including drift) operates with five stretches with different buffer zones for each stream. The stream could be divided into more stretches (of 100 to 500 m) and could be sprayed in a random manner over a selected period. Each spray drift addition is made within 1-5 minutes, depending on the length, assuming that the farmer drives parallel with the stream with a speed of 8 km/hr, or alternatively 5-20 minutes, assuming that he is driving perpendicular to the stream. This option could be combined with option 2 by statistically sampling a wind direction for each stretch. This would lead to a more realistic distribution of drift than option 1. The result is still depending on the time over which the spraying is allowed, but less so, because the individual additions are much more separated than in the present description. “One stretch of 100 m per hour” within 12 hours per day means that the spraying is finished within 3.5 day for the sandy stream, which is the longest. More specific field size statistics and probability of rainfall during different periods can be taken into account. However, the “random manner” should be decided once and for all to avoid that different runs produce different results.

Option 3 is the most realistic option of the above, and therefore recommended. High concentrations reached due to drift at the top end of a stream, caused by small water levels, will still occur. Because the option would limit the buildup seen in the present simulations in downstream points, the concentrations due to drift should be reduced significantly further down. The changes required to implement this option are located in the user interface and in the MIKE 11 setup.

The presently used description of colloid transport in macropores leads to an exaggerated flux of pesticide to groundwater. An ongoing project is working on an improvement of the description. It is recommended to include the improved description in the model when it becomes available.

The present version of the model was built in the 2002- version of MIKE SHE/MIKE 11. The models have been through rather strong revision since then, including an ongoing reprogramming exercise of the advection/dispersion scheme of MIKE 11. It is recommended to change the setup into the new version as and when other changes are to be implemented. Because the old version is not supported at code level anymore, it is extremely difficult to make fundamental changes in the present version.

If the recommendation is followed, special attention should be given to the lower end of the sandy loam catchment and to the calculation of diffusion into the porewater to ensure that they perform as expected.

A question was raised concerning the ability to make longer simulations. As MIKE SHE uses the same water data repeatedly and because the MIKE 11 water files may be doubled, it is possible to extend the simulations. However, presently the time series defining input, crop growth etc. are generated for 8 years only. An extended simulation would then require extension of all the time series used, which is a minor task.