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Agricultural
Water
Management
194
(2017)
172–183
Contents
lists
available
at
ScienceDirect
Agricultural
Water
Management
jou
rn
al
hom
epage:
www.elsevier.com/locat
e/agwat
Research
Paper
Future
changes
in
water
requirements
of
Boro
rice
in
the
face
of
climate
change
in
North-West
Bangladesh
Tapos
Kumar
Acharjeea,c,∗,
Fulco
Ludwiga,
Gerardo
van
Halsemab,
Petra
Hellegersb,
Iwan
Supita
aWater
Systems
and
Global
Change
Group,
Wageningen
University,
The
Netherlands
bWater
Resources
Management
Group,
Wageningen
University,
The
Netherlands
cDepartment
of
Irrigation
and
Water
Management,
Bangladesh
Agricultural
University,
Bangladesh
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
22
June
2017
Received
in
revised
form
12
September
2017
Accepted
13
September
2017
Keywords:
Climate
change
Evapotranspiration
Irrigation
demand
a
b
s
t
r
a
c
t
Understanding
future
changes
in
crop
water
requirements
and
irrigation
demand
in
the
context
of
climate
change
is
essential
for
long-term
water
resources
management
and
agricultural
planning.
This
study
investigates
the
impacts
of
climate
change
on
future
water
requirements
of
dry
season
Boro
rice.
Climate
scenarios
for
four
North-West
districts
of
Bangladesh
were
constructed
from
the
outputs
of
five
global
circulation
models
using
a
combination
of
statistical
downscaling
and
bias
correction.
The
generated
climate
data
were
used
as
input
for
CropWat
to
estimate
water
requirements
of
Boro
rice
for
2050s
and
2080s
(using
30
year
average
climate
data).
Reference
crop
evapotranspiration
(daily
ETo)
is
increasing
in
the
future,
mainly
due
to
higher
temperatures.
Potential
crop
water
requirement
(ETC)
of
Boro
rice,
however,
will
reduce
by
6.5%
and
10.9%
for
RCP
4.5
and
8.5,
respectively
for
2050s;
and
by
8.3%
and
17.6%
for
RCP
4.5
and
8.5,
respectively
for
2080s
compared
to
the
reference
period
(1980–2013).
ETC
will
decrease
because
of
a
lower
number
of
growing
days
due
to
the
phenological
response
of
rice
to
higher
temperatures.
Low
rainfall
accessibility
under
a
shortened
Boro
season
leads
to
an
increase
in
the
amount
of
irrigation
water
required
to
satisfy
crop
evapotranspiration
demand.
Although
daily
water
requirements
will
increase,
the
total
net
irrigation
requirement
of
Boro
rice
will
decrease
by
1.6%
in
2050s
and
7.4%
in
2080s
for
RCP
8.5
scenario
on
average
for
all
models
and
districts.
Estimated
net
irrigation
requirements
showed
high
variations
for
different
climate
models,
mainly
due
to
a
high
variation
in
the
projected
rainfall.
For
improved
water
management
planning,
close
monitoring
and
periodic
evaluations
are
necessary
to
understand
future
directions
of
change
in
rainfall
amounts
and
distribution.
©
2017
Elsevier
B.V.
All
rights
reserved.
1.
Introduction
Human
activities
since
industrialization
have
resulted
in
increased
CO2emissions
causing
anthropogenic
climate
change.
Global
surface
temperature
change
is
projected
to
likely
exceed
1.5 ◦C
for
RCP
4.5
and
2◦C
for
RCP8.5
by
the
end
of
the
21st
cen-
tury,
relative
to
the
average
from
1850
to
1900
(IPCC,
2013).
In
addition
to
global
warming,
climatic
variables
such
as
precipita-
tion,
solar
radiation
and
wind
speed
will
change.
Although
climate
change
is
a
global
phenomenon,
there
are
large
regional
differ-
ences
in
the
impacts.
Projections
show
large
regional
variations
in
∗Corresponding
author
at:
Water
Systems
and
Global
Change
Group,
Wageningen
University,
The
Netherlands.
E-mail
addresses:
tapos.acharjee@wur.nl,
tapos.bau@gmail.com
(T.K.
Acharjee).
future
climate
change.
Hence,
the
consequences
of
climate
change
will
also
be
regionally
specific.
Bangladesh
is
considered
as
one
of
the
most
vulnerable
countries
to
the
impacts
of
climate
change
(IPCC,
2007).
Not
only
flood
risks
will
increase,
but
also
projected
increases
in
rainfall
variability
and
lower
dry
season
rainfall
will
affect
future
crop
production.
Moreover,
climate
change
is
likely
to
affect
future
crop
water
requirements.
Irrigation
requirement
is
very
sensitive
to
climate
change
(Schlenker
et
al.,
2007)
and
particularly
sensitive
to
changes
in
precipitation,
and
temperature
(Frederick
and
Major,
1997).
The
change
in
annual
mean
water
requirements
is
strongly
associated
with
the
change
in
temperature
(McCabe
and
Wolock,
1992).
Estimations
of
evapotranspiration
using
the
Penman–Monteith
formula
are
more
sensitive
to
temper-
ature
and
humidity
compared
to
wind
speed
and
sunshine
hours
(Eslamian
et
al.,
2011).
Yu
et
al.
(2002)
indicated
that
solar
radi-
ation
is
the
most
sensitive
and
wind-speed
the
least
sensitive
http://dx.doi.org/10.1016/j.agwat.2017.09.008
0378-3774/©
2017
Elsevier
B.V.
All
rights
reserved.
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
173
variables
of
the
modified
Penman
formula.
However,
the
use
of
all
climatic
variables
to
estimate
crop
water
and
irrigation
require-
ment
could
provide
a
better
insight
of
consequences
of
climate
change
on
agricultural
water
requirement.
As
the
consequences
of
climate
change
are
regionally
specific,
studies
conducted
at
large
spatial
scales
(e.g.
global,
continental,
or
basin)
often
lack
sufficient
details
to
understand
the
impacts
of
climate
change
on
regional
water
management.
A
study
of
potential
impacts
of
inevitable
cli-
mate
change
on
future
water
requirements,
using
several
global
circulation
model
outputs
to
estimate
water
requirements
of
the
major
crop
of
a
region,
is
important
from
local
water
management
perspective
(Woznicki
et
al.,
2015).
Future
changes
are
always
uncertain.
Therefore,
it
is
difficult
to
predict
the
future
climate.
Using
a
range
of
possible
future
climate
change
scenarios,
rather
than
a
single
projection
is
one
way
of
deal-
ing
with
uncertainties
(Asseng
et
al.,
2009).
As
scenarios
provide
information
for
different
possible
future
changes,
it
can
provide
a
better
insight
rather
than
a
single
prediction.
Global
Circulation
Models
(GCMs)
are
the
basis
for
generating
future
climate
change
scenarios
and
are
used
for
regional
impact
studies
after
applying
downscaling
techniques
to
identify
the
regional
climate
variables
(Smith
and
Pitts,
1997).
GCMs
consider
a
wide
range
of
processes,
including
atmosphere,
ocean
and
land
surface
processes
that
char-
acterize
the
climate
system
and
are
used
to
examine
the
impact
of
increasing
greenhouse
gas
concentrations
on
global
climate
(Houghton
et
al.,
1990).
GCMs
are
the
best
source
of
information
about
regional
climate
change
estimating
changes
in
meteorolog-
ical
variables
in
regional
climate
in
grid
boxes
of
typically
3
or
4◦
in
latitude
and
as
much
as
10◦in
longitude
(Smith
and
Pitts,
1997).
However,
GCMs
do
not
always
accurately
represent
the
climate
at
a
regional
scale
and
misrepresent
the
seasonal
patterns
of
precipi-
tation
in
many
cases
(Robock
et
al.,
1993).
Moreover,
not
all
GCMs
give
a
realistic
scenario
for
a
particular
region
or
country.
Hence,
it
is
wise
to
use
several
GCMs
to
generate
future
climate
change
scenarios
for
impact
studies.
Use
of
a
larger
number
of
GCMs’
out-
puts
better
represent
the
structural
uncertainty
in
climate
models
(Tebaldi
and
Knutti,
2007).
Climate
change
induced
changes
in
the
crop
water
requirements
is
a
major
concern
for
Bangladesh.
Bangladesh
is
facing
challenges
of
rapid
population
growth,
declining
cultivable
land,
inadequate
water
availability
during
the
dry
season,
declining
ground
water
table
(Ahmad
et
al.,
2014;
Salem
et
al.,
2017)
and
extreme
events
such
as
floods
and
droughts.
The
possible
impacts
of
climate
change
could
add
additional
pressure
to
existing
problems.
Bangladesh
will
need
to
produce
more
food
for
its
increasing
population.
There-
fore,
cultivation
of
more
irrigated
high
yielding
crops
could
solve
the
problem.
However,
a
high
water
demand
or
lack
of
water
avail-
ability
due
to
climate
change
may
not
support
the
expansion
of
irrigated
crop
cultivation.
Groundwater
levels
in
the
North-West
part
of
Bangladesh
are
declining
due
to
increasing
groundwater
extraction
for
irrigation
in
the
dry
season
and
recurrent
droughts
(Shahid
and
Hazarika,
2010).
Considering
these
different
aspects,
it
is
important
to
identify
the
possible
future
changes
in
water
requirements
of
dry
season
crops
to
understand
the
consequences
of
climate
change
on
the
longer
run
and
at
regional
scale.
Boro
rice
is
the
major
dry
season
crop
of
Bangladesh,
which
requires
irrigation
from
January
to
April.
The
annual
average
rain-
fall
in
North-West
Bangladesh
ranges
from
1400
to
2000
mm,
with
93%
of
rainfall
occurring
from
May
to
October,
and
only
about
6
percent
during
Boro
rice
growing
season
(Shahid,
2010).
Boro
rice
thus
fully
depends
on
irrigation.
The
total
amount
of
water
used
for
crop
agriculture
has
increased
substantially
over
the
last
few
decades
because
of
intensive
irrigated
agriculture,
especially
in
the
North-Western
part
of
Bangladesh.
Intensive
irrigated
agri-
culture
is
resulting
in
water
shortages
during
dry
periods
(Shahid,
2008).
The
excessive
use
of
groundwater
for
irrigation,
along
with
declining
groundwater
recharge
potential
due
to
urbanization,
are
causing
a
drop
in
groundwater
levels
throughout
the
country.
A
study
by
Dey
et
al.
(2017)
indicates
a
declining
trend
in
ground-
water
levels
in
North-West
Bangladesh,
with
most
depletion
in
Rajshahi
followed
by
Pabna,
Bogra,
Dinajpur
and
Rangpur
because
of
increased
Boro
cultivation
area
and
reduced
recharge.
Now,
it
is
essential
to
understand
the
future
possible
changes
in
agricultural
water
requirements
to
improve
water
resources
management
in
Bangladesh.
Several
studies
have
assessed
the
impact
of
climate
change
on
reference
crop
evapotranspiration
(ETO)
in
Bangladesh.
The
study
by
Mojid
et
al.
(2015)
indicated
a
decreasing
trend
of
ETOdur-
ing
most
of
the
months
of
the
year
for
the
period
1990–2010
in
two
North-West
districts.
From
1980
to
2013,
water
requirements
of
Boro
rice
have
decreased
because
of
increases
in
humidity
and
decreases
in
wind-speed
and
sunshine
hours,
despite
a
warming
of
the
climate
in
North-West
Bangladesh
(Acharjee
et
al.,
2017).
Shahid
(2010)
argued
that
there
will
be
no
appreciable
changes
in
total
irrigation
water
requirement
of
Boro
rice
due
to
climate
change
in
North-West
Bangladesh.
However,
more
in-depth
stud-
ies
are
required
to
assess
the
consequences
of
climate
change
on
crop
water
requirements
and
irrigation
requirements
during
dry
season,
based
on
different
climate
scenarios.
For
a
complete
understanding
of
the
impacts
of
climate
change
on
agricultural
water
requirements,
the
study
of
reference
crop
evapotranspiration
is
insufficient.
Changes
in
temperature
not
only
affect
evapotranspiration,
but
also
affect
the
length
of
the
grow-
ing
season.
Estimations
and
analyses
of
changes
in
potential
crop
water
requirements,
number
of
growing
days,
potential
irriga-
tion
requirements
for
crop
evapotranspiration
and
net
irrigation
requirements
will
expand
our
understanding
of
the
impacts
of
cli-
mate
change
on
regional
water
resources
management.
So
in
this
study,
in
which
we
aim
to
quantify
the
impacts
of
future
climate
change
on
water
requirements
of
Boro
rice,
we
applied
the
Crop-
Wat
model
in
combination
with
downscaled
and
bias
corrected
GCM
outputs
of
different
climatic
parameters,
for
four
North-West
districts.
This
study
will
be
beneficial
for
local
water
managers,
agriculturists,
researchers
and
policy
makers
in
understanding
the
future
consequences
of
climate
change
and
developing
adaptation
strategies
for
local
agricultural
water
management.
2.
Materials
and
method
2.1.
Study
area
Only
five
of
the
16
administrative
districts
in
North-West
Bangladesh
have
a
weather
station.
Four
of
these,
namely
Bogra,
Rajshahi,
Pabna
and
Dinajpur
were
included
in
this
study.
The
North-West
region
extends
from
23◦47N
to
25◦50N
latitude
and
from
88◦01E
to
89◦48E
longitude.
This
part
of
the
country
belongs
to
the
sub-humid
agro-climatic
class.
As
the
total
annual
evapo-
transpiration
is
equal
to
annual
rainfall
in
some
places,
this
region
is
defined
as
very
close
to
dry
(Shahid
et
al.,
2005).
Annual
rain-
fall
varies
from
1400
to
2000
mm.
Meteorological
drought
is
a
very
common
phenomenon
during
the
dry
months
in
this
region
(Shahid
and
Behrawan,
2008).
Recent
drought
events
have
had
a
severe
impact
on
the
economy
of
the
whole
country.
The
North-West
region,
with
its
prolonged
dry
season,
was
affected
more
severely
than
the
rest
of
the
country
(Shahid,
2008).
The
economy
of
this
area
is
mostly
agriculture
based,
with
75%
of
the
land
under
crop
cultivation.
About
31%
of
the
land
is
used
for
single
cropping,
56%
for
double
cropping
and
13%
for
triple
cropping
(Shahid
and
Hazarika,
2010).
Boro
rice
is
the
main
dry
season
crop
in
the
study
area,
and
is
cultivated
on
more
than
70%
of
the
cultivable
area
from
December
to
May.
174
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
2.2.
Data
collection
Crop
data
related
to
dry
season
Boro
rice
has
been
collected
from
Bangladesh
Agricultural
Research
Institute
(BARI).
Collected
data
on
dry
and
wet
crop
co-efficient
(Kc)
values
for
different
growth
stages
(nursery,
land
preparation,
initial,
mid-season
and
late
sea-
son),
rooting
depth,
crop
height
and
critical
depletion
factor
have
been
used
as
input
into
the
CropWat
model.
Soil
data
on
avail-
able
soil
moisture,
maximum
rainfall
infiltration
rate,
maximum
rooting
depth,
initial
soil
moisture
depletion,
initial
available
soil
moisture,
drainable
porosity,
critical
depletion
for
puddle
crack-
ing,
maximum
percolation
rate
after
puddling,
water
availability
at
planting
and
maximum
water
depth,
were
standardized
for
a
medium
average
soil
for
the
study
districts
from
FAO
standard
soil
parameter
values.
2.3.
Climate
models
and
scenarios
Five
General
Circulation
Models
(GCMs)
and
two
emission
sce-
narios
(RCPs)
were
used
to
construct
future
climate
scenarios.
Maximum
and
minimum
temperatures,
rainfall,
wind
speed
and
solar
radiation
for
the
time
series
of
2035–2065
and
2065–2095
were
prepared.
The
GCMs
used
were
the
CNRM-CM5
model,
described
as
developed
by
CNRM-GAME
(Centre
National
de
Recherches
Météorologiques—Groupe
d’études
de
l’Atmosphère
Météorologique)
and
Cerfacs
(Centre
Européen
de
Recherche
et
de
Formation
Avancée)
to
contribute
to
phase
5
of
the
Coupled
Model
Inter-comparison
Project
(CMIP5)
that
includes
atmo-
spheric,
ocean,
land
surface
scheme
and
sea
ice
models
(Voldoire
et
al.,
2013);
the
EC-Earth
model,
is
a
seamless
(forecasting
and
cli-
mate
change
studies
into
a
single
framework)
Earth
System
Model
(Hazeleger
et
al.,
2010);
the
HadGEM2-ES
model,
is
as
a
coupled
Earth
System
Model
that
was
used
by
the
Met
Office
Hadley
Centre
for
the
CMIP5
centennial
simulations;
the
IPSL-CM5A-LR
model,
includes
5
model
components
representing
the
Earth
System
cli-
mate
and
its
carbon
cycle:
LMDz
(atmosphere),
NEMO
(ocean,
oceanic
biogeochemistry
and
sea-ice),
ORCHIDEE
(continental
sur-
faces
and
vegetation),
and
INCA
(atmospheric
chemistry),
coupled
through
OASIS;
the
MPI-ESM-LR
model,
is
a
comprehensive
Earth-
System
Model
that
consists
of
component
models
for
the
ocean,
the
atmosphere
and
the
land
surface.
These
models
were
selected
for
this
study
because
of
their
important
criteria
in
evaluating
the
impacts
of
climate
change,
such
as
ocean-atmosphere
couple,
their
documentation
in
literature,
multi-century
simulation
capability,
and
participation
in
the
Coupled
Model
Inter-comparison
Project
(CMIP)
(Barrow
et
al.,
2004).
Two
different
emission
scenarios,
RCP
4.5
and
8.5
were
used
in
this
study.
RCP
4.5
represents
stabilization
without
overshoot
pathway
to
4.5
W/m2(∼650
ppm
CO2eq.)
at
stabilization
after
2100
and
RCP
8.5
represents
rising
radiative
forcing
pathway
leading
to
8.5
W/m2(∼1370
ppm
CO2eq.)
by
2100
(Van
Vuuren
et
al.,
2011).
These
two
RCPs
were
selected
for
this
study
because
they
represent
realistically
low
and
high
future
climate
change
scenarios.
A
statistical
downscaling
technique
was
used
to
generate
the
future
climate
data
for
maximum
and
minimum
temperatures,
rainfall,
wind
speed
and
solar
radiation.
The
downscaled
model
data
were
bias
corrected
by
comparing
the
past
model
data
with
historical
observed
data.
For
bias
correction,
the
monthly
average
WATCH
Forcing
data
(Weedon
et
al.,
2011)
of
maximum
and
min-
imum
temperatures,
sunshine
hours
and
wind
speed
and
monthly
total
WATCH
Forcing
data
of
rainfall
were
compared
to
monthly
observed
station
data.
Since,
the
relative
humidity
predictions
were
not
available
from
the
GCM
outputs,
they
were
estimated
follow-
ing
the
ratio
between
actual
water
vapour
pressure
and
saturation
vapour
pressure.
The
actual
and
saturation
vapour
pressure
is
a
pure
function
of
temperature
and
can
be
calculated
by
a
common
empirical
interpolation
function
(Holbo,
1981;
WMO,
1979).
For
humid
temperate
climates,
when
temperature
is
at
its
daily
min-
imum,
water
vapour
is
saturated.
Hence,
the
general
assumption
to
estimate
relative
humidity
from
temperature
data
is
to
consider
dew
temperature
as
equal
to
the
minimum
temperature
of
the
day
(Eccel,
2012).
2.4.
Estimation
of
growth
stage
days
The
length
of
the
four
distinguished
growth
stages
of
Boro
rice
were
estimated
for
different
climate
scenarios
following
the
grow-
ing
degree
days
(GDD)
method.
The
following
equation
was
used
to
estimate
GDD:
GDD
=(Tmax +
Tmin)/2−
Tbase (1)
Where,
Tmax is
the
maximum
temperature
(◦C),
Tmin is
the
mini-
mum
temperature
(◦C),
and
Tbase is
the
base
temperature
(10 ◦C
for
rice
plant).
First,
the
GDD
was
estimated
for
four
study
districts
under
five
climate
models
and
two
scenarios.
Later,
the
growth
stage
duration
was
estimated
from
growing
degree
days
and
accu-
mulated
heat
values
at
the
end
of
each
stage.
Growth
stage
duration (days)=
Accumulated
heat
value
at
the
end
of
the
stage (◦C)
GDD
for
the
corrosponding
period (◦C)(2)
The
accumulated
heat
values
at
the
end
of
the
growth
stages
of
Boro
rice
in
the
North-West
zone
of
Bangladesh
were
obtained
from
the
studies
of
Mahmood
(1997).
The
accumulated
heat
values
at
the
end
of
initial,
vegetative,
flowering
and
maturing
stages
for
Bogra,
Rajshahi
and
Pabna
were
80,
528,
1052
and
1291 ◦C,
respectively
and
for
Dinajpur
were
80,
515,
1032
and
1273 ◦C,
respectively.
The
method
of
growing
degree
days
can
consistently
predict
the
growth
stage
days
(Miller
et
al.,
2001).
2.5.
Estimation
of
water
requirements
CropWat
model
developed
by
FAO
was
used
to
estimate
water
requirements
of
Boro
rice.
CropWat
is
used
to
compute
crop
water
requirements
and
irrigation
requirements
based
on
climate,
crop
and
soil
data.
It
has
been
used
extensively
as
a
decision
support
tool
in
an
international
context
to
calculate
regional
irrigation
require-
ments
(Clarke
et
al.,
2001).
This
model
has
also
been
successfully
applied
to
evaluate
impacts
of
climate
change
on
water
require-
ments
in
several
previous
studies
(Chowdhury
et
al.,
2013;
Doria
et
al.,
2006;
Doria,
2010;
Shrestha
et
al.,
2013).
The
FAO
Penman-Monteith
equation
was
used
to
determine
reference
crop
evapotranspiration
for
different
combinations
(5
models,
2
scenarios
and
4
stations)
from
2035
to
2095.
The
statistical
significance
of
future
trends
in
reference
crop
evapo-
transpiration
(daily
ETo)
for
the
periods
2035–2065
and
2065–2095
was
tested
using
the
non-parametric
Mann-Kendall
test.
In
a
Mann-
Kendall
test,
the
data
are
evaluated
as
an
ordered
time
series
and
each
data
value
is
compared
to
all
subsequent
data
values
to
esti-
mate
the
Mann-Kendall
statistic.
The
probability
associated
with
the
Mann-Kendall
statistic
was
computed
to
quantify
the
statistical
significance
of
the
trend.
Water
requirements
of
Boro
rice
were
estimated
in
CropWat
using
statistically
downscaled
bias-corrected
daily
climate
data
from
GCMs
outputs.
The
following
equation
represents
the
esti-
mated
net
irrigation
requirement:
Net
irrigation
requirement
=
ETC−
ER
+
PL
+
N&LP
(3)
Where, ETCis
the
potential
crop
water
requirements
or
total
crop
evapotranspiration,
ER
is
the
effective
rainfall
during
Boro
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
175
Fig.
1.
Potential
crop
water
requirement
of
Boro
rice
in
four
North-West
districts
of
Bangladesh
for
the
base
period,
2050s
and
2080s
using
two
different
RCPs.
Each
bar
show
the
average
for
five
models,
and
error
bar
indicates
the
standard
deviation
of
different
model
estimates.
Table
1
Mann-Kendall
trends
of
estimated
reference
crop
evapotranspiration
during
dry
months
for
2035–2065
and
2065–2095
time
series
in
Rajshahi.
Trends
during
Month
CNRM-CM5
EC-Earth
HadGEM2-ES
IPSL-CM5A-LR
MPI-ESM-LR
RCP
4.5 RCP
8.5 RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
2035–2065 Jan
2.96**
0.41
1.77+0.61
2.41*
2.28*
−1.60
0.82
2.14*
1.26
Feb
1.12
−0.34
1.53
0.27
3.50***
2.92**
2.24*
1.53
2.31*
1.26
Mar
1.29
−1.29
0.85
0.48
2.07*
2.35*
0.85
2.48*
1.09
1.50
Apr
0.99
0.68
0.99
−0.41
1.73+3.43***
−1.22
1.87+2.79**
2.45*
Nov
0.85
−0.03
1.09
0.68
2.96**
1.97*
0.88
0.71
1.56
1.33
Dec
2.35*
−0.95
1.26
0.17
3.03**
2.99**
0.17
1.29
−0.51
1.43
2065–2095 Jan
−0.41
1.63
1.26
1.53
1.97*
4.08***
1.12
0.17
0.51
2.21*
Feb
−1.43
0.92
0.17
1.87+0.68
3.40***
0.51
0.61
0.00
1.63
Mar
−0.27
−0.88
1.50
−0.10
0.88
3.30***
0.41
0.75
−0.85
1.46
Apr
0.68
0.10
1.26
0.58
2.07*
2.04*
0.54
1.36
−0.37
2.11*
Nov
0.48
1.19
2.55*
0.20
2.69**
4.69***
1.97*
0.92
2.21*
0.82
Dec
1.36
2.55*
1.05
0.88
2.45*
4.49***
0.31
−0.20
1.63
0.92
+,
*,
**
and
***
signs
indicate
significant
at
0.10,
0.05,
0.01
and
0.001
level
of
significance,
respectively.
growth
duration,
PL
is
the
amount
of
percolation
loss,
and
N&LP
is
the
amount
of
water
required
for
nursery
and
land
preparation.
ETC−
ER
denotes
the
potential
irrigation
requirement
for
crop
evapotranspiration.
Water
requirements
were
estimated
for
4
districts,
5
models
and
2
RCPs
for
2050s
(average
of
2035–65)
and
2080s
(average
of
2065–95).
Potential
crop
water
requirement
is
calculated
as
total
crop
evapotranspiration
(ETC)
during
the
crop
growing
period
and
takes
into
account
the
changes
in
the
length
of
the
growing
sea-
son.
The
Potential
irrigation
requirement
for
crop
evapotranspiration
is
the
total
crop
evapotranspiration
amount
in
excess
of
effective
rainfall,
i.e. ETC−
ER.
Here,
effective
rainfall
is
the
amount
of
rain-
fall
that
is
effectively
added
and
stored
in
the
soil
for
later
use
by
the
crop,
and
is
derived
as
a
simulation
output
from
CropWat.
Net
irrigation
requirement
takes
into
account
the
amount
of
irrigation
required
for
crop
evapotranspiration,
percolation
loss
and
water
required
for
nursery
and
land
preparation.
All
values
are
derived
from
CropWat
simulations.
In
order
to
assess
the
climate
change
impacts
on
the
water
requirements
of
Boro
rice,
the
influence
of
management
practices
were
excluded
by
modelling
the
rice
growth
in
CropWat
under
a
standardized
schedule
that
provides
irrigation
water
as
required.
The
crop
co-efficient
values
under
dry
condition
were
0.7,
0.3,
0.5,
1.05
and
0.65,
and
wet
condition
were
1.2,
1.05,
1.1,
1.2
and
0.95
for
nursery,
land
preparation,
initial,
mid-season
and
late-season,
respectively.
The
transplanting
date
of
Boro
rice
was
taken
as
10th
of
January
for
all
estimations
(i.e.
for
base
period,
2050s
and
2080s).
The
scheduling
criteria
to
estimate
the
net
irrigation
requirement
was
to
provide
irrigation
at
5
mm
water
depth
above
ground
with
a
refill
to
100
mm
standing
water.
The
estimation
procedure
of
per-
colation
amount,
water
required
for
nursery
and
land
preparation,
etc.
for
a
standard
schedule
in
CropWat
were
discussed
in
detail
in
the
study
by
Acharjee
et
al.
(2017).
3.
Results
3.1.
Future
changes
in
reference
crop
evapotranspiration
All
models
show
increasing
trends
of
reference
crop
evapotran-
spiration
for
most
of
the
dry
months
for
both
the
2035–65
and
2065–95
time
series
in
Rajshahi
(Table
1).
The
HadGEM2-ES
model
shows
a
more
pronounced
increase
of
EToin
comparison
to
other
model
estimates.
For
the
2065–95
time
series,
HadGEM-2ES
model
returns
higher
increasing
trends
of
ETofor
RCP
8.5
scenario
in
com-
parison
to
RCP
4.5
scenario.
For
RCP
4.5
scenario,
the
trends
of
ETo
are
more
pronounced
for
2035–2065
in
comparison
to
the
trends
during
2065–95.
But,
for
RCP
8.5
scenario
it
is
the
opposite,
with
more
pronounced
increases
of
ETofor
the
period
2065–95
com-
pared
to
those
for
the
2035–65
time
series.
Other
climate
models
mainly
show
non-significant
trends,
but
similar
kind
of
charac-
teristics;
e.g.
for
RCP
8.5,
higher
values
of
Mann-Kendall
trends
during
2065–2095
in
comparison
to
the
trends
of
2035–65
time
series.
Similar
results
have
been
found
for
trends
of
reference
crop
176
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
Fig.
2.
Duration
of
growing
days
of
Boro
rice
in
four
North-West
districts
of
Bangladesh
for
the
base
period,
2050s
and
2080s
using
two
different
RCPs.
Each
bar
show
the
average
for
five
models,
and
error
bar
indicates
the
standard
deviation
of
different
model
estimates.
Fig.
3.
Potential
irrigation
requirement
for
crop
evapotranspiration
of
Boro
rice
in
four
North-West
districts
of
Bangladesh
for
the
base
period,
2050s
and
2080s
using
two
different
RCPs.
Each
bar
show
the
average
for
five
models,
and
error
bar
indicates
the
standard
deviation
of
different
model
estimates.
evapotranspiration
for
Bogra,
Pabna
and
Dinajpur
(see
Appendix
A,
Table
A1–A3).
The
results
also
indicate
that,
for
a
moderate
climate
change
scenario
(RCP
4.5)
the
rate
of
increase
in
reference
crop
evapo-
transpiration
will
reduce,
or
even
decrease
in
some
months
(e.g.
CNRM-CM5,
January–March)
during
2065–95
in
comparison
to
the
rate
of
increase
during
2035–65.
For
a
rapid
climate
change
sce-
nario
(RCP
8.5),
the
rate
of
increase
in
ETowill
accelerate
during
2065–95.
However,
for
both
scenarios
the
ETowill
mainly
increase
in
all
study
districts
of
North-West
Bangladesh.
3.2.
Future
changes
in
potential
crop
water
requirement
of
Boro
rice
The
model
estimates
show
a
decrease
in
potential
crop
water
requirement
(ETC)
of
Boro
rice
in
all
study
districts
for
both
RCP
4.5
and
RCP
8.5
compared
to
the
base
period
(Fig.
1).
The
potential
crop
water
requirement
was
highest
during
the
base
period
for
all
districts.
Also,
the
potential
crop
water
requirement
is
comparatively
higher
during
2050s
(average
for
2035–65
climate)
than
during
2080s
(average
for
2065–95
climate).
RCP
8.5
shows
a
steeper
decrease
in
potential
crop
water
requirement
in
compared
to
RCP
4.5.
Therefore,
a
high
end
climate
change
scenario
(RCP8.5)
indicates
a
stronger
decrease
in
potential
crop
water
requirements
compared
to
a
moderate
scenarios
(RCP4.5).
Estimates
of
the
potential
crop
water
requirement
takes
into
account
both
the
changes
in
reference
crop
evapotranspiration
(ETo)
and
the
changes
in
growth
period.
The
duration
of
growth
stage
days
of
Boro
rice
will
reduce
as
a
result
of
increased
tem-
peratures
because,
the
rice
plant
matures
more
quickly
at
higher
temperatures.
All
the
estimates
from
different
climate
models
show
a
decrease
in
the
number
of
total
growing
days
of
Boro
rice
in
the
future
for
both
RCP
4.5
and
RCP
8.5
(Fig.
2).
The
total
growing
period
was
shorter
for
RCP
8.5
compared
to
RCP4.5
due
to
higher
temperatures
under
RCP8.5.
3.3.
Future
changes
in
irrigation
requirement
of
Boro
rice
The
model
estimates
indicate
an
increase
in
future
poten-
tial
irrigation
amount
required
to
satisfy
crop
evapotranspiration
(ETC−
ER)
compared
to
the
base
period
for
both
RCP
4.5
and
8.5
scenarios
(Fig.
3).
However,
some
variations
in
changes
between
different
districts
were
observed.
For
Bogra
and
Dinajpur,
‘ETC−
ER’
showed
the
highest
values
during
2080s
(2065–2095),
which
indicates
a
continuous
increase
till
the
end
of
the
century.
While
for
Rajshahi
and
Pabna,
there
is
an
increase
during
2035–65,
followed
by
a
decrease
during
2065–2095,
which
indicates
a
peak
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
177
Fig.
4.
Effective
rainfall
amount
during
Boro
growing
duration
in
four
North-West
districts
of
Bangladesh
for
the
base
period,
2050s
and
2080s
using
two
different
RCPs.
Each
bar
show
the
average
for
five
models,
and
error
bar
indicates
the
standard
deviation
of
different
model
estimates.
Fig.
5.
Monthly
rainfall
distribution
for
the
base
period,
2050s
and
2080s
in
four
North-West
districts
of
Bangladesh
for
RCP
8.5.
The
green
area
indicates
the
amount
of
rain
water
available
for
Boro
rice,
and
red
area
indicates
the
amount
of
rain
water
not
available
in
2080s
compared
to
base
period.
(For
interpretation
of
the
references
to
color
in
this
figure
legend,
the
reader
is
referred
to
the
web
version
of
this
article.)
during
the
mid
of
the
century.
The
variations
in
different
districts
are
mainly
caused
by
the
variations
in
rainfall
or
rainfall
availability
during
the
Boro
growing
season.
Analysis
of
monthly
rainfall
distribution
for
future
climate
sce-
narios
indicates
a
considerable
decrease
in
the
amount
of
rainfall
that
is
available
during
the
Boro
growing
season
(Fig.
4).
High
vari-
ations
(see
the
error
bars
in
Fig.
4)
in
available
rainfall
amount
during
the
Boro
growing
season
can
be
observed
in
different
model
estimates.
These
high
variations
in
projected
rainfall
represent
a
big
challenge
for
anticipatory
irrigation
and
water
management
planning.
However,
the
considerable
reduction
of
available
rainfall
amount
during
the
Boro
growing
season
is
not
only
the
result
of
reduced
rainfall
during
the
dry
months.
The
reduction
in
total
rice
growth
length
has
a
more
pronounced
effect,
as
it
shortens
the
total
growth
season
with
the
effect
that
the
“late
periods”
of
high
rainfall
(during
May)
start
to
fall
outside
the
rice
growth
period
as
crop
growth
cycles
decrease
(due
to
the
described
GDD
effect)
and
the
planting
date
is
kept
fixed.
The
green
area
in
the
graph
(Fig.
5)
indicates
the
approximate
amount
of
rainfall
available
for
Boro
cul-
tivation
in
2080s
with
same
planting
date
as
now,
and
the
red
area
indicates
the
amount
of
approximate
rainfall
that
is
not
available
for
Boro
rice
cultivation
in
the
2080s
because
of
shortening
of
growing
days
with
fixed
planting
date
for
RCP
8.5.
Estimations
of
net
irrigation
requirements
(ETC−
ER
+
PL
+
N&LP)
show
an
overall
decrease
in
the
future
(Fig.
6).
However,
there
are
high
variations
in
the
change
between
the
different
districts,
RCPs,
models
and
time
periods.
For
all
study
districts,
results
indicate
an
initial
increase
(i.e.,
during
2050s)
but
a
later
decrease
(i.e.,
during
2080s)
in
net
irrigation
requirement
for
RCP
4.5,
and
an
initial
low
decrease
and
later
high
decrease
in
net
irrigation
requirement
for
RCP
8.5.
Therefore,
both
moderate
and
rapid
climate
change
indicate
a
direction
towards
a
long
term
decrease
in
the
net
irrigation
requirements.
A
decreased
amount
178
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
Fig.
6.
Net
irrigation
requirement
of
Boro
rice
in
four
North-West
districts
of
Bangladesh
for
the
base
period,
2050s
and
2080s
using
two
different
RCPs.
Each
bar
show
the
average
for
five
models,
and
error
bar
indicates
the
standard
deviation
of
different
model
estimates.
Fig.
7.
Percolation
amount
during
Boro
growing
season
in
four
North-West
districts
of
Bangladesh
for
the
base
period,
2050s
and
2080s
using
two
different
RCPs.
Each
bar
show
the
average
for
five
models,
and
error
bar
indicates
the
standard
deviation
of
different
model
estimates.
of
total
net
irrigation
requirement
indicates
a
possible
reduction
of
percolation
amounts
and/or
water
requirements
for
nursery
and
land
preparation.
Further
analysis
indicates
a
decrease
of
total
percolation
during
the
crop
growth
period
of
Boro
rice
(Fig.
7).
Due
to
a
shorter
duration
of
growth
stages
(phenological
accelerated
growth
due
to
GDD
impact)
less
days
are
available
in
which
percolation
can
occur,
and
thus
less
irrigation
water
is
required
to
meet
that
percolation.
3.4.
Impacts
of
climate
change
on
water
requirement
components
The
impacts
of
changes
in
different
climate
variables
on
dif-
ferent
water
requirement
components
of
Boro
rice
are
shown
in
Fig.
8.
A
reduction
in
number
of
growing
stage
days
(GSD)
not
only
reduces
potential
crop
water
requirement
(ETC),
but
also
reduces
the
amount
of
percolation
from
the
rice
field
and
amount
of
rainfall
available
for
Boro
rice.
During
recent
decades,
the
potential
crop
water
requirements
(ETC)
decreased
due
to
both
a
reduction
in
EToand
in
the
number
of
growing
days
of
Boro
rice
(Acharjee
et
al.,
2017).
In
the
future,
ETCwill
continue
to
decrease
because
of
shortening
in
growing
days
despite
a
slight
increase
of
daily
water
requirements.
Dur-
ing
recent
decades, ETC−
ER
showed
an
increasing
trend
only
for
Bogra,
and
a
decrease
for
other
studied
districts.
In
the
future,
ETC−
ER
will
increase
for
all
districts
because
of
a
decrease
in
rainfall
availability
during
the
Boro
growing
season.
Therefore,
the
estimated
trend
in
future
reduction
of
net
irrigation
requirement
for
Boro
rice
is
lower
than
the
reducing
trend
observed
over
the
last
decades
(1980–2013)
as
reported
by
Acharjee
et
al.
(2017).
The
percentage
change
(averaged
across
all
climate
models)
in
potential
crop
water
requirement
(ETC),
growing
stage
days
(GSD),
effec-
tive
rainfall
during
Boro
growing
season
(ER),
potential
irrigation
requirement
for
crop
evapotranspiration
(ETC−
ER),
percolation
loss
(PL)
and
net
irrigation
requirement
(ETC−
ER
+
PL
+
N&LP)
are
reported
in
Table
2.
For
both
time
periods
and
RCPs,
the
decrease
in ETCand
growing
stage
days
were
lowest
for
Bogra
and
Rajshahi,
and
highest
for
Dinajpur
and
Pabna;
which
is
in
line
with
the
assessment
of
changes
over
the
last
three
decades
(Acharjee
et
al.,
2017).
4.
Discussion
4.1.
Future
climate
change
impacts
on
reference
crop
evapotranspiration
The
analysis
of
future
trends
indicates
a
considerable
increase
in
maximum
and
minimum
temperatures
and
sunshine
hours
during
the
dry
winter
season.
The
study
by
Rajib
et
al.
(2011)
indi-
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
179
Fig.
8.
Consequences
of
climate
change
on
water
requirement
components
for
Boro
rice
in
North-West
Bangladesh.
The
“”
sign
indicates
an
increase
and
“”
indicates
a
decrease
of
the
parameter.
A
“+”
sign
indicates
that
an
increase
in
a
parameter
will
contribute
to
an
increase
in
the
linked
next
parameter,
or
a
decrease
will
contribute
to
decrease.
A
“−”
sign
indicates
that
an
increase
in
a
parameter
will
contribute
to
decrease
in
the
linked
next
parameter,
or
a
decrease
will
contribute
to
increase.
Table
2
The
percentage
change
in
different
components
for
average
of
five
climate
models.
Scenarios
Comparison
from
base
period
District
Changes
in
different
parameters,
%
ETCGSD
ER
(ETC−
ER)
PL
Net
irrigation
RCP
4.5 by
2050s Bogra
−0.34
−9.06
−24.85
20.05
−6.80
2.03
Rajshahi
−7.30
−12.70
−50.89
15.10
−11.32
0.09
Pabna
−8.21
−13.48
−46.38
15.52
−11.86
1.63
Dinajpur
−10.25
−14.12
−36.30
28.03
−11.75
2.62
Average
−6.52
−12.34
−39.61
19.68
−10.43
1.59
by
2080s Bogra
−4.29
−14.24
−38.63
24.3
−8.39
3.13
Rajshahi
−8.07 −15.47
−43.80
10.23
−12.48
−2.66
Pabna
−10.45
−17.54
−45.20
11.05
−14.06
−4.00
Dinajpur
−10.40
−17.52
−57.32
62.29
−14.82
−0.27
Average
−8.30
−16.19
−46.24
26.97
−12.44
−0.95
RCP
8.5 by
2050s Bogra
−6.81
−13.67
−41.55
22.09
−10.21
−0.08
Rajshahi
−12.79
−15.47
−55.81
9.33
−13.37
−5.33
Pabna
−10.95
−15.51
−51.55
14.43
−13.34
−0.48
Dinajpur
−13.09
−16.73
−60.20
60.03
−13.76
−0.51
Average
−10.91
−15.35
−52.28
26.47
−12.67
−1.60
by
2080s Bogra
−14.70
−22.45
−65.56
27.62
−18.18
−5.46
Rajshahi
−16.67
−21.46
−53.30
2.13
−18.0
−10.35
Pabna
−18.79
−23.33
−60.54
7.45
−19.97
−8.24
Dinajpur
−20.25
−26.14
−73.61
63.56
−21.31
−5.64
Average
−17.60
−23.35
−63.25
25.19
−19.36
−7.42
cates
that
the
dry
winter
months
will
show
steeper
increases
in
temperature
than
the
monsoon
and
pre-monsoon
months
in
the
future.
Increase
in
temperature
will
lead
to
an
increase
in
reference
crop
evapotranspiration
during
the
dry
months.
Increased
poten-
tial
evapotranspiration
could
potentially
exceed
precipitation
and
resulting
in
more
intense
droughts
in
the
North-Western
region
(Islam
et
al.,
2017).
A
study
by
Faisal
and
Parveen
(2004)
indicates
an
increase
in
theoretical
evapotranspiration
requirement
for
rice
and
wheat
in
2050.
According
to
Kirby
et
al.
(2016),
climate
change
is
estimated
to
have
a
larger
impact
on
evapotranspiration
and
runoff
in
Bangladesh
than
irrigation
development.
The
study
by
Acharjee
et
al.
(2017)
showed
a
declining
recent
trend
of
EToover
the
last
three
decades
in
North-West
Bangladesh
using
historical
observed
climate
data.
Increases
in
humidity,
decreasing
wind
speed
and
reduced
sunshine
hours
resulted
in
a
decrease
in
ETo,
despite
an
increase
in
maximum
and
minimum
temperatures.
This
study
explored
possible
future
changes
in
ETo
and
water
requirements
of
Boro
rice
using
downscaled
and
bias
cor-
rected
future
climate
data.
For
both
RCP
4.5
and
8.5,
the
changes
in
minimum
and
maximum
temperatures
mainly
increased
the
daily
ETo.
The
future
changes
in
wind
speed,
humidity
and
sunshine
hours
were
not
strong
enough
to
reduce
the
EToas
observed
over
the
recent
decades
in
the
study
of
Acharjee
et
al.
(2017).
Since,
we
did
not
obtain
humidity
data
from
the
GCMs,
we
estimated
relative
humidity
based
on
future
temperature
data.
This
method
of
humid-
ity
estimation
assumes
that
the
dew
temperatures
are
equal
to
minimum
temperatures.
However,
if
wind
speed,
humidity
and/or
sunshine
hours
follow
similar
trends
as
those
observed
in
recent
decades,
the
decrease
in
water
requirement
in
the
future
would
be
somewhat
more
than
the
estimated
values
in
this
study.
4.2.
Future
climate
change
impacts
on
potential
crop
water
requirement
Our
results
show
that,
for
an
average
over
five
climate
models
and
four
North-West
districts,
potential
crop
water
requirements
will
reduce
by
6.5%
and
10.9%
for
RCP
4.5
and
8.5,
respectively
for
the
2050s;
and
by
8.3%
and
17.6%
for
RCP
4.5
and
8.5,
respec-
tively
for
the
2080s
(Table
2).
A
study
by
Shahid
(2010)
indicates
an
increase
in
daily
water
use,
but
not
any
appreciable
change
in
total
irrigation
requirements
of
Boro
rice
due
to
a
reduction
of
the
irriga-
tion
period
by
approximately
13
days.
Our
results
indicate
that
the
shortening
of
growth
stages
could
be
sufficient
to
counteract
and
ultimately
decrease
the
crop
water
requirement
despite
an
increase
of
daily
ETo.
The
shorter
duration
of
growing
days
of
crops
will
pro-
vide
scope
to
increase
the
cropping
intensity
by
growing
more
crops
180
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
each
year
on
a
single
piece
of
land.
As
the
daily
ETowill
increase
in
the
future,
the
agricultural
water
demand
during
the
dry
season
still
may
increase
if
farmers
take
advantage
of
the
shorter
Boro
sea-
son
to
cultivate
an
increased
number
of
crops.
This
can
cause
an
increase
in
the
yearly
water
demand
for
crop
agriculture.
Such
an
increase
in
water
requirement
is
mainly
caused
by
crop
phenolog-
ical
responses
to
climate
change
and
related
cultivation
changes
in
the
farming
systems.
However,
if
farmers
keep
cultivating
the
same
number
of
crops
in
a
year,
the
total
water
requirement
for
crop
cul-
tivation
will
reduce
in
the
future,
because
the
potential
crop
water
requirements
of
Boro
rice
will
decrease.
Therefore,
the
future
agri-
cultural
management
decisions
regarding
the
number
of
crops,
i.e.
the
cropping
intensity,
are
very
important
for
the
water
resources
management
of
the
region.
4.3.
Future
climate
change
impacts
on
irrigation
requirement
This
study
clearly
reveals
an
increase
in
reference
crop
evapo-
transpiration
and
a
decrease
in
potential
crop
water
requirement
due
to
future
climate
change.
The
exact
amount
by
which
potential
irrigation
requirements
for
crop
evapotranspiration
and
net
irri-
gation
requirement
will
change,
is
still
uncertain
due
to
a
large
variability
in
projected
rainfall
amounts
and
distribution.
However,
all
model
outputs
indicate
changes
of
irrigation
requirements
in
the
same
direction
supporting
the
reliability
of
our
findings
in
terms
of
direction
of
future
changes.
Our
results
indicate
an
increase
in
the
amount
of
irrigation
water
required
to
satisfy
crop
evapotran-
spiration
because
of
reduced
rainfall
availability,
but
a
decrease
of
net
irrigation
requirement
(except
for
RCP
4.5
in
2050s)
because
of
lower
percolation
due
to
shorter
growing
period.
A
study
by
Karim
et
al.
(2012)
also
indicated
a
lower
demand
of
irrigation
water
in
the
Northern
region
of
Bangladesh
in
the
future
with
a
13%
increase
in
projected
soil
moisture.
According
to
Faisal
and
Parveen
(2004),
if
the
water
availability
for
crop
production
remains
unchanged
in
the
dry
season,
then
there
should
be
enough
water
to
meet
the
irrigation
requirements
in
2030
and
2050.
A
non-significant
increasing
trend
of
pre-monsoon
rainfall
and
a
reducing
trend
in
winter
rainfall
were
found
in
Bangladesh
during
1969–2003
(Shahid,
2009).
The
increase
in
pre-monsoon
rainfall
will
reduce
irrigation
requirement
during
flowering
and
maturing
stages
of
Boro
rice,
while
the
decrease
in
winter
rainfall
will
increase
the
irrigation
requirements
during
initial
and
development
stages
of
Boro
rice.
The
estimation
of
irrigation
requirement
using
Crop-
Wat
by
Shrestha
et
al.
(2013)
for
Nepal
also
indicated
variations
in
irrigation
requirement
in
the
projected
time
windows,
with
peak-
ing
values
at
the
early
stages
and
decreasing
in
the
mid
or
later
stages.
Using
the
CropWat
model
for
tropical
paddy
in
Malaysia,
Tukimat
et
al.
(2017)
showed
that
climate
change
will
reduce
irri-
gation
demand
by
0.9%
per
decade.
The
main
challenge
of
climate
change
however
will
be
to
manage
the
increased
variability
in
future
irrigation
water
demand.
Tukimat
et
al.
(2017)
also
indi-
cated
that
an
increased
rainfall
will
exceed
the
evapotranspiration
loss
and
lead
to
reduced
irrigation
demand.
Our
study,
however,
shows
that,
a
reduction
in
growing
stage
days
will
mainly
lead
to
a
reduction
of
total
crop
water
demand
and,
depending
on
changes
in
rainfall,
the
net
irrigation
demand
can
reduce
or
increase.
Both
studies
indicate
that
the
major
challenge
in
water
resources
man-
agement
will
be
to
manage
the
uncertainty.
Future
reductions
in
net
irrigation
requirement
indicate
that,
if
the
groundwater
irrigated
area
or
the
number
of
crops
do
not
further
increase
in
the
future,
Boro
rice
cultivation
in
the
area
will
not
put
additional
pressures
on
the
groundwater
resources.
A
study
by
Kirby
et
al.
(2015)
indicated
that
the
rate
of
decline
in
groundwater
tables
would
reduce
and
could
even
attain
a
new
equilibrium
assuming
no
further
increase
in
the
groundwater
irrigated
area.
According
to
Mainuddin
et
al.
(2015),
the
impact
of
climate
change
on
the
irrigation
requirements
of
dry
season
Boro
rice
is
small,
and
projected
to
increase
by
a
max-
imum
of
3%
for
2050.
Our
study
also
confirms
a
small
impact
(an
increase
of
1.59%
for
RCP
4.5
and
a
decrease
of
1.6%
for
RCP
8.5
sce-
nario
by
2050s)
on
net
irrigation
requirement
but
larger
impacts
or
changes
on
individual
components
such
as
ETo,ETC,
effective
rainfall
and ETC−ER.
However,
as
those
higher
individual
com-
ponent
changes
or
impacts
compensate
for
each
other
(Fig.
8)
the
overall
combined
effect
is
a
relatively
small
change
in
net
irrigation
requirements.
4.4.
Implications,
limitations
and
scope
for
future
research
Some
future
agricultural
water
management
measures,
e.g.
the
preparation
of
a
suitable
crop
calendar,
identification
of
opti-
mum
planting
date,
etc.
can
be
planned
based
on
the
estimates
of
reference
crop
evapotranspiration
and
potential
crop
water
requirement.
Anticipatory
planning
of
local
and
national
level
water
management
to
adapt
to
climate
change
should
include
flex-
ible
measures
related
to
irrigation
demand
management
to
deal
with
future
uncertainties.
Therefore,
for
long-term
future
water
management
planning,
development
of
adaptation
strategies
or
pathways
in
relation
to
various
possible
rainfall
conditions
and
related
changes
in
net
irrigation
requirement
could
be
effective
to
cope
with
future
climate
change.
In
particular
the
effect
of
potential
shifts
in
growing
season,
and
intensification
of
cropping
patterns,
as
facilitated
by
projected
reductions
in
crop
growth
periods,
needs
further
attention.
Estimates
of
different
water
requirement
com-
ponents
(i.e.
ETO,ETC,ETC−
ER
and
Net
irrigation)
can
better
reflect
on
future
possible
consequences
of
climate
change,
instead
of
an
overall
estimate
(i.e.
Net
or
Gross
irrigation
requirement)
and
therefore,
could
be
more
suitable
for
adaptation
planning
to
deal
with
an
uncertain
future.
Although
climate
change
could
reduce
the
water
requirements
of
rice,
it
may
negatively
affect
yields.
According
to
Karim
et
al.
(2012),
the
yield
of
their
studied
rice
cultivar
will
be
hampered
by
33%
later
this
century
in
Bangladesh.
Development
of
suitable
rice
varieties
could
be
helpful
in
this
regard.
If
farmers
identify
and
use
any
late-maturing
cultivars
of
rice
that
ensure
higher
production
under
the
future
climate
change,
the
net
irrigation
requirement
may
increase
as
the
duration
of
growth
period
will
increase.
The
water
requirements
at
actual
field
condition
may
differ
from
our
estimated
values
because
of
high
diversity
and
com-
plexity
of
soils
in
Bangladesh,
including
differences
between
the
physiographic
regions,
within
the
soil
topo-sequences,
between
and
within
the
neighbouring
fields,
and
in
areas
of
shifting
cul-
tivation
(Brammer
and
Nachtergaele,
2015).
Also,
the
actual
field
level
estimation
may
vary
because
of
changes
in
crop
variety,
water
application
methods,
soil
management,
and
crop
density.
As
the
physical
geography
of
Bangladesh
is
very
diverse
(Brammer,
2016a)
impacts
of
climate
change
are
not
uniform
throughout
the
country
and
distribution
of
rainfall
could
differently
affect
different
regions,
some
finer
scale
studies
to
understand
the
changes
in
future
water
requirements
can
be
helpful
for
better
agricultural
diversification
and
water
management
planning.
In
addition
to
the
climatic
variables,
the
physiological
response
of
the
crop
to
the
increased
concentration
of
CO2also
plays
an
important
role
in
the
evapotranspiration
estimation.
However,
this
has
not
been
considered
in
this
study
and
needs
further
research.
Elevated
CO2concentration
decreases
evapotranspiration
and
increases
water
use
efficiency
(Baker
and
Allen,
1993;
Morison,
1985).
Therefore,
consideration
of
elevated
CO2concentration
in
the
atmosphere
is
probably
not
contradictory,
but
will
strengthen
the
final
conclusion
of
this
study.
As
both
the
rainfall
amount
and
distribution
are
important
for
irrigation
management,
and
it
is
very
difficult
to
exactly
predict
the
future
rainfall
amounts
and
distribution,
a
close
monitoring
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
181
and
evaluation
are
needed
to
identify
the
direction
of
changes
in
rainfall.
Based
on
that,
it
would
be
possible
to
take
anticipatory
decision
related
to
agricultural
water
management
to
cope
with
climate
change.
Therefore,
this
study
further
recommends
to
set
up
some
good
quality
meteorological
stations
in
Bangladesh
(if
possi-
ble,
near
crop
fields)
for
close
monitoring
and
regular
evaluation
of
changes
in
climatic
parameters.
Brammer
(2016b)
also
recom-
mended
to
install
more
meteorological
stations,
especially
in
rural
areas
to
recognize
climate
change
with
greater
certainty.
The
study
by
Hossain
et
al.
(2016)
recommends
a
proper
drought
early
warn-
ing
system
in
the
North-West
Bangladesh,
which
would
also
be
possible
by
a
close
monitoring
and
evaluation
of
changes
in
climatic
parameters.
5.
Conclusion
Climate
change
will
lead
to
an
increase
in
reference
crop
evap-
otranspiration,
but
will
not
increase
the
overall
water
requirement
of
Boro
rice,
because
of
crop
phenological
response
to
increased
temperature
that
reduces
the
number
of
growing
days.
All
model
estimates
showed
a
decreased
future
potential
crop
water
require-
ment
(ETC)
of
Boro
rice
for
all
study
districts.
For
RCP
8.5
scenario
by
2050s,
the
potential
crop
water
requirements
will
reduce
by
10.9%
and
net
irrigation
requirement
will
reduce
by
1.6%
compared
to
the
base
period
(1980–2013).
The
variations
in
the
amount
of
decrease
in ETCfor
different
models
are
low,
compared
to
the
variations
in
estimated
net
irrigation
requirements.
Large
differ-
ences
between
climate
models
in
estimates
of
future
rainfall
result
in
high
variations
in
estimated
future
water
requirements.
High
variations
in
projected
rainfall
potentially
limits
the
anticipatory
irrigation
and
water
management
planning.
Therefore,
close
mon-
itoring
and
evaluation
to
detect
the
direction
of
future
change
and,
thereafter,
development
of
flexible
adaptation
strategies
are
essen-
tial
to
deal
with
climate
change;
so
that
prompt
decisions
can
be
taken
when
the
future
becomes
more
visible.
Acknowledgement
The
authors
sincerely
acknowledge
the
Nuffic
NICHE-BGD-
155
project
for
granting
the
fellowship
to
Tapos
Kumar
Acharjee
for
his
PhD
study
at
Wageningen
University
and
Research,
the
Netherlands.
Appendix
A.
Table
A1
Mann-Kendall
trends
of
estimated
reference
crop
evapotranspiration
during
dry
months
for
2035–2065
and
2065–2095
time
series
in
Bogra.
Trends
during Month
CNRM-CM5
EC-Earth
HadGEM2-ES
IPSL-CM5A-LR
MPI-ESM-LR
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
2035–2065 Jan
2.11*
0.48
2.11*
0.51
2.48*
3.06**
−1.73+1.39
1.39
1.67+
Feb
1.05
−0.27
1.94+0.65
2.99**
4.22***
1.87+1.56
1.70+2.07*
Mar
1.02
−1.16
1.02
0.54
2.89**
2.96**
0.58
2.35*
0.41
1.73+
Apr
0.51
0.58
0.92
−0.54
1.12
2.18*
−1.43
1.56
2.52*
2.14*
Nov
0.34
−0.24
1.29
1.36
2.75**
2.35*
0.54
0.48
1.67+1.87+
Dec
1.87+−1.12
1.50
0.48
3.20**
2.35*
−0.03
1.05
−0.88
0.88
2065–2095 Jan
−0.24
1.80+1.70+1.73+−0.14
3.88***
0.82
0.07
1.16
2.52*
Feb
−1.43
1.05
0.92
1.29
1.26
3.50***
0.54
0.78
0.37
1.80+
Mar
−0.20
−0.75
1.36
−0.10
1.36
3.13**
0.48
1.29
−0.68
1.94+
Apr
0.61
0.07
1.26
0.68
1.94+1.26
0.58
1.50
−0.31
2.62**
Nov
0.20
1.33
2.72**
0.85
1.56
4.01***
1.97*
1.16
1.53
0.71
Dec
0.20
3.03**
1.56
1.09
2.11*
4.05***
0.27
0.48
0.99
1.39
+,
*,
**
and
***
signs
indicate
significant
at
0.10,
0.05,
0.01
and
0.001
level
of
significance,
respectively.
Table
A2
Mann-Kendall
trend
of
estimated
reference
crop
evapotranspiration
during
dry
months
for
2035–2065
and
2065–2095
time
series
in
Pabna.
Trends
during
Month
CNRM-CM5
EC-Earth
HadGEM2-ES
IPSL-CM5A-LR
MPI-ESM-LR
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
2035–2065 Jan
2.21*
0.14
1.87+0.34
2.69**
2.79**
−1.36
1.19
1.84+1.60
Feb
0.92
−0.68
1.53
0.17
3.26**
3.50***
2.18*
1.53
1.97*
1.77+
Mar
0.95
−1.33
0.75
0.71
2.18*
2.24*
0.99
2.79**
0.95
0.88
Apr
0.48
0.41
0.78
−0.61
1.26
2.65**
−1.67+1.56
2.35*
2.48*
Nov
0.24
−0.54
1.19
0.82
2.65**
1.56
0.88
1.05
1.56
1.90+
Dec
1.77+−1.36
1.50
0.17
2.92**
2.96**
−0.07
1.36
−0.92
0.75
2065–2095 Jan
−0.65
1.53
1.67+1.33
1.36
4.11***
0.78
0.27
1.19
2.14*
Feb
−1.56
0.61
0.07
1.63
0.61
3.50***
0.37
0.54
0.24
1.53
Mar
−0.54
−0.92
1.50
−0.03
0.92
2.96**
0.03
0.95
−1.02
1.56
Apr
0.34
0.00
1.12
0.58
1.84+1.43
0.03
1.12
−0.58
2.21*
Nov
0.03
1.12
2.62**
0.61
2.62**
4.66***
1.53
1.05
2.01*
0.92
Dec
0.03
3.03**
0.85
0.75
1.84+4.25***
−0.03
−0.17
1.22
1.16
+,
*,
**
and
***
signs
indicate
significant
at
0.10,
0.05,
0.01
and
0.001
level
of
significance,
respectively.
Table
A3
Mann-Kendall
trend
of
estimated
reference
crop
evapotranspiration
during
dry
months
for
2035–2065
and
2065–2095
time
series
in
Dinajpur.
Trends
during
Month
CNRM-CM5
EC-Earth
HadGEM2-ES
IPSL-CM5A-LR
MPI-ESM-LR
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
2035–2065 Jan
1.43
0.03
1.97*
0.00
2.75**
2.41*
−2.07*
1.12
0.65
0.58
Feb
1.43
−0.10
2.48*
0.75
1.19
2.62**
0.17
1.56
1.26
1.90+
182
T.K.
Acharjee
et
al.
/
Agricultural
Water
Management
194
(2017)
172–183
Table
A3
(Continued)
Trends
during
Month
CNRM-CM5
EC-Earth
HadGEM2-ES
IPSL-CM5A-LR
MPI-ESM-LR
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
RCP
4.5
RCP
8.5
Mar
1.12
−1.19
1.09
0.27
2.07*
2.72**
0.88
1.67+0.44
1.29
Apr
0.14
0.65
0.61
−0.92
1.53
2.79**
−1.67+1.39
2.41*
2.18*
Nov
2.18*
0.61
0.78
2.35*
3.43***
1.73+0.14
0.14
1.77+2.21
Dec
1.87+−0.71 1.26
0.37
3.26**
2.21*
−0.14
0.65
−0.07
0.44
2065–2095 Jan
0.14
3.20**
1.39
1.84+−0.17
2.14*
0.78
−0.51
1.22
3.20**
Feb
−1.12
1.05
1.29
2.01*
1.26
3.09**
0.41
0.58
−0.14
1.46
Mar
0.14
−0.71
1.22
−0.68
1.39
2.48*
0.61
0.54
−0.78
2.07*
Apr
0.58
−0.31
0.71
0.82
1.73+1.63
0.75
1.53
−0.17
2.72**
Nov
0.00
3.50***
2.45*
0.58
1.09
2.79**
2.48*
1.12
0.44
1.63
Dec
1.09
3.50***
0.75
0.92
1.36
3.81***
0.03
1.09
0.61
2.62**
+,
*,
**
and
***
signs
indicate
significant
at
0.10,
0.05,
0.01
and
0.001
level
of
significance,
respectively.
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