PROBLEMS IN PROBLEMS IN DETECTING TREND IN DETECTING TREND IN HYDROMETEOROLOGICAL HYDROMETEOROLOGICAL SERIES FOR CLIMATE SERIES FOR CLIMATE CHANGE STUDIES CHANGE STUDIES Jasna Plavšić 1 and Zoran Obušković 2 1 University of Belgrade – Faculty of Civil Engineering 2 Energoproject – Hydroengineering 16. naučno savetovanje SDHI/SDH, 22-23. oktobar 2012, Donji Milanovac
24
Embed
P ROBLEMS IN DETECTING TREND IN HYDROMETEOROLOGICAL SERIES FOR CLIMATE CHANGE STUDIES Jasna Plavšić 1 and Zoran Obušković 2 1 University of Belgrade –
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
PROBLEMS IN DETECTING PROBLEMS IN DETECTING TREND IN TREND IN
HYDROMETEOROLOGICAL HYDROMETEOROLOGICAL SERIES FOR CLIMATE SERIES FOR CLIMATE
CHANGE STUDIESCHANGE STUDIES
Jasna Plavšić1 and Zoran Obušković2
1University of Belgrade – Faculty of Civil Engineering2Energoproject – Hydroengineering
16. naučno savetovanje SDHI/SDH, 22-23. oktobar 2012, Donji Milanovac
Climate changeClimate change
• Global warming and increased concentrations of greenhouse gases
Hansen et al, Proc. Natl. Acad. Sci., (2006)
Copenhagen Diagnosis (2009)
Climate change – we knowClimate change – we know
Radionica - Klimatske Promene - 2010 www.slobodansimonovic.com
Copenhagen Diagnosis (2009)
Climate change – we knowClimate change – we know
Radionica - Klimatske Promene - 2010 www.slobodansimonovic.com
Climate change – we knowClimate change – we know
Radionica - Klimatske Promene - 2010 www.slobodansimonovic.com
Church and White , Geophysical Research Letters, (2006)Cazenave et al, Global and Planetary Change, (2009)
Climate change impactsClimate change impacts
• Questions:– Change projections?– Impact on water
resources?
IPCC (2007)
Impact of climate change on water Impact of climate change on water resourcesresources
Estimation of climate change impacts
Future climate scenarios + hydrologic
models
Statistical trends
fairly complicated approach; propagation of uncertainty
simple calculations; but:
How to prove presence of a trend? How to interpret the trend?
Trend detectionTrend detection
• Starting point: hydrometeorological series are considered stationary– stationarity is well defined and departures from
stationary indicate changes• Trend detection vs. identification of non-stationarities– trend in mean is just one type of non-stationarities– false trend detection in time series where other non-
stationarities are present• slow changes (long memory) can look like trend when observed
in shorter periods– significance of trends can decrease in series with long
memory and high serial correlation
Practical aspects of trend analysis – Practical aspects of trend analysis – choice of variableschoice of variables• Runoff– mean flows, floods, low
flows – annual and monthly
values– time of occurrence of
annual maximum flood– ice start and end dates,
number of days with ice
• Precipitation– annual and monthly
precipitation– daily precipitation
annual maxima– number of rainy days– etc.
Practical aspects of trend analysis – Practical aspects of trend analysis – choice of stationschoice of stations• Trend analysis is valid if performed on adequate
series– time series should be long enough for reliable
statistical analysis• WMO recommends 30-year statistics for describing climate
(eg. standard climatological period 1961-1990)• series used for analysis of change in climate should be
much longer than 30 years– series should reflect natural flow regime with no
human interventions within the basin– data from a station should be checked for accuracy
and consistency (rating curves etc.)
Tests for trendTests for trend
• Linear regression: X = a + bt
slope significance?
Tests for trendTests for trend
• Non-parametric tests– data need not be drawn from a (normal) distribution– some test assume data independence
• Most popular: Mann-Kendall test
– H0: no monotonic decreasing or increasing trend
– H0 is rejected when S significantly departs from 0
– serial correlation decreases detection power
Other test for detecting changes in Other test for detecting changes in time seriestime series
Tests for change in the mean Z-test, t-test, Pettitte test
Tests for change in variance F-test
Tests for change in distribution
Mann-Whitney, Kolmogorov-Smirnov
Tests for randomness Run test
Tests for serial correlation Bartlett’s test
Tests for trend Mann-Kendall, Spearman rho, linear regression slope
ExampleExample
• Runoff, precipitation and temperatures in the Drina Basin – Brodarevo/Lim– Drina/Radalj
Energoprojekt- Hidroinženjering
2011, 2012
ExampleExample• Precipitation and runoff cycles– cumulative standardized deviation from the
mean
ExampleExample
• Runoff– no
significant trend
MEAN ANNUAL FLOWS
ANNUAL MAXIMUM FLOODS
LOW FLOWS (annual minimum monthly flows)
Radalj
ExampleExample
• Runoff– Significant
decreasing trend in mean annual flow
MEAN ANNUAL FLOWS
ANNUAL MAXIMUM FLOODS
LOW FLOWS (annual minimum monthly flows)
Brodarevo
ExampleExample
• Temperatures– 8 met.
stations
Results of trend Results of trend analysisanalysis• Temperatures– change in 2035
– in accordance with other studies
BeraneKolašin
Bijelo Polje
Brodarevo
Prijepolje
Bajina Bašta
Loznica
Radalj
Zlatibor
Pljevlja
Žabljak
1.5
1.5
1.3
1.1
1.0
1.01.3
1.5
ExampleExample
• Precipitation– 10 stations
Results of trend Results of trend analysisanalysis• Precipitation:– % change in 2035
– other studies: absence of trend or weak increasing or decreasing trends
– change in seasonal distribution of precipitation, with opposite tendencies for summer and winter seasons Berane
Kolašin
Bijelo Polje
Brodarevo
Prijepolje
Bajina Bašta
Loznica
Radalj
Zlatibor
Pljevlja
Žabljak
14.7%
-16.5%
22.7%
-0.6%
5.9%0.4%
15.2%
9.9%5.0%
6.5%
ConclusionsConclusions
• Trend detection – problems:– Series of different lengths can exhibit different,
even opposite, trends– Spatial inconsistency of the stations are
considered separately– Presence of non-stationarities makes trend
detection more difficult– Opposite changes in different seasons result in
insignificant changes at annual level
ConclusionsConclusions
• River basins with heavily modified flow regime (such as reservoirs) require detailed and careful analysis based on climate and hydrologic modelling with consideration of water management practices