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No. 185: Significance of probable underestimated precipitation on the calculated supply of water, nitrogen and phosphorus to the sea

Thodsen, H., Tornbjerg, H., Blicher-Mathiesen, G., Højbjerg, A.L., Stiesen, S. & Troldborg, L. 2020. Betydning af sandsynligvis underestimeret nedbør på den beregnede tilførsel af vand, kvælstof og fosfor til havet. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 32 s. - Teknisk rapport nr. 185


The modelled difference between two scenarios:

i) original precipitation scenario = DMI’s (Danish Meteorological Institute) 10x10km grid precipitation 1990-2018 unchanged

ii) modified precipitation scenario = DMI 10x10 km grid precipitation modified).

for the period 2011-2018 at national level (see section 3) shows that:

  • Freshwater runoff is 0-4% higher in the modified precipitation scenario than in the original precipitation scenario.
  • Nitrogen input is 0-4% higher in the modified precipitation scenario than in the original precipitation scenario.
  • Phosphorus input is 0-3% higher in the modified precipitation scenario than in the original precipitation scenario.

This means that the effect of the inconsistently measured and likely underestimated precipitation is that the freshwater runoff in the period 2011 – 2018 on average is underestimated by 0-4% at national level, that the nitrogen load is underestimated by 0-4%, and that the phosphorus load is underestimated by 0-3%, calculated using the methods used in Thodsen et al., (2019a). Underestimation of the nitrogen and phosphorus load to the sea is mainly due to underestimation of the freshwater runoff. Based on a subdivision of Denmark in to 8 regions the freshwater runoff, nitrogen and phosphorus loads are underestimated by 0-14% in the period.

For individual years, the difference will be greater, just as there will be greater differences on decreasing geographical scales.

The uncertainty of the results is significant, and there will be significant variations within the country. The greatest percentage uncertainty will be found in completely unmonitored catchment areas without point sources, while the least uncertainty is related to catchments with a high proportion of monitored catchment area.

The effect is only related to partially monitored (catchment with incomplete measurement time series) and unmonitored catchment areas.

The effect on the reference freshwater runoff for the Danish River Basin Management Plans 2021-2027, which is calculated for the period 1990-2018, is estimated to be -3 to 3% of the current reference runoff.

As a basis for the analysis, it is assumed that the current evaporation in the periods 1990-2010 and 2011-2018 is the same, even though the DMI has calculated that the potential evaporation is increased by approx. 4.5% in the last period. It is estimated that the effect of including such a change in potential evaporation is of low importance to the final results of the report. In addition, it is assumed that the cause of inhomogeneity in the precipitation data is found in the period after 2010. However, the DMI does not exclude the possibility of adjusting the precipitation data from the period before 2011.