The impact of climate change on regional water
balances in Bangladesh: supplementary material
J.M. Kirby • M. Mainuddin • F. Mpelasoka • M.D. Ahmad • W. Palash • M.E. Qadir •
S.M. Shah-Newaz • M.M. Hossain
The diagrams below summarize model simulations of regional water balances. The first five show
changes in a water balance flux term (total landscape evapotranspiration including irrigation, runoff,
drainage below the root zone, baseflow to rivers, and outflows from groundwater other than the direct
contribution to evapotranspiration). The analogous figure for irrigation water use is shown in Fig. 4 of
the main paper. The last figure shows changes in pre-monsoon groundwater levels.
The leftmost group of lines in each diagram shows the variation from the base case (historical
1985 – 2010 climate, current irrigation development) annual average of the 10th and 90th percentile
annual values. The next leftmost group of points and lines in each diagram shows the change from the
base case annual average of having no irrigation – that is, it shows the impact of irrigation
development. The next group of points show the smallest deviation from the base case amongst six
climate change scenarios for any water balance model. The six climate change scenarios are the 10th,
50th and 90th percentile GCM model results from 18 GCMs, for each of the RCP 4.5 and RCP8.5
scenarios. The difference between the minimum and maximum deviations gives us a measure of
climate change model and scenario uncertainty. The final group of points show the change with
greater surface water use, and is the larger deviation from the base case of the two surface water
development scenarios (though they generally gave fairly similar results). For all the above cases (no
irrigation, minimum and maximum climate change deviation, and increased surface water use), the
variation amongst water balance models is shown by the bars which give the range from the greatest
to least deviation value in the water balance simulations.
ET deviation from base case, bcm
5
4
Climate
No irrigation
variability
Min change with
Max change with
climate change
climate change
Increase surface
water use
3
2
1
GCM model
& climate scenario
uncertainty
0
-1
-2
-3
Water balance
model sensitivity
-4
-5
NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE
Fig S1 Deviation from a base case of calculated total regional evapotranspiration for the five regions.
All panels show deviations from the annual average (1985-2010) water use calculated for the
historical climate. The left-most panel (“Climate variability”) shows the deviation of the 10th and 90th
percentile years. The second panel (“No irrigation”) indicates the large increase in
evapotranspiration resulting from irrigation development, plotted as the deviation of the no
irrigation scenario. The third (“Min change with climate change”) and fourth (“Max change with
climate change”) panels show the minimum (most negative) and maximum (most positive)
deviations calculated under the climate change scenarios. The final panel (“Increase surface water
use”) indicates the increase in evapotranspiration calculated to result from increased surface water
use, plotted as the greater deviation of the two surface water development scenarios. The second
and subsequent panels show the range of results calculated for five different water balance models
(bars associated with each point).
Runoff deviation from base case, bcm
15
Climate
No irrigation
Min change with
Max change with
climate change
climate change
variability
Increase surface
water use
10
5
GCM model
& climate scenario
uncertainty
0
Water balance
model sensitivity
-5
-10
-15
NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE
Fig S2 As for Fig S1, but for runoff. Note that runoff is calculated to have been greater with no
irrigation; this results from groundwater extraction for irrigation leading to lower groundwater levels
(Fig. S5) and hence greater induced recharge, leaving less water in excess of that required to
recharge the groundwater.
Drainage deviation from base case, bcm
4
Climate
No irrigation
Min change with
Max change with
climate change
climate change
variability
2
Increase surface
water use
GCM model
& climate scenario
uncertainty
0
-2
-4
-6
Water balance
model sensitivity
-8
NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE
Fig S3 As for Fig S1, but for drainage below the root zone (which is equated with inflows into
groundwater).
Baseflow deviation from base case, bcm
1
Climate
No irrigation
variability
Min change with
Max change with
climate change
climate change
Increase surface
water use
0
GCM model
& climate scenario
uncertainty
Water balance
model sensitivity
-1
NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE
Fig S4 As for Fig S1, but for baseflow to the rivers
Groundwater outflows deviation from base case,
bcm
4
2
Climate
No irrigation
variability
Min change with
Max change with
climate change
climate change
Increase surface
water use
GCM model
& climate scenario
uncertainty
0
-2
-4
-6
Water balance
model sensitivity
-8
NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE
Fig S5 As for Fig S1, but for groundwater outflows
Irrigation water use deviation from base case, bcm
4
Climate
No irrigation
variability
Min change with
Max change with
climate change
climate change
Increase surface
water use
3
2
1
0
-1
-2
Water balance
model sensitivity
GCM model
& climate scenario
uncertainty
-3
-4
-5
NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE NW NC NE SW SE
Fig S6 As for Fig S1, but for pre-monsoon changes in groundwater levels. Note that pre-monsoon
groundwater levels are calculated to have been greater with no irrigation; this results from
groundwater extraction for irrigation leading to lower groundwater levels.