Quantifying Drought Tolerance in Root Crops- The FAO AquaCrops Model's Perspective on Jamaican Sweet Potatoes, Ipomoea batatas

1. Quantifying Drought Tolerance in Root Crops- The FAO
AquaCrop Model’s Perspective on Jamaican Sweet potatoes,
Ipomoea batatas
Collaborators:
Dale Rankine, Jane Cohen, Michael Taylor, Andre Coy and Tannecia Stephenson
September 14, 2016
JaREEACH Climate Smart Agriculture Symposium

2. Contents
1. Introduction
 Sweet Potato- An Important Root Crop
The Challenges with Modelling Crop Yields
2. Methodology
3. Results
4. Conclusions
5. Next Steps
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3.  Sweet potato, Ipomoea batatas (L.), is a 5-month root crop, a dicotyledonous herbaceous
trailing vine and the only economically important member of the family Convolvulaceae.
 The crop is the 6th most important globally and is propagated from cuttings sown in the
Caribbean during the period September to December.
 This period is coincident with the late rainfall season in Jamaica. The crop rainfall requirement
is 750-1250 mm; of this, about 500 mm should occur during the first third of the crop life
 Sweet potato is drought tolerant but…
 Crop most sensitive to dry conditions at the tuber initiation stage (about 40-50 days after
planting) but requires less water as it nears maturity (CARDI 2010; Stathers et al. 2013).
 Central to pursuit of reducing imports of, and reliance on externally grown wheat and cereals
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1. Introduction- Sweet Potato An Important Root Crop
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4. 1. Introduction- Challenges with Modelling Crop Yields
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Annual drought index (SPI-12) versus mean annual Sweet potato yields
(detrended) for Jamaica 1961-2009)
Source: FAOSTAT; Climate Studies Group, Mona
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1961
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1975
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1981
1983
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1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
Yield(t/ha)
SPI-12
SPI-12 Detrended Yield
Annual drought index (SPI-12) versus mean annual Sweet potato
yields for Jamaica (1961-2009)
Source: FAOSTAT; Climate Studies Group, Mona
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1995
1997
2000
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Yield(t/ha)
SPI
SPI-12 Yield
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Wavelet analysis (Cazelles et al. (2007), of area harvested data (1961-2009) suggests strongest periodic signal at
2-6 years (1985-2010) The high value of the power curve also indicates that this is a statistically significant
cyclical pattern.
Source: Climate Studies Group Mona (CSGM)
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1. Introduction- Challenges with Modelling Crop Yields

6. 2. Methodology: Research Design Summarised
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AquaCrop Model Explained
•Biomass=WP x ΣTr [Biomass]
•ET=E +Tr
•WP normalised for ET and CO2
• Y=B x HI [Yield]
•Robust, Accurate yet simple
Devon
Ebony
Park
Passley
GardensBodles
*
* *
* On site weather
station
Randomised Complete Blocks
(RCBs)
Parameters:
•Rainfall
•Temperature
•Relative
Humidity
•Solar Radiation
•Wind
•ETo
Canopy cover, Biomass (above & Below)
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7. 2. Methodology- An Analogue Approach for Climate Change Data
The method: Attempts to produce data representative of extremes climate such as could occur
under climate change in the absence of long-terms records
• Uses data from an existing station (NMIA) for a specified period (1996-2013)
• Ranks growing season (Sept 1- Dec 31) temperature and rainfall into quartiles
• Selects a baseline (the median for both temperature and rainfall) using an ensemble average
of three years (2000, 2001, 2012)
• Two alternative future climates were chosen from the extremes of the ranked data (relative to
baseline): Warm and Dry (2006); and Cool and Wet (2010).
• Simulations were done for Rainfed and Irrigated cultivation
• The irrigation schedule was generated in AquaCrop and based on a maximum allowable
depletion of readily available water (RAW) of 50% and an irrigation depth of 15 mm.
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8. 2.Methodology-Analogue
Scenario
(year)
Total Seasonal
(Sept 1-Dec 31)
Rainfall (mm)
Crop Season Mean
Temperature (°C)
Comparison
relative to
Baseline
Baseline
(Mean of
2000, 2001, &
2012)
453.8 ± 46.2 28.4 ± 0.1
Warm and
Dry (2006) 116.9 29.1
Rainfall: -74%
Temperature:
+0.7°C
Cool and Wet
(2010)
800.8 27.3
Rainfall :
+77%
Temperature:
-1.0°C
(a) Rainfall and (b) Temperature for
NMIA(1996-2013)
Baseline and alternative climates
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a
b

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Varietal differences:
•Colour (Flesh and skin)
•Texture
• Foliage
Source: CARDI 2010
2. Methodology: Five Varieties of Sweet potato
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10. 3. Results: Drought Tolerance in Devon, Manchester
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Ideal growing conditions
favours higher yields
(both treatments) relative
to other zones
• Temperature, (warm
days, cool nights) ,
• Rainfall (above 1200
mm),
• Elevation (high),
• Soil: well drained
Chudliegh Soil
Rainfed vs Irrigated Yields (t/ha) 2013
Variety Rainfed Irrigated
Ganja 12.07 ± 5.36 22.48 ± 0.14
Uplifta 15.84 ± 4.57 8.39 ± 0.14
Yellow Belly 21.26 ± 4.57 5.0 ± 0.14
Varietal Mean 16.70 ± 2.80 11.96 ± 0.08
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11. 3. Results: Drought Tolerance in Ebony Park and Portland
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Passely Gardens, Portland- 2011/2012
Irrigated Yield
(t ha-1)
Rain-fed Yields
(t ha-1)
Variety 2012 2012
Ganja 9.14 ± 1.47 4.50 ± 0.22
Uplifta 4.33 ± 1.14 6.28 ± 3.22
Yellow Belly 8.53 ± 1.14 2.26 ± 0.87
Fire on Land 8.92 ± 1.14 5.92 ± 1.49
Clarendon 6.47 ± 1.14 3.35 ± 0.76
Varietal
Mean 7.48 ± 0.55 4.60 ± 0.95
Ebony Park, Clarendon-2013 Yields (t/ha-1)
Variety Irrigated (t/ha) Rain-fed ( t ha-1)
Ganja 31.49 ± 0.08 9.70 ± 0.08
Uplifta 10.33 ± 0.12 5.73 ± 0.09
Yellow Belly 18.31 ± 0.09 5.04 ± 0.09
Varietal Mean 21.11 ± 0.06 6.74 ± 0.05
•Waterlogging, high rainfall inhibits growth in Portland
•Low rainfall thwarts growth in at Ebony, in rain-fed
production; high benefits from irrigation
•Highest overall yield recorded in irrigated Ganja at
Ebony Park
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12. 3. Results: Overall Assessment of Drought Tolerance
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Irrigation
Rainfall
High Medium Low
High
Ganja/
Fire on
Land
Ganja / Fire
on Land
Clarendon /
Ganja
Medium Ganja Uplifta Ganja
Low Ganja
Yellow
Belly/ Ganja Ganja
•The Ganja Variety appears the most drought
tolerant and ‘adaptable’ variety
•Uplifta variety does well under medium (750-
1250 mm) water availability
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13. 3. Results: AquaCrop a Model to widely Test Drought Tolerance
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CanopyCover(%)
DAP
Devon, Manchester: Rain-fed (2013)
Simulated Measured
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Biomass(t/ha)
DAP
Simulated Measured
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40
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100
0 10 20 30 40 50 60 70 80 90 100
CanopyCover(%)
DAP
Ebony Park, Clarendon-Irrg. (2013)
Simulated Measured
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0 10 20 30 40 50 60 70 80 90 100
Biomass(t/ha)
DAP
Simulated Measured
• Parameterization of Sweet potato
in AquaCrop- Original contribution
• Excellent agreement between
simulated and measured canopy
cover (CC)
• Model exhibits good skill in the
simulation of biomass at both
locations and for the two
treatments
• When CC is well simulated, so
also is Biomass. All based on
dry weights
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14. 3. Results: Validation of AquaCrop- Model Performance Summarised
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•Deviation = {(Simulated- Measured)/Measured} *100
•Biomass estimation within 30% of ‘actual’ values for 4 of 6 simulations (further refined)
• Yields: Deviation for 3 out of 6 treatments < 40 % (highest for rain-fed)
Final (Total) Biomass (t/ha) Tuber Yield (t/ha)
Year Treatment Measured Simulated Deviation Measured Simulated Deviation
Devon
2012
Irrigated 14.5 ± 6.3 10.3 -29.2 11.2 ± 5.7 5.2 -53.1
Rain-fed 28.4 ± 18.5 7.7 -72.7 20.8 ± 14.9 3.9 -81.1
2013
Irrigated 16.9 ± 5.7 13.5 -20.1 6.3 ± 4.9 6.7 7.5
Rain-fed 17.8 ± 2.4 13.5 -24.2 6.7 ± 2.0 6.7 0.24
Ebony Park
2013
Irrigated 27.8 ± 2.8 11.2 -51.8 11.0 ± 5.8 6.7 -39.5
Rain-fed 14.6 ± 4.2 10.6 -8.6 2.6 ± 1.4 7.2 158.6
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15. 3. Results: Testing Drought Tolerance under Climate Change
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PERCENTAGE(%)CHANGE
SCENARIO/RCP
Yield Changes (%) Warm and Dry Climate
Rainfed Irrigated
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PERCENTAGE(%)CHANGE
SCENARIO/RCP
Yield (t/ha) changes (%)-Cool & Wet Climate
Rainfed Irrigated
• Warmer and drier conditions resulted in earlier maturity, declines in biomass and yield while cooler and wetter
conditions favoured production, but suggested longer maturity period.
• Elevated CO2 (under A2, B2 SRES and RCP 2.6, 4.5, 6.0 and 8.5), had a net benefit for both yield in both the
warm and dry; and cool and wet climates.
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16. 4. Conclusions
 Agroecology: sweet potato production is affected by agroecology but the crop is versatile and
adaptations and could be pursued as one to reduce reliance on imported wheats and ceareals.
 Drought Tolerance: deep roots and extensive trailing vines are among the properties that make
sweet potato tolerant to drought, but yields are considerably reduced by dry conditions (especially up
to 42 DAP)
 Parameterization: of root and tuber crops in AquaCrop is challenging (perhaps more so than for
other types of crops). Experience allows for much wider application
 Model Performance:
 Fairly accurate prediction of sweet potato crop growth under both rain-fed and irrigated conditions.
 Canopy cover was reasonably well simulated by the model but some divergence was noted for biomass
and yield.
 The overall simulation of biomass was good with deviations of less than 30% for four out of six
simulations and season-long performance of the model was commendable
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17. • Warm and dry (cool and wet) conditions were found to be least (most) favourable to
future production of sweet potato, but overwatering was also found to be
counterproductive.
• The results suggest that elevated CO2 benefits future production with yield increases
ranging to a high of over 20%.
• The benefits however seem to taper off as 2050 is approached.
• Reduced stomatal conductance seem to contribute to a reduction in transpiration and
coupled with the increased biomass and yield gave significantly higher (maximum of
89%) water use efficiency.
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4. Conclusions

18. 5. Next Steps
 Further refinements of model parameters to increase accuracy of predictions;
 Testing in other environments: soil, climates, other scenarios of water limitation;
 Downscaling of climate model outputs using weather generator to provide multiple
scenarios of future climates;
 Expansion to other important crops;
 Training and capacity building;
 Routine incorporation of crop modelling into operations of agriculture sector.
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19. THANK YOU
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