Potential effects of expanding
irrigation on crop production
under climate change
Masashi OKADANIES, Japan

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.

CROVER model
Climate DataCrop Growth andYield Formation

Crop Growth Simulation
with considering
Cultivation Managements

Water Resource(Temp, Precip., Solar Rad.,
Humidity, Wind Speed)
Precipitation

PRYSBI-2

Evapotranspiration

H08

Natural Water
Circulation and Human
Activities (Industrialand Domestic Waters,
Dam Operations etc.）

Irrigataion
River flow
RunoffReservoir

Irrigation Process

Considering ofEquilibrium with Crop
Water Demand and Supply

Withdrawal

Okada et al. [2015] JAMES The coupled model CROVER was created by embedding the PRYSBI-2 large-area cropmodel into the global water resources model H08.

PRYSBI-2 large-area crop model
PRYSBI2

 PRYSBI-2 has 3 major components:(1) Crop growth was based on
basic process-based models,(2) Land-surface hydrologicaldynamics (SWAT) and(3) Soil carbon dynamics (RothC).
 PRYSBI-2 can address 5 crops(maize, rice, soybean, spring wheat
and winter wheat)

RothC

Sakurai et al. [2014] Sci. Rep.

 The parameter values were calibrated by using a Bayesian method for eachcrop in individual grid cells.
 the spatial uncertainties of the model parameters was considered byestimating the posterior distribution of the parameters from thehistorical agricultural data.

H08 global hydrological model H08 model can simulate both natural andanthropogenic water flow globally on a daily basis.
 H08 has 6 components:(1) Land surface hydrology,(2) River routing,(3) Crop growth,(4) Reservoir operation,(5) Environmental flow requirement estimation and(6) Anthropogenic water withdrawal.

Humanactivities

452 large reservoirs （4,140 km3）
Hanasaki et al. [2006] JH
Hanasaki et al. [2008a,b] HESSNaturalwater cycle

CROVER modelPrecipitation

PRYSBI-2Evapotranspiration

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 water
withdrawal.
 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
 It simulates the large-scale terrestrial hydrological cycle and crop growth depending on
climate, soil properties, landuse, crop cultivation management, socio-economic water
demand, and reservoir operation management.

Simulation setting
 This study applied three types of future scenarios (climate change, irrigation
area expansion and fertilizer scenarios) to the CROVER model.
 The simulation operated at a 1.125-degree resolution and on a daily time step.
Climate change scenario

Fertilizer scenario

Precipitation

PRYSBI-2Evapotranspiration

H08

Irrigataion
River flow
RunoffReservoirIrrigation area expansion scenario
Withdrawal

Adaptation effectdue to irrigated area
expanding
Yield

Irrigation
adaptation
2000

2050

2100

Irrigation area expansion scenario•

The historical data was derived from HID product [Siebert et al. [2015] HESS].

•The future irrigated area was extrapolated from historical irrigation pattern.

•However, the future expansion of irrigated area was stayed when the sum of theirrigated and rainfed areas was greater than the current harvested area (MIRCA2000[Portmann et al. [2010] GBC).

Simulated yield and irrigation adaptation effect（Rice）
 Yield change： At the end of this century, the yield was projected to decrease on themost regions.
 Irrigation adaptation effect： Although the yield decrease was effectively improved by
the irrigation adaptation in Southeast and South Asia, the expanded irrigated areaslightly worsened the yield in East Asia.

Change in irrigated water and crop production
with the expanding irrigated area（Rice）
 Irrigated water was projected to decrease at the north part of China, although theirrigated area expands.
 It brought decrease in rice production.

?

?
?

Change in irrigated water and crop production
with the expanding irrigated area（Maize）
 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.

The future increase in irrigation water demands in maize-growing areas will
decrease the irrigation water available in rice-growing areas.
→ irrigation expansion will lead to increased competition for water amongcrops and decreased rice production in China.

Conclusion• Our assessment reveals that expanding irrigation can be a usefuladaptation measure in some areas where a single or a few major crops
are grown under irrigated 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 among crops.
• These findings emphasize the importance of considering inter-cropwater-use competition and the associated changes in the water
availability when planning irrigation expansion to adapt to climate change.
• Our analysis demonstrates a complex water-food nexus using irrigation
expansion and crop production under a changing climate as an example.

Thank you for your attention

Change of global average temperature[℃]

Climate change scenario
108
6

GFDL-ESM2M
HadGEM2-ES

4

IPSL-CM5A-LR

2

MIROC-ESM-CHEM
NorESM1-M

0-2
1981

2001

2021

2041
Year

2061

2081

This study used the bias-corrected five GCMs outputs under the RCP 8.5 scenario.
• GFDL-ESM2M• HadGEM2-ES• IPSL-CM5A-LR• MIROC-ESM-CHEM• NorESM1-M

Fertilizer scenario
 The annual applied N fertilizer was assumed to depend on economic growth.
 The global relationship was derived from the country mean per capita GDP and the gridbased crop-specific N fertilizer circa 2000.

GDP per capita:
World bank, IMF, UN
N fertilizer:
Mueller et al. [2012] Nat.

Fertilizer scenario
 The effect of increase in the fertilizer on crop yield was parameterized as technicalcoefficient. In this study, we did not operate the soil carbon dynamics component.
 The parameter was previously assimilated from historical yield data with PRYSBI-2 notusing the fertilizer data.

N fertilizer:
Mueller et al. [2012] Nat.
Technical coefficient:
Model parameterFertilizer scenario
 Based on these relationships, the technical coefficient in the future was calculatedusing the future country mean per capita GDP data under SSP2 scenario [IIASA 2016],
via estimating the applied N fertilizer.

2100-2010 (SSP2)

Change of technical coefficientChange of river inflow with the expanding irrigated area
 With the expanding irrigated area, the available water for irrigation on the adjacent
lower grid cell projected to decrease.
 The substantial decrease was estimated at the north part of China.

With the expanding irrigated area, slightly worsened the yield in East Asia results from• the increase of irrigation water uptake on maize field with the significant expanding
irrigated area at the north part of China,• the increase of irrigation water uptake on the upstream area in each river watershed.

Simulated yield and irrigation adaptation effect（Maize）
Of course, the irrigation adaptation provides benefits to crop production.
 Yield change： At the end of this century, although the yield was projected to increase inAsia, the yield decrease in Europe.
 Irrigation adaptation effect： The yield decrease was effectively mitigated by theirrigation adaptation in Europe.

Change in irrigated water and crop production with theexpanding irrigated area（Maize）
 Irrigated water was projected to increase around France with the expanding irrigatedarea, brought increase in maize production.

Future distribution of irrigated area in major crops 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.