UCL DEPARTMENT OF GEOGRAPHY
Climate change impacts on a Nile headwater catchment:
the River Mitano, Uganda
Daniel Kingston, Richard Taylor, Martin Todd, Julian Thompson
Department of Geography, University College London
The River Mitano
•
Basin size: 2098km2
•
Headwater catchment of the Nile
•
Located in southwestern Uganda
•
River drains from relatively high peaks
(~2500m) to Lake Edward (975m) in the
East African Rift valley
•
79% catchment land use is agrarian
•
Previous work: Mileham et al.
–
–
–
RCM-driven soil-moisture balance model
for SRES A2 scenario from HadCM3
GCM
For 2070-99 time horizon
Mileham 2008
Mitano climatology and hydrology
•
Humid-tropical climate
– Mean annual precipitation
(1965-79): 1190mm
– Bi-model annual regime with
wet seasons from March-May,
and Sept-Nov
– Monthly temperature (and
evaporation) relatively constant
throughout year
– PET > precipitation in 9 out of
12 months
•
Discharge lags precipitation by
2-6 weeks
Mileham 2008
Scenarios analysed
• 1-6 °C increase in global mean temperature for HadCM3
• 2 °C increase across all seven GCMs
– UKMO HadCM3 & HadGEM1, CCCMA, CSIRO, IPSL, MPI and
NCAR
• All four SRES scenarios on HadCM3 (2040-69)
• SRES A1b across all seven GCMs (2040-69)
• Not probabilistic, but ‘envelopes of non-discountable
change’
Hydrological model
• SWAT (Soil and Water Assessment Tool)
– Physically-based semi-distributed river basin scale model
– Widely used and freely available
– Runs within Arc GIS packages
• Manually calibrated for 1961-90 period
– Using CRU TS3 0.5° lat/lon resolution gridded monthly climate
data
– Weather generator
• Validated 1991-2005
Model set-up
• Basin defined from 3 arc-second SRTM DEM data
• Land-cover derived from FAO Africover data set
– and modified to conform with internal SWAT land-classes
• Soil data from FAO global data base
– No local data
• Potential evaporation (PET) calculated using the
Hargreaves equation
– (temperature-based)
Model calibration
•
Calibration period 1961-90
Spearman correlation
coefficient: 0.61
Nash-Sutcliffe coefficient: 0.06
– Issues with CRU observational
data…
25
20
obs
model
15
10
5
0
J
F
M
A
M
J
J
A
S
O
N
D
60
Discharge (cumecs)
•
•
discharge (cumecs)
30
50
40
obs
model
30
20
10
0
0
20
40
60
% exceedence
80
100
Model calibration (2)
•
•
6 precipitation gauges within the catchment for 1965-1980
For this period, the difference between gauged and gridded (CRU)
precipitation data is correlated with model discharge error
40
100
30
20
50
10
0
0
•
79
19
-150
77
19
-100
75
19
73
19
71
19
69
19
67
19
-50
ppt
flow
1965-1980 modelled river discharge
– Nash-Sutcliffe = 0.21
– Correlation coefficient = 0.71
•
1991-2005 validation consistent with calibration period
-10
-20
-30
discharge (cumecs)
150
65
19
precip (mm)
– coefficient = 0.40
Blue line= gridded
minus gauged
precipitation
Brown line = model
minus observed river
discharge
•
annual flow anomaly (%) from
baseline
Prescribed increase in global mean temperature,
on HadCM3
With increasing global mean
temperature:
– Increasing late-season flow
– Decreasing early season flow
Annual runoff:
– No linear trend – balance
between decreasing early season
flow and increasing late season
flow
– Relatively small overall changes
until 6 °C threshold
5
0
1
2
3
4
5
6
-5
-10
-15
°C change in global mean temperature
30
-3 -1
Discharge (m s )
•
10
baseline
25
1 °C
20
2 °C
15
3 °C
10
4 °C
5 °C
5
6 °C
0
1
2
3
4
5
6
7
8
9
10 11 12
HadCM3 prescribed warming:
temperature vs precipitation
Impact of temperature changes
greater than precipitation in first
wet season
Precipitation-dominated signal
in early part of second wet
season
Both are important for end of
second wet season
•
•
Temperature climate change signal
35
Discharge (cumecs)
•
baseline
1deg
2deg
3deg
4deg
5deg
6deg
30
25
20
15
10
5
0
j
f
m
j
j
a
s
o
n
d
baseline
25
1 °C
20
2 °C
15
3 °C
10
4 °C
5
5 °C
0
6 °C
1
2
3
4
5
6
7
8
9
10 11 12
Discharge (cumecs)
35
30
-3 -1
a
Precipitation climate change signal
Overall climate change signal
Discharge (m s )
m
baseline
1deg
2deg
3deg
4deg
5deg
6deg
30
25
20
15
10
5
0
j
f
m
a
m
j
j
a
s
o
n
d
HadCM3 prescribed warming:
groundwater climate change signal
% contribution of groundwater and
lateral flow to total discharge:
• Link to strong temperature signal
in discharge
• Little change otherwise
30
-3 -1
– Little change at 2 °C
– Decreasing GW flow in 1st wet
season at 4 & 6 °C
Overall climate change signal
Discharge (m s )
•
baseline
25
1 °C
20
2 °C
15
3 °C
10
4 °C
5
5 °C
0
6 °C
1
Lateral flow contribution to streamflow
2
3
4
5
6
7
8
9
10 11 12
Groundwater contribution to streamflow
100
100
80
80
%
60
40
40
20
20
0
0
1
2
3
4
5
6
7
8
9
10
11
12
baseline
2deg
4deg
6deg
60
%
baseline
2deg
4deg
6deg
1
2
3
4
5
6
7
8
9
10 11 12
HadCM3 prescribed warming:
groundwater climate change signal
2 degree
baseline
25
25
20
15
surfaceQ
lateralQ
10
gwaterQ
mm
mm
20
surfaceQ
lateralQ
gwaterQ
15
10
5
5
0
0
1
2
3
4
5
6
7
8
9
10
11
12
1
2
3
4
5
7
8
9
10
11
12
6 degrees
25
25
20
20
15
surfaceQ
10
lateralQ
gwaterQ
5
mm
mm
4 degrees
6
surfaceQ
15
lateralQ
10
gwaterQ
5
0
0
1
2
3
4
5
6
7
8
9
10
11
12
1
2
3
4
5
6
7
8
9
10
11
12
2 °C prescribed warming for all 7 GCMs
annual flow anomaly (%) from
baseline
• No consistency in direction
of change between GCMs
– NCAR and CSIRO are the
wettest and driest
(respectively)
80
60
40
20
0
HadCM3
CCCMA
CSIRO
IPSL
NCAR
MPI
HadGEM
-20
-40
• …either in seasonality or
annual total
GCM
35
-3 -1
Discharge (m s )
30
baseline
HadCM3
CCCMA
CSIRO
IPSL
NCAR
MPI
HADGEM1
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10 11 12
2 °C prescribed warming:
temperature vs precipitation
Temperature climate change signal
•
Impact of changing temperature
less than impact of changing
precipitation
Uncertainty in temperature
change between GCMs is less
than for precipitation
– Agreement in direction of
temperature change for 10
months of the year
baseline
hadcm3
cccma
csiro
ipsl
mpi
ncar
hadgem
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11 12
Precipitation climate change signal
35
Discharge (cumecs)
•
Discharge (cumecs)
35
baseline
hadcm3
cccma
csiro
ipsl
mpi
ncar
hadgem
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
12
annual flow anomaly (%) from
baseline
SRES scenarios on HadCM3
for 2040-2069
• A1b, A2, B1 very similar:
– increasing seasonality
– increasing total annual flow
25
20
15
10
5
0
A1b
A2
B1
B2
SRES emissions scenario
30
25
Discharge
• B2 also shows increasing
total annual flow, but in the
context of reduced early
season flow
30
baseline
a1b
a2
b1
b2
20
15
10
5
0
j
f
m
a
m
j
j
a
s
o
n
d
annual flow anomaly (%) from
baseline
SRES A1b across all GCMs
for 2040-2069
• No consistency in direction
of change between GCMs
80
60
40
20
0
HadCM3
CCCMA
CSIRO
IPSL
NCAR
MPI
HadGEM
-20
GCM
– …either in seasonality or
annual total
35
30
baseline
hadcm3
cccma
csiro
ipsl
ncar
mpi
hadgem1
discharge
25
20
15
10
5
0
j
f
m
a
m
j
j
a
s
o
n
d
Summary:
uncertainty envelopes
2 °C prescribed warming (all GCMs)
35
35
30
30
25
25
20
20
flow
flow
HadCM3 1-6 °C prescribed warming
15
15
10
10
5
5
0
0
j
f
m
a
m
j
j
a
s
o
n
j
d
HadCM3 SRES scenarios
m
a
m
j
j
a
s
o
n
d
SRES A1b (all GCMs)
35
35
30
30
25
25
20
flow
flow
f
15
20
15
10
10
5
5
0
0
j
f
m
a
m
j
j
a
s
o
n
d
j
f
m
a
m
j
j
a
s
o
n
d
Solid line=baseline; dotted lines indicate upper and lower bounds of climate change signal
Summary
•
•
Mixed results, but some common themes
GCM uncertainty > climate sensitivity and emissions uncertainty
– Consistent with findings of others
•
Emissions uncertainty relatively small for 2040-69
– but emissions scenarios likely to diverge towards 2100
•
•
•
•
No scenario shows substantial decrease in 2nd wet season discharge
Little change in annual low flow period
No consistent changes in early season 1st wet season flow at 2 °C or
A1b between GCMs
Only 1 scenario shows notable decrease in mean annual flow (CSIRO
at 2 °C )
Further work
SRES A1b (2040-2069)
GLOBAL MODEL
Model structure
– Global hydrological model
• Some agreement of changes in
seasonality and annual runoff
60
runoff (mm)
•
baseline
50
cccma
40
ipsl
30
mpi
20
ncar
10
– Mileham et al:
hadcm3
0
• For 2070-99, A2 scenario:
– Annual mean recharge
increase by 14%
– Annual mean runoff
increase by 84%
J
F
M
A
M
J
J
A
S
O
N
D
CATCHMENT MODEL
60
baseline
•
Model parameterisation
– Manual vs auto-calibration
techniques
runoff (mm)
50
cccma
40
ipsl
30
mpi
20
ncar
10
hadcm3
0
J
F
M
A
M
J
J
A
S
O
N
D
Further work (2)
•
Consideration of non-climatic
pressures on water resources:
– impacts of changing basin population
alongside climate change
– Land-use change (from SRES
scenarios)
– Ecosystem requirements
(environmental flows)
•
Incorporation of measure of
storage/reliability
– Theoretical reservoir…? (McMahon et
al. 2007)