Varying benefits of irrigation expansion forcrop production under a changing climate
Masashi OKADA
National Institute for Environmental Studies

This research was carried with great aid of Dr. Hijioka (NIES),
Dr. Iizumi, Dr. Sakurai, Dr. Nishimori (NARO),
Dr. Sakamoto (PWRI), and Ms. Kotoku (Univ. Tsukuba)
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Background• Replacing rainfed cropping systems with irrigated systems has
been deemed an effective agricultural adaptation measure
under climate change.
• However, few agricultural impact assessments haveconsidered changes in the water availability because ofclimate change and water-use competition among crops.
• Here, we assessed future global crop production under achanging climate and expanded irrigation using the largescale crop-river coupled model CROVER.

2CROVER modelPrecipitation

PRYSBI-2

Evapotranspiration

H08

Irrigataion
River flow

RunoffReservoir

Withdrawal

Okada et al. [2015] JAMES➢ The CROVER model was developedby combining 3 components fromPRYSBI-2:(1) Crop growth,(2) Land surface hydrology,(3) Soil carbon dynamics
and 3 components from H08:
(1) River routing,(2) Reservoir operation,(3) Anthropogenic waterwithdrawal.
➢ It operates at a 1.125-degree
resolution at a daily time step.

A grid cell is separated into 11 land-use categories-5 crops (maize, rice, soybean, spring wheat, winter wheat)- 2 conditions (irrigated, rainfed)- 1 vegetation➢ Such the crop model that can simulate crop growth considering dynamic terrestrial
water cycling under changing climate and land use, and varying water availability are3
LPJml and CROVER only.

Methods• This study applied two types of future scenarios to the CROVER model.
• The study assessed the potential effects of expanding irrigation on crop production
within the context of climate change.
Climate change scenario [Iizumi et al. 2017]
Precipitation

PRYSBI-2

Evapotranspiration

H08

Irrigataion

River flow
RunoffReservoir

Irrigation area expansion scenario(developed by this study)

Withdrawal

Yield

Yield increase
due to irrigatedarea expanding
2000

2050

2100

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Case 1.
Expanding irrigation leads to increased production➢ Yield change： At the end of this century, the yield decrease in Europe.
➢ Irrigation adaptation effect： The yield decrease was effectively mitigated by theirrigation expansion in Europe.

Maize
RCP8.5Maize
RCP8.55Case 1.
Expanding irrigation leads to increased production➢ Irrigated water was projected to increase around France with the expanding irrigatedarea, brought increase in maize production.

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Case 1.
Expanding irrigation leads to increased production➢ Irrigated areas of the other crops than maize are little.
➢ Water use for irrigation on maize field hardly compete with those on the other crops.

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Case 2.
Expanding irrigation does not lead to increased production➢ Southeast and South Asia: the rice yield increased because of the irrigationexpansion.
➢ East Asia: the effect of expanding irrigation is negligibly small, even though theirrigated area expands.

RCP8.5Case 2.
Expanding irrigation does not lead to increased production➢ Irrigated water was projected to decrease at the north part of China, although theirrigated area expands.
➢ It brought decrease in rice production.

?

?
?

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Case 2.
Expanding irrigation does not lead to increased production➢ At the north part of China, the irrigated area of maize was estimated notable expansion.
➢ With the notable expanding irrigated area, the applied water to the maize field wasprojected to increase significantly.

In E Asia, irrigation is already extensively used for multiple crops.
Irrigation expansion will lead to increased competition for water among crops
and decreased rice production

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Case 3. Geographical patterns of projected precipitation
change influence the benefits of irrigation expansion• In this case study, we used a single GCM(GFDL-ESM2M) to simulate contrasting
precipitation changes between the higher
and lower areas of a watershed.
• The GCM shows that;• precipitation increases in the higher
watershed area,• precipitation decreases in the lower
watershed area.

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Case 3. Geographical patterns of projected precipitation
change influence the benefits of irrigation expansion
Rainfed: water deﬁcit is anticipated to increase due to the decreased
precipitation in the plain area.
Irrigated: water deﬁcit changes little because the increased precipitationin the mountainous areas of the watershed provides additional waterresources.
Change in probability of occurrence of soil water deficit
in the plain area over the Indus River watershed

12Conclusion• Our assessment using a crop-river coupled model reveals
three interesting cases;✓ that expanding irrigation can be a useful adaptation measure insome areas where a single or a few major crops are grown underirrigated conditions (e.g., Europe),✓ but that irrigation expansion produces only limited crop production
increases in areas where irrigation is already extensively used for
multiple crops (e.g., East Asia) due to competitive water use amongcrops.
✓ and that increased precipitation in the higher mountainous areas ofa watershed can reduce yield losses caused by precipitation
decreases in lower watershed areas (e.g., the Indus Riverwatershed).

• These ﬁndings emphasize the importance of consideringintercrop water use competition and the associated changes
in the water availability in addition to future precipitation
changes when planning irrigation expansion to adapt toclimate change.
13Thank you for your attention

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