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Perennial rhizomatous grasses as bioenergy feedstock in SWAT:
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parameter development and model improvement
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Running Head: SWAT improvements for bioenergy simulation
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Elizabeth M. Trybula1,2, Raj Cibin1, Jennifer L. Burks2, Indrajeet Chaubey*1,3, Sylvie M.
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Brouder2, Jeffrey J. Volenec2
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1
Purdue University Department of Agricultural and Biological Engineering
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2
Purdue University Department of Agronomy
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3
Purdue University Department of Earth, Atmospheric, and Planetary Sciences
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* Corresponding Author: Phone: (765) 494-3258, Fax: (765) 496-1115, Email:
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ichaubey@purdue.edu
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KEYWORDS
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Soil and Water Assessment Tool, hydrologic model, model parameterization, perennial
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rhizomatous grass, switchgrass, Miscanthus, bioenergy feedstock
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TYPE OF PAPER: Original Research
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SUPPLEMENTAL INFORMATION
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Figure S1. Harvest path (a) and plot quadrat before (b) and after (c) Miscanthus residue
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collection (2011); 25-35% of plot aboveground biomass remained on the field after harvest for
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Miscanthus and switchgrass.
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biomass sample.
Harvester yield was 70-95% of hand harvested aboveground
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b.
One Week Moving Average
(Emergence Period, Annual Inset)
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2009
2010
2011
15
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Annual
30
5
20
0
10
0
-5
60
70
80
90
100
110
Julian Day
100
Base Temperature 8 C
300
20
15
10
Annual
30
5
20
0
10
0
-5
60
70
80
90
100
110
d.
-10
120
0
Julian Day
100
200
300
Base Temperature 10 C
2500
2500
2000
2000
1500
1500
1000
1000
500
500
0
0
0
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200
Two Week Moving Average
(Emergence Period, Annual Inset)
Heat Units (C)
Heat Units (C)
c.
-10
120
0
Moving Average of Average Daily Air Temperature (C)
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Moving Average of Average Daily Air Temperature (C)
a.
100
200
Julian Day
300
0
100
200
300
Julian Day
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Figure S2. One- (a) and two-week (b) moving averages during the period of emergence (grey
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band) for WQFS and central Illinois studies provided reasonable ranges for Midwestern base
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temperature 8° C (c) and 10° C (d) to estimate corresponding heat units to maturity
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Figure S3. Leaf area index development curves for observed values (2010 to 2011); whisker
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boxplots demonstrate first and third quartiles, minimum and maximum values. Dotted line is the
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mean, solid line is the median. Solid dots values exceed twice the interquartile range
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Figure S4.
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outputs with SWAT database continuous corn growth values for total biomass production (a),
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leaf area index (b), total plant nitrogen (c), and total plant phosphorus (d).
Comparison of Miscanthus and switchgrass suggested parameter crop growth
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Figure S5. Comparison of simulated Miscanthus growth using existing SWAT 2009 code and
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code changes proposed in this study for total biomass production (a), total plant nitrogen (b), leaf
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area index (c), nitrogen uptake (d), evapotranspiration (e), and total plant phosphorus (f).
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Changes in the SWAT model were implemented to better represent physiological characteristics
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of perennial rhizomatous grasses.
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Table S1. Major and Minor changes to source code in SWAT
Type of
Change
File
Code
Readplant.f
if (blai(ic) >= 13.0) blai(ic) = 13.0
Comment
Modified from 10 to
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Adjusted n-fraction for stover removal=
Harvestop.f
Deleted root decomposition fractions for PRG
Major
Root fraction calculation equation changed
Grow.f
The equation was
wrong
No nutrient uptake in stress (temp or water or
aeration stress)=1
Extended LAI development to have DLAI>1
Nlch.f
if (vv > 1.e-10) co = Max(vno3 / vv, 0.)
Soil_chem.f
sol_orgn(j,i) = 10000. * (sol_cbn(j,i) / 14.) * wt1
Hruyr.f
Header.f
Sim_inityr.f
Minor
Harvgrainop.f
Killop.f
Sim_initday.f
Zero0.f
subyr.f
Virtual.f
There was some
difference between
windows and linux
simulation.
CN ratio changed back
to 14
pdvas(66) = yldanu(j)
pdvas(67) = BioYld(j)
&
"LAI"," YLDt/ha"," BioYLDtha","
GrnYLDtha", &
&
"LATNO3kg/h","GWNO3kg/ha","
GrainT/ha"," BiomT/ha", &
BioYld = 0.
GrainYld = 0.
GrainYld(j) = GrainYld(j) + yield / 1000.
!bio_yrms(j) = bio_yrms(j) + bio_ms(j) / 1000.
sub_grain = 0.
sub_Biom = 0.
sub_grain = 0.
sub_Biom = 0.
pdvab(19) = sub_grain(sb)
pdvab(20) = sub_Biom(sb)
sub_grain(sb)= sub_grain(sb) + GrainYld(j) *
hru_fr(j)
sub_Biom(sb)= sub_Biom(sb) + BioYld(j) *
hru_fr(j)
Commented it
accounts biomass
twice in harvest and
kill. - this can affect
the kill only operation
Initialization
sub_grain and
sub_biom added to
output subbasin level
yields
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Table S2. Relative sensitivity of model outputs to crop growth parameters, ranked by greatest
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sensitivity of harvested biomass yield.
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approach zero.
Parameter
Parameter Definition1
BIO_E
Decreased sensitivity indicated as absolute values
Harvested
Biomass
Yield
Water
Yield
Nitrate Total
Load
Phosphorus
Load
Radiation Use Efficiency in
ambient CO2
0.908
0.252
-1.076
0.502
T_BASE
Base Temperature
0.269
-0.533
-1.435
-0.167
T_OPT
Optimal Temperature
-0.256
0.016
0.593
-0.072
EXT_COEF
Light Extinction Coefficient
0.133
0.042
-0.281
0.072
BLAI
Maximum Leaf Area Index (LAI)
0.131
-0.279
-0.468
-0.096
DLAI
Point in growing season when
LAI declines
0.098
-1.161
-1.482
-0.574
FRGRW1
Fraction of growing season
coinciding with LAIMX1
-0.089
0.169
0.257
0.048
LAIMX2
Fraction of BLAI corresponding
to second point on optimal leaf
area development curve
0.076
-0.106
0.234
0.048
FRGRW2
Fraction of growing season
coinciding with LAIMX2
-0.066
0.093
0.203
0.048
LAIMX1
Fraction of BLAI corresponding
to first point on optimal leaf area
development curve
0.061
-0.114
-0.172
-0.048
CNYLD
Plant nitrogen fraction in
harvested biomass
-0.052
-0.013
-1.342
-0.024
PLTNFR3
Plant nitrogen fraction at maturity
(whole plant)
-0.033
-0.01
0.422
-0.023
GSI
Maximum stomatal conductance
0.002
-0.296
-0.148
-0.167
PLTNFR1
Plant nitrogen fraction at
emergence (whole plant)
0.002
0.001
-0.148
0
0.001
0
0.46
0
PLTNFR2
Plant nitrogen fraction at 50%
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maturity (whole plant)
CHTMX
Maximum Canopy Height
0
0.019
0
0
FRGMAX
Fraction of GSI corresponding to
the second point on the stomatal
conductance curve
0
-0.016
0.008
0
VPDFR
Vapor pressure deficit
0
0.007
-0.008
0
RSDCO_PL
Plant residue decomposition
coefficient
0
0
0.008
0
PLTPFR3
Plant phosphorus fraction at
maturity (whole plant)
0
0
0
0.323
CPYLD
Plant phosphorus fraction in
harvested biomass
0
0
0
0.163
USLE_C
Minimum Crop Factor for Water
Erosion
0
0
0
0.072
RDMX
Maximum Rooting Depth
0
0
0
0
PLTPFR1
Plant phosphorus fraction at
emergence (whole plant)
0
0
0
0
PLTPFR2
Plant phosphorus fraction at 50%
maturity (whole plant)
0
0
0
0
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Short Summary of Key Parameters for Crop Growth in SWAT
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development, seasonal leaf area development, and plant nutrient uptake. Once optimal growth
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values are established, site-specific conditions limiting crop growth are simulated in SWAT via
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water, nutrient and temperature stress attributed to soil type, nutrient availability and weather
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characteristics.
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temperature inputs. Daily heat units (HU) accumulate throughout the growing season, serving as
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an inherent scheduling component of day-to-day biomass production and leaf area development
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to crop maturity. A crop-specific base air temperature (T_BASE) serves as proxy for necessary
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soil moisture-temperature characteristics to trigger initiation of seasonal regrowth.
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biomass accumulation is a direct function of intercepted photosynthetically active radiation
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(PAR) and crop-specific radiation use efficiency (RUE, listed as BIO_E).
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determines the amount of intercepted radiation, and leaf area development is modeled using an
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optimal leaf area development curve. The curve shape and bounds are defined in the model
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using six parameters calculated from measured leaf area index (LAI). Daily nutrient uptake is
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simulated by the model as a function of the plant nutrient demand and soil nutrient availability.
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The daily nutrient demand is a function of biomass developed to date and nutrient requirements.
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Nutrient requirements are characterized at three growth stages (emergence, development, and
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maturity), represented by nutrient fraction parameters for nitrogen (PLTNFR) and phosphorus
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(PLTPFR). Crop yield output is a function of biomass, harvest index (HI), and harvest efficiency
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(HEFF).
Optimal crop growth in the SWAT model is driven by three key elements: biomass
Simulated plant growth is initiated and regulated by average daily air
Daily
Plant leaf area
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