Download The impact of climate change on rainfall runoff statistics in the

11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
The impact of climate change on rainfall runoff statistics in the
Emscher-Lippe region
H.-R. Verworn1, S. Krämer1*, M. Becker2 and A. Pfister2
1
Institute of Water Resources Management, Hydrology and Agricultural Engineering,
Leibniz University of Hanover, Appelstr. 9a, D-30167 Hanover, Germany.
2
Emschergenossenschaft / Lippeverband, Kronprinzenstr 24, D-45128 Essen, Germany.
*Corresponding author, e-mail [email protected]
ABSTRACT
With the global increase of temperature a change in rainfall pattern is expected. Though it is
often argued that rainfall extremes causing flooding in urban and rural areas tend to become
more frequent, it is not yet clear whether an increase is to be expected for all rainfall and
runoff related features. In addition to a former study concerning extreme rainfall statistics, in
this paper the rainfall patterns of dry and wet spells are analysed. On the runoff side the trends
for various runoff criteria (peak flows, duration curves, sewer surcharge and CSO volumes)
are investigated on the basis of rain gauge data from a dense network with long time series in
the Emscher-Lippe region in Germany. Simulations with these time series have been carried
out for a mixed rural and urban catchment of 82 km² as well as for an urban sewer system
(850 ha, 45% impervious). The results for the various criteria were quite different. Though in
most cases an increase within the last two decades was visible, nearly none of the trends
proved to be significant when tested. The presented results indicate that immediate
modifications of design procedures and management strategies are not required. But aware of
the projections of climate modelling for the next hundred years, it seems reasonable to give
priority to no regret strategies concerning water management tasks.
KEYWORDS
Climate change; rainfall; runoff; statistics; urban drainage
INTRODUCTION
The global increase of temperature is a proven fact. This increase is responsible for higher
dynamics in the atmospheric processes. As a consequence, a change in rainfall pattern is
expected. It is often argued that rainfall extremes, which cause flooding in urban and rural
areas, appear to be more frequent today and in future compared to former times. But the
change in rainfall characteristics differs from region to region and depends on what feature of
the rain is regarded (yearly, monthly, daily totals; extremes of various durations). Whilst
climate models predict an increase of rainfall for the southern Scandinavian region, the
Mediterranean region gets less rainfall and has become dryer in the last decades. For
Germany, especially in the southern and western parts an increase in monthly rainfall during
winter times is expected at the magnitude of 25 % whereas during summer times rainfall will
become slightly less (Schönwiese et al. 2003).
For the design and the assessment of urban drainage system performance it is common
practice to use intensity duration frequency (idf) curves. They are derived from statistical
analysis of rainfall patterns of recorded historical data. To investigate the impact of climate
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
change on extreme rainfall characteristics, Pfister and Verworn (2002) analysed rainfall time
series of 42 rain gauge stations in the Emscher/Lippe region (western Germany) from the
period 1931-2001 for durations from 15min to 24h. A similar study was performed by
Arnbjerg-Nielsen (2006) based on 41 rain gauge time series (1979 - 2000) recorded in
Denmark for durations of 10min and 6h. The results from both studies show that the statistical
parameters of the time series have often a non unique local trend which is both, either positive
or negative. Whilst for the German case no statistical significance has been proven for any
duration or recurrence interval, the Danish data base revealed a statistical significant increase
of rainfall for the 10 min duration, but only for the eastern part of the country.
The event based frequency of floods, however, does generally not match the duration based
frequency of extreme rainfalls. Therefore a study was performed to investigate the effect of
climate change on the runoff side.
DATA BASE
The study is based on rain gauge time series of 15 stations within the Emscher-Lippe region
from the period of 1937-2006 (Fig. 1). The stations were divided into two groups, WEST and
EAST, to smoothen out the effects of local peculiarities but investigate regional variations.
From each group a cluster was selected to cover roughly the size of the two catchments which
are shown in Fig. 2.
Figure 1. Rain gauge locations with groups (East and West) and clusters (EGla and ApKu)
For both catchments the rain clusters were transferred to cover the catchment areas
maintaining their spatial arrangement. The selected locations of the rain gauges of both
clusters are shown in Fig. 2. For the investigations of the effects on the runoff side two
different approaches were used. First, a continuous long term simulation has been performed
with the hydrological model NASIM for the Boye catchment (a mixed rural and urban
catchment of 82 km²). Second, a time series simulation using the hydrologic and
hydrodynamic model HYSRAD/EXTRAN was performed for the urban catchment
Picksmühlenbach (850 ha, 45 % impervious, 4400 sewer elements). These simulations were
carried out for both clusters.
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Impact of climate change on rainfall runoff statistics in the Emscher-Lippe region
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Figure 2. Boye catchment (left, with flow simulation nodes; only the one at the end of the
catchment is considered here (encircled)) and sewer network Picksmühlenbach (right,
Thiessen polygon borders) with rain gauge locations.
RAINFALL ANALYSIS
In addition to the findings concerning the extreme value statistics (Pfister and Verworn,
2002) the yearly totals as well as wet and dry spells were investigated to make sure that there
are no station specific peculiarities that may influence the results of the simulations.
Yearly totals
Yearly totals in that region range from 420 to 1160 mm with a mean of about 800 mm. The
trend for the yearly totals showed differences from -0.1 mm/a to +2.4 mm/a with a mean of
1.34 mm/a. For a detailed trend analysis the time series of all stations were investigated
separately for summer and winter half-years.
Rainfall total summer [mm]
700
R 2684
R 2822
R 2841
600
500
400
300
200
1950
1960
1970
1980
1990
2000
2010
Figure 3. Rainfall totals for summer half-years with trend lines
Figs.3 and 4 show the results for the three stations of the EGla cluster. The summer rainfall
totals show minimal negative and positive changes, none of them significant. The winter
rainfall totals, however, show all positive changes, two of them significant, making it clear
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
that the change in yearly totals is entirely due to the changes in winter. The increases of the
half-year totals are mainly due to the low values before 1980. If only the years from 1980
onwards are analysed, there are non-significant negative as well as positive changes. Though
the time series were thoroughly checked and processed, it may well be that the low values
before 1980 are due to insufficient measurements of all precipitation. All data used here are
purely recording rain gauge data. For reliable deductions daily readings from collecting nonrecording rain gauges (including snowfall) should be used. Generally these years were colder
and more precipitation fell as snow. This poorly detected by recording rain gauges which
were the only data used here.
Rainfall total winter [mm]
700
R 2684
R 2822
600
R 2841
500
400
300
200
1950
1960
1970
1980
1990
2000
2010
Figure 4. Rainfall totals for winter half-years with trend lines
Dry and wet spells
The time series of all stations were separated in dry and wet periods, and the lengths of the
periods were analysed. To avoid very small rain intensities to be counted as “events”,
intervals with less than 0.02 mm/min were counted as dry, and a wet period had to have at
least 0.1 mm of rain. Occasional zero rain intervals were rated as rain fall pauses as long as no
more than 3 of them in succession (15 min) occurred.
These thresholds were necessary because the data used (equidistant 5-min data) came from
different measurement devices and were processed in various ways. The data from analog
recordings had to be digitized, which was mainly done by individually or automatically
selecting many or few characteristic points of the hyetographs, and chopped into 5-min slices.
Data from tipping bucket gauges in many cases produced minutes with 0.1 or 0.2 mm of rain
and minutes without rain in between tips.
The analysis of the data of all stations showed a very similar behaviour of all stations except
one which was excluded. Especially the stations represented in the two clusters had all very
similar results which ensured that the results of the subsequent analysis of the decades were
not corrupted by station specifics. The analysis by decades was carried out for all stations
showing slightly different results. The results for station 2841, which is a member of cluster
EGla and had the most prominent differences, are shown here as an example (Figs. 5 and 6).
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Impact of climate change on rainfall runoff statistics in the Emscher-Lippe region
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1000
1997-2006
1987-1996
Frequency of exceedence [1/a]
1977-1986
100
1967-1976
1957-1966
10
Dry spells
Wet spells
1
0,1
10
100
1000
10000
100000
Dry and wet spell duration [min]
Figure 5. Frequencies of exceedence for dry and wet spell durations (Station 2841)
Dry spells. For the dry spells the short durations increase over the decades whereas the longer
durations are not so frequent as before. The few very long dry spell durations show no
systematic behaviour as they are due to the few stochastic appearances of longer periods
without any rain. Additionally, it cannot be excluded that they are influenced by gauges
failures not detected and corrected. This trend has been recognized for the other stations as
well, though in many cases not so prominent as in this example.
Frequency of exceedence [1/a]
1000
100
10
1997-2006
1987-1996
1977-1986
1
1967-1976
1957-1966
0,1
0,1
1
10
100
Wet spell amount [mm]
Figure 6. Frequencies of exceedence for wet spell amounts (Station 2841)
Wet spell durations. The wet spell durations of the same station show no systematic change in
frequency for short rain periods. The results for the last decade, however, show the
frequencies over nearly the whole range of durations. Especially for durations of 90 min to 5
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hours (300 min) there is a clear increase of frequencies over the decades. These tendencies,
however, are not found at all other stations. For most of them there is no clear change over the
decades, and various decades are showing the highest frequencies in various duration spans.
Wet spell amounts. The wet spells have also been analysed for their rainfall amounts. The
frequencies of wet spell amounts are shown in Fig. 6 for all decades. The highest values are
those of the last decade which significantly stand out from those of all other decades. This is
clearly visible in the span between 2 an 10 mm rainfall. Events with 5mm or higher which are
detected 20 times a year in all decades up to 1996, occur more than 30 times a year in the last
decade. For the same frequency (20 times a year) the event total threshold increases from 5
mm to 6 mm (red arrow). These results correspond well with the increase of wet spell
durations and the yearly rainfall totals that are highest for the last decade with 1977-1986
following second. Again, the differences shown here are detectable but not so pronounced for
many of the other stations.
FLOW ANALYSIS
The analysis for the results of the long term simulation of the Boye catchment (Fig. 2, left)
was performed for flows which exceeded a certain time per year (duration exceedence curves)
and for the frequency of peak flows which is not presented here.
Duration exceedence curves
The non-equidistant time series if flows which resulted from the simulation with NASIM
were transformed to equidistant 1-min data. For each year all 1-min flow data were sorted by
size. Herewith the flow data are detached from their consecutiveness. Fig. 7 (left) shows the
graph of the sorted data for the year of 1991. The total number of values corresponds to the
minutes of a year (525 600), but only the largest 20160 are shown which corresponds to a
time equivalent of 14 days.
Duration of exceedence:
1 day
3 days
7 days
14 days
Exceeded flow [m³/s]
15
10
5
0
1950
1960
1970
1980
1990
2000
Figure 7. Yearly duration curves: definition (left) and time series of exceeded flows for
various durations (right)
Thresholds may then be set anywhere to find out how many data point lie above the threshold
value (= for how many minutes higher flows occur). All thresholds chosen here are multiples
of 1440 minutes and are therefore given as days to avoid the large minute values.
For the chosen threshold durations of 1 to 14 days the corresponding flows of each year are
shown as time series in Fig. 7 (right) where the flow values for 1991 from Fig. 7 (left) can be
found. These time series were then analysed for trend. All regression lines show an increase.
The significance of these trends was investigated with various tests (Helsel and Hirsch, 1992).
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Impact of climate change on rainfall runoff statistics in the Emscher-Lippe region
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
The Cox-Stuart sign tests revealed no significant trends as the changes in direction are too
frequent to deduct an increase from them. The Mann-Kendall test showed highly significant
increases for all durations apart from 1 day. The same results were achieved for the times
series computed with the ApKu cluster. The test of the regression coefficients of the trend
lines shown in Fig. 7 (right) resulted in confidence intervals shown in Fig. 8. On the 90%
level all increases are significantly higher than zero.
Figure 8. Confidence intervals of the regression coefficient (slope) for various durations
SEWER SYSTEM ANALYSIS
The analysis for the results of the simulation of all relevant events for the combined sewer
system Picksmühlenbach (Fig. 2, right) was performed for the features “surcharge” and “CSO
volume”. “Surcharge” in this paper means the surcharge of manholes when the water level
reaches the surface and flooding starts. The amount of water getting out of the manholes is not
regarded with the simple counting of these cases. Regarding surcharge the following analysis
criteria were defined:
- Number of surcharge events: all events were counted for which surcharge occurred at least
at one manhole.
- Number surcharge cases: the number of surcharged manholes for each event is counted.
When decades are considered, the results for all events within the decade are added up.
Regarding combined sewer overflow (CSO) from detention tanks the following analysis
criteria were defined:
- Number of overflow events: all events were counted for which CSO occurred
- Total overflow volume: the overflow volumes of all events within a decade are added up
- Time series of overflow volumes: overflow volumes per event in chronological order
Surcharge results
Fig. 9 shows the number of events for which surcharge occurred (in blue) and the total
number of surcharge cases (in red) per decade. The results for both clusters show an increase
of surcharge events in the last or the last two decades. For the total number of surcharges,
however, there is no trend detectable. In many cases the results are opposed, between groups
as well as between criteria. The total number of surcharges is independent of the number of
surcharge events, but dependent on the magnitude of the particular events. This fact is
obvious from the surcharge cases per event shown in Fig. 10: the three highest cases per event
for cluster ApKu appear 1987 and 1993. The regression lines for both clusters show opposite
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11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
slopes with an increase of 1.75 cases per year and a decrease of 0.3 cases per year,
respectively.
40
8000
Events with manhole surcharge (ApKu)
7000
Events with manhole surcharge (EGla)
Total count of manholes surcharged (ApKu)
30
6000
Total count of manholes surcharged (EGla)
25
5000
20
4000
15
3000
10
2000
5
1000
0
Total count of manholes surcharged
No. of events with manhole surcharge
35
0
1957 - 1966
1967 - 1976
1977 - 1986
1987 - 1996
1997 - 2006
Figure 9. Number of surcharge events and total surcharge occurrences per decade
1600
No. of surcharged manholes per event
ApKu
1400
EGla
1200
1000
800
600
400
200
0
1957
1962
1967
1972
1977
1982
1987
1992
1997
2002
2007
Figure 10. Number of surcharge occurrences per event in chronological order
CSO results
The investigations were carried out for three detention basins, only the results for one of them
are shown here. Fig. 11 shows the number of CSO events (blue) and the total CSO volume
(red) per decade for both clusters. For both variables a trend is obvious. Similar to the
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Impact of climate change on rainfall runoff statistics in the Emscher-Lippe region
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
surcharges, the overflow volumes do not correspond well with number of overflow events,
especially in the decade 1987-1996.
60
360000
50
290000
45
40
220000
35
30
150000
Overflow volume per decade [m³]
No. of overflow events per decade
Basin Hassel
No. of overflow events (ApKu)
No. of overflow events (EGla)
Overflow volume (ApKu)
Overflow volume (EGla)
55
25
20
80000
1957 - 1966
1967 - 1976
1977 - 1986
1987 - 1996
1997 - 2006
Figure 11. Number of CSO events and total overflow volumes per decade
30000
Basin Hassel
Overflow volume per event [m³]
EGla
∆V = 0,3 m³/a
25000
ApKu
∆V = –10,0 m³/a
20000
15000
10000
5000
0
1957
1962
1967
1972
1977
1982
1987
1992
1997
2002
2007
Figure 12. Overflow volume per event in chronological order (right)
The high overflow volume of cluster ApKu in the decade 1987-1996 is definitely not due to
extreme events with high overflow volumes but to an increased number of overflow events
with medium high CSO volumes. In contrary to the findings of the decade investigation the
regression lines for the time series of overflow volumes show no or rather negative trends
(Fig. 12).
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CONCLUSIONS
The rainfall runoff processes in two different types of catchments have been analysed for
various variables and criteria, mainly to find out whether any trends due to climatic change
may be detected. The findings were not unanimous. The winter rainfall totals showed a severe
increase but there are doubts concerning data base. The distribution of dry and wet spells
showed small variations and no significant change. Only the changes in wet spells amount
within the last decade were noticeable. There is, however, no clear indication that this is due
to climatic change. Other findings for the last decades indicate that it may well be one of the
normal fluctuations or irregularities.
On the runoff side trends may be detected or not dependent on what criteria, variables or
analysis procedures are chosen. Over all, even for the trends confirmed by significance tests,
doubts remain because no continuous increase over the decades could be detected. One might
well argue that the impact of climatic change only started to become noticeable recently and
that the change is dramatic. On the other hand the results can be interpreted as proof of the
high stochastic and variability of the rainfall process and especially of the extreme events.
One of the reasons for this investigation was the question whether design and assessment
guidelines for urban drainage and integrated water management in the Emscher-Lippe area
have to be modified. The findings presented in this paper lead to the conclusion that there is
no immediate need for such actions. The indications from this investigation show a tendency
towards higher values for certain criteria, but it is not proved, that these tendencies may
continue in the near future – as predicted by climate modelling. The uncertainty of these
models is high, but dependent on the design objective and the vulnerability and envisaged life
cycle of new facilities, the possibility of upgrading should be considered even now.
Nevertheless, the changes within the last decades were not so severe as not to await the near
developments before re-analysing and re-evaluating.
REFERENCES
Arnbjerg-Nielsen, K. (2006). Significant change of extreme rainfall in Denmark. Wat. Sci.Tech, 54(6-7), 1-6
Helsel, D.R., and Hirsch, R.M. (1992). Statistical methods in water resources. Elsevier Publishers, Amsterdam.
Pfister, A. and H.-R. Verworn (2002). Trenduntersuchungen von Starkregen im Emscher-Lippe-Raum (Trend
investigations of extreme rainfall in the Emscher-Lippe region). Korrespondenz Abwasser 49(8), 11011104 (in German)
Schönwiese C.D., Bader S., Böhm R., Claussen M., Cubasch U., Fischer H., Gärtner U., Graßl H, Rahmstorf und
J. Sündermann (2003). Climate Statement. German Met. Soc. (DMG), Austrian Met. Soc. and Swiss
Met. Soc., www.DWD.de (in German).
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Impact of climate change on rainfall runoff statistics in the Emscher-Lippe region