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Changes in River Special Runoff in Estonia during the Period of Minimum Discharge in Summer 1951-2016

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

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

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

  4. Changes by trend of spatial mean precipitation (mm) in Estonia during 1966-2010 Highprecipitationperiod 1975-2006

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

  6. 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?

  7. Drasticchanges in winter No changes in summer

  8. What about minimum runoff? • Monthlymeanrunoffcanbeaffectedbyshortbutintensiveflashfloods • Minimumrunoffas a mesure of wateravailability in landscape • Are there any changes in the summer runoff minimum? • Summer minimumrunoffperiod?

  9. Data • Estonian Weather Service • 16 stations + spatialaverage • Dailydata • Period 1951-2016 • Specialrunoff (l/s*km²) • May-August • Lineartrends and regimeshifts • Result: notmuchchanges…

  10. 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

  11. Except for May RainyMayorshift in springflood? 6 of 16 stations have negative trend in minimum value

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

  13. Except for July in small rivers

  14. How to define and analyse summer low water period? Lenght of theperiod and meanvalue

  15. 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

  16. Significancelevel: 0.1 Cut-offLenght: 30 (days) HWP:1

  17. Lenght: 188 days, Weightedspecial runoff:1.72

  18. It looks nice, however… • Largeinterannualvariance • Differentrivers – differentregimes • i.e. setting 0.1/30/1 isnotuniversal • Results: • No statisticallysignificanttrends • Lenght and weightedspecialrunoff are correlated (r = -0.21…-0.56) • Shortertheperiod, highertherunoff

  19. Tendences In linewithclimatepredictions: Morewater in NorthernEurope Morewater in thesystem. Howmuch?

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