Comment on the results
There is a large variation for when the spring flood starts between years. But based on the stations examined, a change is seen in that it comes earlier in the year. Since the beginning of the 20th century, it is generally an advance of 5 to 15 days.
How is the starting date of spring flood defined?
When the snow melts towards spring, the flow in the waterways increases. This is called spring flow. In central and northern Sweden this happens one time, but south of the Dalälven winter flows occur because there are no sustained freezing temperatures in the winter, which gives some flows even in winter. Therefore, this indicator only exists for the Dalälven and the north.
The start date for the spring flood has been calculated as the first day of the year when the flow in the watercourse rises above the mean water discharge (MQ) after the lower winter flow.
Why is the indicator important?
The spring flood means a big change in and around the waterways. The flow goes from being very low to being high and coastal areas are flooded. When the timing of this event changes, animals and nature in and around the waterways are affected.
The fact that the spring flood occurs earlier also affects, among other things, winter tourism and the hydropower companies.
How has the indicator been calculated?
From 1910 to 1940, around ten stations were used. Since then, around 20 stations have been used. The stations selected have long measurement series and are located in unregulated watercourses, see the map above. In some cases, there are gaps in the time series due to missing data or the fact that a start date could not be obtained.
- The start date for the spring flood has been calculated as the first day of the year when the flow in the watercourse rises above the mean water discharge (MQ) after the lower winter flow.
- Mean water discharge (MQ) is the mean value of the 24-hour water discharge during the entire measurement series.
- Average start date is the average value over all years included in the indicator.
Spring flood in the future
In a warmer climate, snow will melt earlier in the year. However, the variations between years will still be large.
It will also become more common that there will not be a clear spring flood start. We can already see that the area that previously had clearly low winter flows and spring floods is decreasing. Instead there will be higher winter flows and a lower or non-existent spring flood for southern Sweden and we can see that the border is moving north.