Changes in river special runoff in Estonia during the period of minimum discharge in summer 1951-2016 Mait Sepp Zaragoza, 12 February 2018

Changes by trend of mean, maximum and minimum temperature in Estonia in 1966-2010 II 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

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

High precipitation period Changes by trend of spatial mean precipitation (mm) in Estonia during 1966-2010 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 1975-2006

(Ennet, Reihan, Järvet, Pihelgas, 2016) Estonian surface runoff in 1924 – 2014 (catchment area 45227 km2) (Ennet, Reihan, Järvet, Pihelgas, 2016)

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?

Drastic changes in winter No changes in summer

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?

Data Estonian Weather Service 16 stations + spatial average Daily data Period 1951-2016 Special runoff (l/s*km²) May-August Linear trends and regime shifts Result: not much changes…

Very few statistically significant trends or regime shifts Some regime shifts that coinside with 1975-2006 high water period General tendencies show increase in minimum runoff

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?

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

Except for July in small rivers

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

STARS method Regime shifts S. N. Rodionov (Uni. Of Washington, Seattle) NOAA Bering Sea Centre https://www.beringclimate.noaa.gov/regimes/ The method is based on the sequential application of the Student's t-test

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

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

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

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