1. Changes in river special runoff in Estonia during the period of minimum discharge in summer
Mait Sepp
Zaragoza, 12 February 2018

3. Changes by trend of mean, maximum and minimum temperature in Estonia in 1966-2010II
III IV V VI
VII
VIII IX X XI
XII
Year
Mean
5.6
2.5
1.3
3.0
1.0
-0.2
2.2
1.6
1.5
0.3
2.0
1.8
Max
4.6
2.3
1.4
4.0
1.1
-0.6
2.4
1.7
1.9
Min
6.6
3.1
1.2
0.5
0.4
0.9

4. Snow cover duration in Tartu 1893/95-2014/15

5. High precipitation period
Changes by trend of spatial mean precipitation (mm) in Estonia during I II
III IV V VI
VII
VIII IX X XI
XII
Annual 23 16 11
-19 -4 41 26
-25 1 3 83
High precipitation period

6. (Ennet, Reihan, Järvet, Pihelgas, 2016)
Estonian surface runoff in – 2014 (catchment area km2)
(Ennet, Reihan, Järvet, Pihelgas, 2016)

7. So, what about the runoff in summer?
Summer runoff as a proxy for drought
Higher air temperature in summer -> higher evaporation -> less water in rivers
Shorter snow cover duration and earlier snowmelt-> less water in spring -> prolonged summer minimum period
But precipitation in summer has increased?

11. Drastic changes in winter
No changes in summer

12. What about minimum runoff?
Monthly mean runoff can be affected by short but intensive flash floods
Minimum runoff as a mesure of water availability in landscape
Are there any changes in the summer runoff minimum?
Summer minimum runoff period?

13. Data Estonian Weather Service 16 stations + spatial average Daily data
Period
Special runoff (l/s*km²)
May-August
Linear trends and regime shifts
Result: not much changes…

15. Very few statistically significant trends or regime shifts
Some regime shifts that coinside with high water period
General tendencies show increase in minimum runoff

16. Rainy May or shift in spring flood?
Except for May
6 of 16 stations have negative trend in minimum value
Rainy May or shift in spring flood?

17. Except for June
9 of 16 stations show positive trend in monthly mean and minimum runoff
June has become rainy

18. Except for July in small rivers

19. How to define and analyse summer low water period?
Lenght of the period and mean value

20. STARS method
Regime shifts
S. N. Rodionov (Uni. Of Washington, Seattle)
NOAA Bering Sea Centre
The method is based on the sequential application of the Student's t-test

21. Cut-off Lenght: 30 (days)
Significance level: 0.1
Cut-off Lenght: 30 (days)
HWP:1

22. Weighted special runoff:1.72
Lenght: 188 days,
Weighted special runoff:1.72

23. It looks nice, however… Large interannual variance
Different rivers – different regimes
i.e. setting 0.1/30/1 is not universal
Results:
No statistically significant trends
Lenght and weighted special runoff are correlated (r = -0.21…-0.56)
Shorter the period, higher the runoff

24. Tendences In line with climate predictions:
More water in Northern Europe
More water in the system.
How much?