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Nordic hydrology 5 months from now: What can we tell you?

Eylert Ellefsen
Archived blog post. This blog post has been transferred from our previous blogging platform. Links and images may not work as intended.

The Nordic hydrological balance has weakened by about 10 TWh since the beginning of the year. Our models still show it's more likely than not that the balance will have recovered to above normal levels by the end of July.

Although we're only in the 2nd half of February, we would dare to offer an outlook on how the hydrological balance may evolve over the next five months based on the latest 20 years of historical variables.

Key observations

  • In the last 20 years, there's been a 50 TWh spread in the Nordic hydrological balance at the end of July – ranging from +20 TWh to -30 TWh
  • In the same period, the Nordic area's accumulated precipitation for weeks 9-30 has varied from 97 to 37 TWh
  • Our precipitation scenarios show that the hydrological balance by the end of week 30 (1 August) could reach the historic highs of the last 20 years, despite the weakening of the balance so far this year.

Weekly hydrological balance

Energy Quantified (EQ) has generated a synthetic weekly hydrological balance for the Nordic area for 2001-2020 based on today's hydropower capacity.

The chart below shows the boundary curves (max, min, 95- and 5-percentile) and the 2021-curve up until the end of week 8.

Historically we see a variation between +20 TWh and -30 TWh by the end of July (week 30).

The variation band is not symmetrical, partly because the water reservoirs' spare capacity is about 20 TWh relative to the normal level. This is more or less the upper limit for the hydrological balance after the melting season, but the downside is not similarly limited.

Weekly hydrological balance (TWh) from 2002 to 2021 (min, max, 5- and 95-percentiles).

As part of our overall product development, Energy Quantified (EQ) has made climate scenarios for energy precipitation for the 40 years from 1980-2019 for all countries across Europe.

The table and charts below show the Nord Pool area's accumulated precipitation curve for weeks 9-30 (duration curve):

Accumulated precipitation in Nord Pool for weeks 9-30 over a forty-year period (1980-2019).
Accumulated precipitation for weeks 9-30 over a forty year period (1980-2019). Ordered from the driest to the wettest scenario.

It reveals that the max-min variation is +27 TWh to -32 TWh relative to the average of 69.5 TWh. That shows the potential variation band of the precipitation for these 40 years, but we consider it more relevant to look at the 95-5 percentiles (36 years), and the 75-25 percentiles (20 years).

In 20 out of 40 years (75-25 percentiles), the variation band is about ±7 TWh (about 10% of average).

We see an interesting skewness in the higher variations, where the minimum scenario has a very high difference to the 5-percentile.

This scenario is the arid year 2018, which was 14 TWh drier than the 5-percentile. We don't see that large difference between the max and 95-percentile (5 TWh difference).


The chart below shows the probability distribution of the precipitation deviation from average. We have included a smoothened curve better to show the distribution skewness. This curve is close to being symmetrical around zero, while we see some more samples of the positive extremes than the negative extremes.

It is nearly 10% probability that we will see more precipitation 25 TWh or more above average.

Probability distribution for precipitation for weeks 9-30 over a forty year period (1980-2019). The distribution is plotted as deviation from the average precipitation in the climatic period.

Variation band of the hydrological balance until end week 30

The historical max/min variations indicate that we still might reach the hydrological balance's historical extremes by summertime.

Based on the 40-year precipitation scenarios, EQ has predicted the corresponding hydrological balance until the end of week 30. We have assumed 105% of normal precipitation for all scenarios in our production forecast except for the 3-4 highest precipitation years where we have assumed 110% of normal from week 15. In general, we have then assumed a production philosophy to normalize the hydrological balance.

Forecasted weekly hydrological balance in Nord Pool from now until end of week 30.

This chart shows that the hydrological balance may still reach the 2002-2021 period's historical max-min boundaries.

The 95- and 5-percentiles again show that the upside seems more likely than the downside. The maximum level has a 5% probability of being realized (21 TWh), while the 5% probability on the downside represents about -14 TWh. 

The curve representing the average of forecasts is close to zero as expected, confirming the method's validity.

The forecasted hydrological balances are not symmetrical in the extreme scenarios as there are more extreme high than low scenarios.

Final words

Contact us for more information if you're also interested in these related topics:

  • Snow and groundwater levels
  • Inflow
  • Net precipitation energy
  • Seasonal normals
  • Weather spreads for every market in Europe

– Eylert Ellefsen,

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