Models for estimating crop growth and Yield - Crop Growth Assessment and Modelling, Crop Production | Crop Production Notes- Agricultural Engineering PDF Download

 Page 1


8.1 Introduction
A continuous simulation erosion model, such as WEPP, requires a plant growth component in order
to simulate the growth of plants and their impact on the hydrologic and erosion processes. This chapter
describes the growth models used within the WEPP computer program to predict the development of
cropland and rangeland plants. The purpose of the growth models is to predict temporal changes in plant
and residue variables such as canopy cover, canopy height, root development, and biomass produced by
the plants which is removed during a harvest operation or ends up as surface residue material. Cropland
and rangeland plant growth are simulated in separate submodels of the WEPP model.
The plant growth component provides information to the water balance component (Chapter 5)
which allows estimation of daily water use by the plants and extraction of water from the soil layers.
Canopy cover and height information are passed to the erosion component (Chapter 11) for use in
estimation of interrill soil detachment. The amount of residue remaining after harvest, or residue created
by leaf drop during senescence is sent to the residue decomposition and management component (Chapter
9) of the WEPP model. Crop yield predicted by the plant growth component is available as a model
output, and the user may alter the biomass production and predicted crop yield through cautious
adjustment of the plant-speci?c input parameters.
Several plant management options are available to the user, including harvesting for grain or silage
for cropland annual plants, hay harvest and livestock grazing for cropland perennial plants, and burning,
herbicide application and livestock grazing for rangeland situations. Management options related to
residues produced by a plant are discussed in the following chapter (Chapter 9).
This chapter has been organized into ?ve sections. Sections 8.2 and 8.3 discuss plant growth and
management options for cropland simulations, respectively. Sections 8.4 and 8.5 discuss plant growth
and management options for rangeland simulations. Management and decomposition of residues
resulting from the plant growth described in this chapter are discussed in Chapter 9.
8.2 Cropland Plant Growth Model
The crop model in WEPP was modi?ed to make it similar to the EPIC crop model (Williams et al.,
1989). WEPP uses EPIC concepts of phenological crop development based on daily accumulated heat
units, harvest index for partitioning grain yield, Montieth’s approach (Montieth, 1977) for determining
potential biomass, and water and temperature stress adjustments. However, the nutrient cycling routines
in EPIC are not included. A single model is used for simulating several crops by changing model
parameters. WEPP is capable of simulating crop growth for both annual and perennial plants. Annual
crops grow from planting date to harvest date or until accumulated heat units equal the potential heat
units for the crop. Perennial crops maintain their activity throughout the year, although the plant may
become dormant after frost.
Phenological development of the crop is based on daily heat unit accumulation. Heat units are
computed using the equation:
HU
i
=
2
T
mx,i
+ T
mn,i
h hhhhhhhhhh - T
b, j
[8.2.1]
Page 2


8.1 Introduction
A continuous simulation erosion model, such as WEPP, requires a plant growth component in order
to simulate the growth of plants and their impact on the hydrologic and erosion processes. This chapter
describes the growth models used within the WEPP computer program to predict the development of
cropland and rangeland plants. The purpose of the growth models is to predict temporal changes in plant
and residue variables such as canopy cover, canopy height, root development, and biomass produced by
the plants which is removed during a harvest operation or ends up as surface residue material. Cropland
and rangeland plant growth are simulated in separate submodels of the WEPP model.
The plant growth component provides information to the water balance component (Chapter 5)
which allows estimation of daily water use by the plants and extraction of water from the soil layers.
Canopy cover and height information are passed to the erosion component (Chapter 11) for use in
estimation of interrill soil detachment. The amount of residue remaining after harvest, or residue created
by leaf drop during senescence is sent to the residue decomposition and management component (Chapter
9) of the WEPP model. Crop yield predicted by the plant growth component is available as a model
output, and the user may alter the biomass production and predicted crop yield through cautious
adjustment of the plant-speci?c input parameters.
Several plant management options are available to the user, including harvesting for grain or silage
for cropland annual plants, hay harvest and livestock grazing for cropland perennial plants, and burning,
herbicide application and livestock grazing for rangeland situations. Management options related to
residues produced by a plant are discussed in the following chapter (Chapter 9).
This chapter has been organized into ?ve sections. Sections 8.2 and 8.3 discuss plant growth and
management options for cropland simulations, respectively. Sections 8.4 and 8.5 discuss plant growth
and management options for rangeland simulations. Management and decomposition of residues
resulting from the plant growth described in this chapter are discussed in Chapter 9.
8.2 Cropland Plant Growth Model
The crop model in WEPP was modi?ed to make it similar to the EPIC crop model (Williams et al.,
1989). WEPP uses EPIC concepts of phenological crop development based on daily accumulated heat
units, harvest index for partitioning grain yield, Montieth’s approach (Montieth, 1977) for determining
potential biomass, and water and temperature stress adjustments. However, the nutrient cycling routines
in EPIC are not included. A single model is used for simulating several crops by changing model
parameters. WEPP is capable of simulating crop growth for both annual and perennial plants. Annual
crops grow from planting date to harvest date or until accumulated heat units equal the potential heat
units for the crop. Perennial crops maintain their activity throughout the year, although the plant may
become dormant after frost.
Phenological development of the crop is based on daily heat unit accumulation. Heat units are
computed using the equation:
HU
i
=
2
T
mx,i
+ T
mn,i
h hhhhhhhhhh - T
b, j
[8.2.1]
where HU, T
mx
, and T
mn
are the values of heat units, maximum temperature, and minimum temperature
in
o
C on day i, and T
b
is the crop-speci?c base temperature in
o
C (no growth occurs at or below T
b
)of
crop j. A heat unit index (HUI) ranging from 0 at planting to 1 at physiological maturity is computed as
follows:
HUI
i
=
PHU
j
k =1
S
i
HU
k
h hhhhhh [8.2.2]
where HUI is the heat unit index for day i and PHU is the potential heat units required for maturity of
crop j.
8.2.1 Potential Growth
Interception of photosynthetic active radiation (PAR) is estimated with Beer’s law (Monsi and
Saeki, 1953):
PAR
i
= 0.02092 (RA)
i
( 1.0 - e
-0.65 LAI
)
i
[8.2.3]
where PAR is photosynthetic active radiation (MJ
.
m
-2
), RA is solar radiation (Ly), LAI is leaf area index,
and subscript i is the day of the year. Potential biomass production per day is estimated with the equation
(Montieth, 1977):
? B
p,i
= 0.0001 BE
j
(PAR)
i
[8.2.4]
where ? B
p,i
is the potential increase in total biomass on day i (kg
.
m
-2
), and BE
j
is the crop parameter for
converting energy to biomass for crop j (kg
.
MJ
-1
). The potential increase in total biomass is adjusted
daily according to the growth constraints. The adjusted daily total biomass production (? B
i
)is
accumulated through the growing season (B
m
).
B
m
=
i =1
S
ndays
? B
i
[8.2.5]
where ndays is the total number of days from the starting day.
8.2.2 Canopy Cover and Height
Canopy cover and height for annual and perennial crops are calculated as functions of vegetative
biomass:
C
c
= 1 -e
-ß
c
B
m
[8.2.6]
where C
c
is canopy cover (0-1). The variable ß
c
is de?ned as:
ß
c
=
ln
I J L 1 -
ß
2
R
w
hhh M J O -ß
1
h hhhhhhhhhh [8.2.7]
where R
w
is the row width (m), ß
1
is a plant-dependent constant, and ß
2
is the maximum canopy width at
physiological maturity. ß
c
is an input parameter (BB). For crops not grown in rows, R
w
is set equal to
the plant spacing (P
s
).
Page 3


8.1 Introduction
A continuous simulation erosion model, such as WEPP, requires a plant growth component in order
to simulate the growth of plants and their impact on the hydrologic and erosion processes. This chapter
describes the growth models used within the WEPP computer program to predict the development of
cropland and rangeland plants. The purpose of the growth models is to predict temporal changes in plant
and residue variables such as canopy cover, canopy height, root development, and biomass produced by
the plants which is removed during a harvest operation or ends up as surface residue material. Cropland
and rangeland plant growth are simulated in separate submodels of the WEPP model.
The plant growth component provides information to the water balance component (Chapter 5)
which allows estimation of daily water use by the plants and extraction of water from the soil layers.
Canopy cover and height information are passed to the erosion component (Chapter 11) for use in
estimation of interrill soil detachment. The amount of residue remaining after harvest, or residue created
by leaf drop during senescence is sent to the residue decomposition and management component (Chapter
9) of the WEPP model. Crop yield predicted by the plant growth component is available as a model
output, and the user may alter the biomass production and predicted crop yield through cautious
adjustment of the plant-speci?c input parameters.
Several plant management options are available to the user, including harvesting for grain or silage
for cropland annual plants, hay harvest and livestock grazing for cropland perennial plants, and burning,
herbicide application and livestock grazing for rangeland situations. Management options related to
residues produced by a plant are discussed in the following chapter (Chapter 9).
This chapter has been organized into ?ve sections. Sections 8.2 and 8.3 discuss plant growth and
management options for cropland simulations, respectively. Sections 8.4 and 8.5 discuss plant growth
and management options for rangeland simulations. Management and decomposition of residues
resulting from the plant growth described in this chapter are discussed in Chapter 9.
8.2 Cropland Plant Growth Model
The crop model in WEPP was modi?ed to make it similar to the EPIC crop model (Williams et al.,
1989). WEPP uses EPIC concepts of phenological crop development based on daily accumulated heat
units, harvest index for partitioning grain yield, Montieth’s approach (Montieth, 1977) for determining
potential biomass, and water and temperature stress adjustments. However, the nutrient cycling routines
in EPIC are not included. A single model is used for simulating several crops by changing model
parameters. WEPP is capable of simulating crop growth for both annual and perennial plants. Annual
crops grow from planting date to harvest date or until accumulated heat units equal the potential heat
units for the crop. Perennial crops maintain their activity throughout the year, although the plant may
become dormant after frost.
Phenological development of the crop is based on daily heat unit accumulation. Heat units are
computed using the equation:
HU
i
=
2
T
mx,i
+ T
mn,i
h hhhhhhhhhh - T
b, j
[8.2.1]
where HU, T
mx
, and T
mn
are the values of heat units, maximum temperature, and minimum temperature
in
o
C on day i, and T
b
is the crop-speci?c base temperature in
o
C (no growth occurs at or below T
b
)of
crop j. A heat unit index (HUI) ranging from 0 at planting to 1 at physiological maturity is computed as
follows:
HUI
i
=
PHU
j
k =1
S
i
HU
k
h hhhhhh [8.2.2]
where HUI is the heat unit index for day i and PHU is the potential heat units required for maturity of
crop j.
8.2.1 Potential Growth
Interception of photosynthetic active radiation (PAR) is estimated with Beer’s law (Monsi and
Saeki, 1953):
PAR
i
= 0.02092 (RA)
i
( 1.0 - e
-0.65 LAI
)
i
[8.2.3]
where PAR is photosynthetic active radiation (MJ
.
m
-2
), RA is solar radiation (Ly), LAI is leaf area index,
and subscript i is the day of the year. Potential biomass production per day is estimated with the equation
(Montieth, 1977):
? B
p,i
= 0.0001 BE
j
(PAR)
i
[8.2.4]
where ? B
p,i
is the potential increase in total biomass on day i (kg
.
m
-2
), and BE
j
is the crop parameter for
converting energy to biomass for crop j (kg
.
MJ
-1
). The potential increase in total biomass is adjusted
daily according to the growth constraints. The adjusted daily total biomass production (? B
i
)is
accumulated through the growing season (B
m
).
B
m
=
i =1
S
ndays
? B
i
[8.2.5]
where ndays is the total number of days from the starting day.
8.2.2 Canopy Cover and Height
Canopy cover and height for annual and perennial crops are calculated as functions of vegetative
biomass:
C
c
= 1 -e
-ß
c
B
m
[8.2.6]
where C
c
is canopy cover (0-1). The variable ß
c
is de?ned as:
ß
c
=
ln
I J L 1 -
ß
2
R
w
hhh M J O -ß
1
h hhhhhhhhhh [8.2.7]
where R
w
is the row width (m), ß
1
is a plant-dependent constant, and ß
2
is the maximum canopy width at
physiological maturity. ß
c
is an input parameter (BB). For crops not grown in rows, R
w
is set equal to
the plant spacing (P
s
).
H
c
=
I L 1 -e
-ß
h
B
m
M O H
cm
[8.2.8]
where H
c
is the canopy height (m), H
cm
is the maximum canopy height (m), and ß
h
is a plant-dependent
constant.
8.2.3 Senescence
When the fraction of growing season (F
gs
) is equal to the fraction of the growing season when
senescence begins (GSSEN), canopy cover (C
c
) starts declining linearly for a given time period (S
p
). The
daily decline in canopy cover can be predicted with the equation:
? C
c
= C
cm
I J L S
p
1 - f
cs
h hhhhh M J O [8.2.9]
where ? C
c
is the daily loss of canopy cover (0-1), C
cm
is canopy cover fraction at maturity (0-1), f
cs
is the
fraction of canopy cover remaining after senescence, and S
p
is the number of days between the beginning
and end of leaf drop. f
cs
and S
p
are user inputs to the model. Canopy cover is adjusted using:
C
c (i)
= C
c (i -1)
-? C
c
.
[8.2.10]
where C
c (i)
is the canopy cover for the current day i, and C
c (i -1)
is the canopy cover for the previous day.
Because leaves are falling during the senescence period, live above-ground biomass (B
m
) decreases
while ?at residue mass (M
f
) increases. The daily decline in above-ground biomass due to senescence
(? B
ms
) is predicted using the equation:
? B
ms
= B
mx
I J L S
p
1 - f
bs
h hhhhh M J O [8.2.11]
where B
mx
is the above-ground biomass at crop maturity (kg
.
m
-2
) and f
bs
is the fraction of above-ground
biomass remaining after senescence. f
bs
is a user input to the model. Above-ground biomass is then
adjusted using the following equation:
B
m (i)
= B
m (i -1)
-? B
m
[8.2.12]
Flat residue mass is increased by same amount (the change in vegetative biomass:
M
f (i)
= M
f (i -1)
+ (B
m (i -1)
- B
m (i)
)
[8.2.13]
where M
f (i -1)
is ?at residue mass of the previous day, and B
m (i -1)
is vegetative biomass of the previous
day.
8.2.4 Growth Limitations
The potential biomass predicted with Eq. [8.2.4] is adjusted daily if one of the plant stress factors
(water or temperature) is less than 1.0 using the equation:
? B
i
= (? B
p,i
)(REG)
[8.2.14]
where REG is the crop growth regulating factor (the minimum of the water and temperature stress
factors).
Page 4


8.1 Introduction
A continuous simulation erosion model, such as WEPP, requires a plant growth component in order
to simulate the growth of plants and their impact on the hydrologic and erosion processes. This chapter
describes the growth models used within the WEPP computer program to predict the development of
cropland and rangeland plants. The purpose of the growth models is to predict temporal changes in plant
and residue variables such as canopy cover, canopy height, root development, and biomass produced by
the plants which is removed during a harvest operation or ends up as surface residue material. Cropland
and rangeland plant growth are simulated in separate submodels of the WEPP model.
The plant growth component provides information to the water balance component (Chapter 5)
which allows estimation of daily water use by the plants and extraction of water from the soil layers.
Canopy cover and height information are passed to the erosion component (Chapter 11) for use in
estimation of interrill soil detachment. The amount of residue remaining after harvest, or residue created
by leaf drop during senescence is sent to the residue decomposition and management component (Chapter
9) of the WEPP model. Crop yield predicted by the plant growth component is available as a model
output, and the user may alter the biomass production and predicted crop yield through cautious
adjustment of the plant-speci?c input parameters.
Several plant management options are available to the user, including harvesting for grain or silage
for cropland annual plants, hay harvest and livestock grazing for cropland perennial plants, and burning,
herbicide application and livestock grazing for rangeland situations. Management options related to
residues produced by a plant are discussed in the following chapter (Chapter 9).
This chapter has been organized into ?ve sections. Sections 8.2 and 8.3 discuss plant growth and
management options for cropland simulations, respectively. Sections 8.4 and 8.5 discuss plant growth
and management options for rangeland simulations. Management and decomposition of residues
resulting from the plant growth described in this chapter are discussed in Chapter 9.
8.2 Cropland Plant Growth Model
The crop model in WEPP was modi?ed to make it similar to the EPIC crop model (Williams et al.,
1989). WEPP uses EPIC concepts of phenological crop development based on daily accumulated heat
units, harvest index for partitioning grain yield, Montieth’s approach (Montieth, 1977) for determining
potential biomass, and water and temperature stress adjustments. However, the nutrient cycling routines
in EPIC are not included. A single model is used for simulating several crops by changing model
parameters. WEPP is capable of simulating crop growth for both annual and perennial plants. Annual
crops grow from planting date to harvest date or until accumulated heat units equal the potential heat
units for the crop. Perennial crops maintain their activity throughout the year, although the plant may
become dormant after frost.
Phenological development of the crop is based on daily heat unit accumulation. Heat units are
computed using the equation:
HU
i
=
2
T
mx,i
+ T
mn,i
h hhhhhhhhhh - T
b, j
[8.2.1]
where HU, T
mx
, and T
mn
are the values of heat units, maximum temperature, and minimum temperature
in
o
C on day i, and T
b
is the crop-speci?c base temperature in
o
C (no growth occurs at or below T
b
)of
crop j. A heat unit index (HUI) ranging from 0 at planting to 1 at physiological maturity is computed as
follows:
HUI
i
=
PHU
j
k =1
S
i
HU
k
h hhhhhh [8.2.2]
where HUI is the heat unit index for day i and PHU is the potential heat units required for maturity of
crop j.
8.2.1 Potential Growth
Interception of photosynthetic active radiation (PAR) is estimated with Beer’s law (Monsi and
Saeki, 1953):
PAR
i
= 0.02092 (RA)
i
( 1.0 - e
-0.65 LAI
)
i
[8.2.3]
where PAR is photosynthetic active radiation (MJ
.
m
-2
), RA is solar radiation (Ly), LAI is leaf area index,
and subscript i is the day of the year. Potential biomass production per day is estimated with the equation
(Montieth, 1977):
? B
p,i
= 0.0001 BE
j
(PAR)
i
[8.2.4]
where ? B
p,i
is the potential increase in total biomass on day i (kg
.
m
-2
), and BE
j
is the crop parameter for
converting energy to biomass for crop j (kg
.
MJ
-1
). The potential increase in total biomass is adjusted
daily according to the growth constraints. The adjusted daily total biomass production (? B
i
)is
accumulated through the growing season (B
m
).
B
m
=
i =1
S
ndays
? B
i
[8.2.5]
where ndays is the total number of days from the starting day.
8.2.2 Canopy Cover and Height
Canopy cover and height for annual and perennial crops are calculated as functions of vegetative
biomass:
C
c
= 1 -e
-ß
c
B
m
[8.2.6]
where C
c
is canopy cover (0-1). The variable ß
c
is de?ned as:
ß
c
=
ln
I J L 1 -
ß
2
R
w
hhh M J O -ß
1
h hhhhhhhhhh [8.2.7]
where R
w
is the row width (m), ß
1
is a plant-dependent constant, and ß
2
is the maximum canopy width at
physiological maturity. ß
c
is an input parameter (BB). For crops not grown in rows, R
w
is set equal to
the plant spacing (P
s
).
H
c
=
I L 1 -e
-ß
h
B
m
M O H
cm
[8.2.8]
where H
c
is the canopy height (m), H
cm
is the maximum canopy height (m), and ß
h
is a plant-dependent
constant.
8.2.3 Senescence
When the fraction of growing season (F
gs
) is equal to the fraction of the growing season when
senescence begins (GSSEN), canopy cover (C
c
) starts declining linearly for a given time period (S
p
). The
daily decline in canopy cover can be predicted with the equation:
? C
c
= C
cm
I J L S
p
1 - f
cs
h hhhhh M J O [8.2.9]
where ? C
c
is the daily loss of canopy cover (0-1), C
cm
is canopy cover fraction at maturity (0-1), f
cs
is the
fraction of canopy cover remaining after senescence, and S
p
is the number of days between the beginning
and end of leaf drop. f
cs
and S
p
are user inputs to the model. Canopy cover is adjusted using:
C
c (i)
= C
c (i -1)
-? C
c
.
[8.2.10]
where C
c (i)
is the canopy cover for the current day i, and C
c (i -1)
is the canopy cover for the previous day.
Because leaves are falling during the senescence period, live above-ground biomass (B
m
) decreases
while ?at residue mass (M
f
) increases. The daily decline in above-ground biomass due to senescence
(? B
ms
) is predicted using the equation:
? B
ms
= B
mx
I J L S
p
1 - f
bs
h hhhhh M J O [8.2.11]
where B
mx
is the above-ground biomass at crop maturity (kg
.
m
-2
) and f
bs
is the fraction of above-ground
biomass remaining after senescence. f
bs
is a user input to the model. Above-ground biomass is then
adjusted using the following equation:
B
m (i)
= B
m (i -1)
-? B
m
[8.2.12]
Flat residue mass is increased by same amount (the change in vegetative biomass:
M
f (i)
= M
f (i -1)
+ (B
m (i -1)
- B
m (i)
)
[8.2.13]
where M
f (i -1)
is ?at residue mass of the previous day, and B
m (i -1)
is vegetative biomass of the previous
day.
8.2.4 Growth Limitations
The potential biomass predicted with Eq. [8.2.4] is adjusted daily if one of the plant stress factors
(water or temperature) is less than 1.0 using the equation:
? B
i
= (? B
p,i
)(REG)
[8.2.14]
where REG is the crop growth regulating factor (the minimum of the water and temperature stress
factors).
Water Stress -- The water stress factor is computed by considering supply and demand in the
equation
WS =
E
P
l =1
S
nl
u
l
h hhhh [8.2.15]
where WS is the water stress factor (0-1), u
l
is plant water use in soil layer l (mm), nl is the number of soil
layers, and E
P
is the potential plant evaporation (mm). The value of E
P
is predicted in the
evapotranspiration component of WEPP (Chapter 5).
Temperature Stress -- The temperature stress factor is computed with the equation:
TS = sin
I J L 2
p
hh
T
o
-T
b
T
a
- T
b
h hhhhhh M J O [8.2.16]
where TS is the temperature stress factor (0-1), T
a
is the average daily temperature (°C), T
b
is the base
temperature for the crop (°C), and T
o
is the optimum temperature for the crop (°C).
8.2.5 Crop Yield
The economic yield of most grain and tuber crops is a reproductive organ. Crops have a variety of
mechanisms which insure that their production is neither too great to be supported by the vegetative
components nor too small to insure survival of the species. As a result, harvest index (economic
yield/above-ground biomass) of unstressed crops is often relatively stable across a range of environmental
conditions. Crop yield for annual crops is estimated using the harvest index concept, which is adjusted
throughout the growing season according to water stress constraints.
YLD
j
= (HIA
j
)(B
AG
)
[8.2.17]
where YLD is crop yield (kg
.
m
-2
), HIA is adjusted harvest index at harvest, and B
AG
is cumulative
above-ground biomass (kg
.
m
-2
) before senescence occurs. Harvest index increases nonlinearly from zero
at planting using the equation:
HI
i
= HIO
j
(HUFH
i
- HUFH
i -1
)
[8.2.18]
where HI
i
is the harvest index on day i, HIO
j
is the harvest index under favorable growing conditions for
crop j, and HUFH is the heat unit factor that affects harvest index for day i and the previous day i -1.
The harvest index heat unit is computed with the equation:
HUFH
i
=
HUI
i
+ e
(6.50 - 10.0 HUI
i
)
HUI
i
hhhhhhhhhhhhhhhhhhh [8.2.19]
The constants in Eq. [8.2.20] are set to allow HUFH
i
to increase from 0.1 at HUI
i
= 0.5 to 0.92 at
HUI
i
=0.9. This is consistent with economic yield development of grain crops which produce most
economic yield in the second half of the growing season.
Most grain crops are particularly sensitive to water stress from shortly before until shortly after
anthesis (Doorenbos and Kassam, 1979). Optimum conditions for growth may reduce harvest index
slightly if dry matter accumulation is large and economic yield is limited by sink size. The harvest index
Page 5


8.1 Introduction
A continuous simulation erosion model, such as WEPP, requires a plant growth component in order
to simulate the growth of plants and their impact on the hydrologic and erosion processes. This chapter
describes the growth models used within the WEPP computer program to predict the development of
cropland and rangeland plants. The purpose of the growth models is to predict temporal changes in plant
and residue variables such as canopy cover, canopy height, root development, and biomass produced by
the plants which is removed during a harvest operation or ends up as surface residue material. Cropland
and rangeland plant growth are simulated in separate submodels of the WEPP model.
The plant growth component provides information to the water balance component (Chapter 5)
which allows estimation of daily water use by the plants and extraction of water from the soil layers.
Canopy cover and height information are passed to the erosion component (Chapter 11) for use in
estimation of interrill soil detachment. The amount of residue remaining after harvest, or residue created
by leaf drop during senescence is sent to the residue decomposition and management component (Chapter
9) of the WEPP model. Crop yield predicted by the plant growth component is available as a model
output, and the user may alter the biomass production and predicted crop yield through cautious
adjustment of the plant-speci?c input parameters.
Several plant management options are available to the user, including harvesting for grain or silage
for cropland annual plants, hay harvest and livestock grazing for cropland perennial plants, and burning,
herbicide application and livestock grazing for rangeland situations. Management options related to
residues produced by a plant are discussed in the following chapter (Chapter 9).
This chapter has been organized into ?ve sections. Sections 8.2 and 8.3 discuss plant growth and
management options for cropland simulations, respectively. Sections 8.4 and 8.5 discuss plant growth
and management options for rangeland simulations. Management and decomposition of residues
resulting from the plant growth described in this chapter are discussed in Chapter 9.
8.2 Cropland Plant Growth Model
The crop model in WEPP was modi?ed to make it similar to the EPIC crop model (Williams et al.,
1989). WEPP uses EPIC concepts of phenological crop development based on daily accumulated heat
units, harvest index for partitioning grain yield, Montieth’s approach (Montieth, 1977) for determining
potential biomass, and water and temperature stress adjustments. However, the nutrient cycling routines
in EPIC are not included. A single model is used for simulating several crops by changing model
parameters. WEPP is capable of simulating crop growth for both annual and perennial plants. Annual
crops grow from planting date to harvest date or until accumulated heat units equal the potential heat
units for the crop. Perennial crops maintain their activity throughout the year, although the plant may
become dormant after frost.
Phenological development of the crop is based on daily heat unit accumulation. Heat units are
computed using the equation:
HU
i
=
2
T
mx,i
+ T
mn,i
h hhhhhhhhhh - T
b, j
[8.2.1]
where HU, T
mx
, and T
mn
are the values of heat units, maximum temperature, and minimum temperature
in
o
C on day i, and T
b
is the crop-speci?c base temperature in
o
C (no growth occurs at or below T
b
)of
crop j. A heat unit index (HUI) ranging from 0 at planting to 1 at physiological maturity is computed as
follows:
HUI
i
=
PHU
j
k =1
S
i
HU
k
h hhhhhh [8.2.2]
where HUI is the heat unit index for day i and PHU is the potential heat units required for maturity of
crop j.
8.2.1 Potential Growth
Interception of photosynthetic active radiation (PAR) is estimated with Beer’s law (Monsi and
Saeki, 1953):
PAR
i
= 0.02092 (RA)
i
( 1.0 - e
-0.65 LAI
)
i
[8.2.3]
where PAR is photosynthetic active radiation (MJ
.
m
-2
), RA is solar radiation (Ly), LAI is leaf area index,
and subscript i is the day of the year. Potential biomass production per day is estimated with the equation
(Montieth, 1977):
? B
p,i
= 0.0001 BE
j
(PAR)
i
[8.2.4]
where ? B
p,i
is the potential increase in total biomass on day i (kg
.
m
-2
), and BE
j
is the crop parameter for
converting energy to biomass for crop j (kg
.
MJ
-1
). The potential increase in total biomass is adjusted
daily according to the growth constraints. The adjusted daily total biomass production (? B
i
)is
accumulated through the growing season (B
m
).
B
m
=
i =1
S
ndays
? B
i
[8.2.5]
where ndays is the total number of days from the starting day.
8.2.2 Canopy Cover and Height
Canopy cover and height for annual and perennial crops are calculated as functions of vegetative
biomass:
C
c
= 1 -e
-ß
c
B
m
[8.2.6]
where C
c
is canopy cover (0-1). The variable ß
c
is de?ned as:
ß
c
=
ln
I J L 1 -
ß
2
R
w
hhh M J O -ß
1
h hhhhhhhhhh [8.2.7]
where R
w
is the row width (m), ß
1
is a plant-dependent constant, and ß
2
is the maximum canopy width at
physiological maturity. ß
c
is an input parameter (BB). For crops not grown in rows, R
w
is set equal to
the plant spacing (P
s
).
H
c
=
I L 1 -e
-ß
h
B
m
M O H
cm
[8.2.8]
where H
c
is the canopy height (m), H
cm
is the maximum canopy height (m), and ß
h
is a plant-dependent
constant.
8.2.3 Senescence
When the fraction of growing season (F
gs
) is equal to the fraction of the growing season when
senescence begins (GSSEN), canopy cover (C
c
) starts declining linearly for a given time period (S
p
). The
daily decline in canopy cover can be predicted with the equation:
? C
c
= C
cm
I J L S
p
1 - f
cs
h hhhhh M J O [8.2.9]
where ? C
c
is the daily loss of canopy cover (0-1), C
cm
is canopy cover fraction at maturity (0-1), f
cs
is the
fraction of canopy cover remaining after senescence, and S
p
is the number of days between the beginning
and end of leaf drop. f
cs
and S
p
are user inputs to the model. Canopy cover is adjusted using:
C
c (i)
= C
c (i -1)
-? C
c
.
[8.2.10]
where C
c (i)
is the canopy cover for the current day i, and C
c (i -1)
is the canopy cover for the previous day.
Because leaves are falling during the senescence period, live above-ground biomass (B
m
) decreases
while ?at residue mass (M
f
) increases. The daily decline in above-ground biomass due to senescence
(? B
ms
) is predicted using the equation:
? B
ms
= B
mx
I J L S
p
1 - f
bs
h hhhhh M J O [8.2.11]
where B
mx
is the above-ground biomass at crop maturity (kg
.
m
-2
) and f
bs
is the fraction of above-ground
biomass remaining after senescence. f
bs
is a user input to the model. Above-ground biomass is then
adjusted using the following equation:
B
m (i)
= B
m (i -1)
-? B
m
[8.2.12]
Flat residue mass is increased by same amount (the change in vegetative biomass:
M
f (i)
= M
f (i -1)
+ (B
m (i -1)
- B
m (i)
)
[8.2.13]
where M
f (i -1)
is ?at residue mass of the previous day, and B
m (i -1)
is vegetative biomass of the previous
day.
8.2.4 Growth Limitations
The potential biomass predicted with Eq. [8.2.4] is adjusted daily if one of the plant stress factors
(water or temperature) is less than 1.0 using the equation:
? B
i
= (? B
p,i
)(REG)
[8.2.14]
where REG is the crop growth regulating factor (the minimum of the water and temperature stress
factors).
Water Stress -- The water stress factor is computed by considering supply and demand in the
equation
WS =
E
P
l =1
S
nl
u
l
h hhhh [8.2.15]
where WS is the water stress factor (0-1), u
l
is plant water use in soil layer l (mm), nl is the number of soil
layers, and E
P
is the potential plant evaporation (mm). The value of E
P
is predicted in the
evapotranspiration component of WEPP (Chapter 5).
Temperature Stress -- The temperature stress factor is computed with the equation:
TS = sin
I J L 2
p
hh
T
o
-T
b
T
a
- T
b
h hhhhhh M J O [8.2.16]
where TS is the temperature stress factor (0-1), T
a
is the average daily temperature (°C), T
b
is the base
temperature for the crop (°C), and T
o
is the optimum temperature for the crop (°C).
8.2.5 Crop Yield
The economic yield of most grain and tuber crops is a reproductive organ. Crops have a variety of
mechanisms which insure that their production is neither too great to be supported by the vegetative
components nor too small to insure survival of the species. As a result, harvest index (economic
yield/above-ground biomass) of unstressed crops is often relatively stable across a range of environmental
conditions. Crop yield for annual crops is estimated using the harvest index concept, which is adjusted
throughout the growing season according to water stress constraints.
YLD
j
= (HIA
j
)(B
AG
)
[8.2.17]
where YLD is crop yield (kg
.
m
-2
), HIA is adjusted harvest index at harvest, and B
AG
is cumulative
above-ground biomass (kg
.
m
-2
) before senescence occurs. Harvest index increases nonlinearly from zero
at planting using the equation:
HI
i
= HIO
j
(HUFH
i
- HUFH
i -1
)
[8.2.18]
where HI
i
is the harvest index on day i, HIO
j
is the harvest index under favorable growing conditions for
crop j, and HUFH is the heat unit factor that affects harvest index for day i and the previous day i -1.
The harvest index heat unit is computed with the equation:
HUFH
i
=
HUI
i
+ e
(6.50 - 10.0 HUI
i
)
HUI
i
hhhhhhhhhhhhhhhhhhh [8.2.19]
The constants in Eq. [8.2.20] are set to allow HUFH
i
to increase from 0.1 at HUI
i
= 0.5 to 0.92 at
HUI
i
=0.9. This is consistent with economic yield development of grain crops which produce most
economic yield in the second half of the growing season.
Most grain crops are particularly sensitive to water stress from shortly before until shortly after
anthesis (Doorenbos and Kassam, 1979). Optimum conditions for growth may reduce harvest index
slightly if dry matter accumulation is large and economic yield is limited by sink size. The harvest index
is affected by water stress using the equation:
HIA
i
=
1.0 + WSYF
j
(FHU
i
) (0.9 - WS
i
)
HI
i
hhhhhhhhhhhhhhhhhhhhhhhhhhh [8.2.20]
where HIA is the adjusted harvest index, WSYF is a crop parameter expressing drought sensitivity
(assumed to be a constant 0.01 in the WEPP model), FHU is a function of crop stage, and WS is the water
stress factor for day i. Notice that harvest index may increase slightly on days with WS values greater
than 0.9. The maximum value for HIA
i
is limited to HI
i
within the WEPP code. The crop stage factor,
FHU, is estimated with the equation:
FHU
i
= sin
2
p
hh
I J L 0.3
HUI
i
- 0.3
h hhhhhhhhh M J O 0.3 < HUI
i
< 0.9
[8.2.21]
FHU
i
= 0.0 HUI
i
= 0.3 or HUI
i
= 0.9
8.2.6 Yield Adjustment
Currently, the crop growth model in WEPP does not account for biomass and yield variation due to
nutrient, pest, or other management factors. The impact of these factors on erosion rates has to be
estimated and crop yield can be adjusted in one of two different ways. The recommended approach is to
alter crop yields through careful direct adjustments to the BE
j
and HI
j
user input parameters for the
speci?c crop. An alternative method is to use an algorithm which allows the WEPP user to adjust BE
j
indirectly though inputting of an optimum crop yield (yop
in
), assuming the plant experiences no growth
stresses. At the start of the simulation, the model calculates an optimum yield (yop
calc
) based on Eq.
[8.2.3] and [8.2.4] for potential growth (no stress). The biomass conversion factor is then adjusted with
the equation:
BE
adj
=
yop
calc
yop
in
h hhhhhh BE
j
[8.2.22]
where BE
adj
is the adjusted biomass conversion factor for crop j (kg
.
MJ
-1
), yop
in
is the optimum crop
yield input by the user (kg
.
m
-2
), and yop
calc
is the optimum crop yield calculated by the model (kg
.
m
-2
).
During a WEPP simulation, BE
adj
can then be used in Eq. [8.2.4] and the potential growth stressed
according to Eq. [8.2.15].
8.2.7 Root Growth
Ratios to describe partitioning between root biomass and above-ground vegetative biomass (root to
shoot ratios) are used to grow plant roots for all annual and perennial crops. Total root mass (B
rt
) on any
day (i) is predicted with the equation:
(B
rt
)
i
= (B
rt
)
i -1
+? B
i
(R
sr
)
j
[8.2.23]
where R
sr
is the root to shoot ratio, a plant-dependent constant.