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7.0.1.0 PS‑1: BIOPHYSICAL PRODUCTION POTENTIAL
Objectives Learning objectives and requirements
  • Defining Production Situation
  • Understanding Light response curves at various temperatures
  • Maximum Assimilation (AMAX) to temperature curve interpretations
  • Analysis of production situation PS‑1
Production Situation

Production situation PS‑1 represents a land‑use system with the least possible analytical complexi­ty; all land qualities which can be influenced by a farmer through irrigation and drainage, use of fer­tilizers, weeding and control of pests and diseases are assumed to be optimum. The production calculated for production situation PS‑1 is the highest production possible on a farmer's field. It is the 'biophysical production potential'.

The biophysical production potential is determined by the solar radiation and temperature du­ring the growing period and by the physio­logical characteris­tics of the crop. Analysis of production situation PS‑1 is based on the same principles as calculation of net biomass pro­duction for agro‑ecological zoning but the procedure is dynamic and consi­derably more detailed.

The basic methods to calculate production potentials are described in Land use systems analysis by P.M. Driessen and N.T. Konijn (1992). Chapter 8 contains the formulas for PS1. The complete book can be downloaded here; to read chapter 8 (page 108 and following), please use the bookmarks in the PDF.


Photosynthesis The fundamental process behind plant growth is assimilation, i.e. reduction of atmospheric CO2 to carbohydrates, (CH2O)n. Assimilation requires energy; it is a unique capability of green plants that they can capture solar energy and use it in assimilation:

CO2 + H2O + solar energy ‑‑> 1/n(CH2O)n + O2 (1a)

Conversion of (CH2O)n to CO2 and H2O occurs also. This process is known as respiration; it releases chemical energy which can be used by the plant.

 

Pathways of photosynthesis The rate of assimilation under conditions of light saturation and op­ti­mum temperature differs among plants. Three different pathways of photosynthesis exist of which two have practical importance.
  • one group of plants produces C3H6O3 as the first assimilate; plants in this group are called C3‑plants after the length of the carbon chain of the first assimilate
  • plants in the second group produce C4H8O4 as the first assimilate; they are the C4‑plants.

An important difference between C3‑plants and C4‑plants is that respiration in the sunlit photosynthetic organs (photorespiration) is considerable in C3‑plants and negli­gible in C4‑plants.

Losses of assimilates incurred in photo­res­pi­ration increase with temperature and intensity of light. This has practical consequences.

  • C4‑plants make more efficient use of intercepted solar radiation than C3‑plants at high light intensity (there is little dif­ference at low light intensity)
  • C4‑plants reach their maximum rate of assimilation rate between 25 and 35 oC
    whereas C3‑plants perform best between 15 and 25 oC (Black, 1973).

Not surprisingly, C4‑plants stem predominantly from the tropics. Most C3‑crops (not all) have their origin in more tem­pe­rate regions. Representa­tives of both groups are included in Table 1.

Table 1: Photosynthetic mechanism for various crops (Driessen and Konijn, 1992)

CropC3/C4CropC3/C4
BarleyC3Pigeon beanC3
CassavaC3PotatoC3
ChickpeaC3RiceC3
CottonC3SesameC3
CowpeaC3SorghumC4
GroundnutC3SoyaC3
JuteC3Sugar-caneC4
LentilC3SunflowerC3
MaizeC4Sweet potatoC3
MilletC4TobaccoC3
Mung beanC3WheatC3

 

Effects of light intensity and temperature on assimilation The amount of solar energy at the outer extremity of the atmosphere varies with the latitude of the site and the time of year. Approxi­mately half the total global radiation is photosynthetically active radiation (PAR). The trans­parancy of the atmosphere determines how much radiation reaches the canopy. Light response curves relate irradiance with gross assi­mi­lation. Light response curves are described by only two parameters.

-light use efficiency at low light intensity (EFF)

-maximum rate of assimilation (AMAX).

AMAX (kg ha-1 h-1) is the gross rate of assimilation at light saturation; AMAX is co-determined by photorespiration and is much greater for C4‑crops than for C3‑crops. AMAX is strongly temperature‑dependent; EFF decreases by only 1% for every degree of tempe­ra­ture increase in C3‑plants, and even less in C4‑plants. For practical purposes EFF is a constant with a value of some 0.5 kg ha-1 h-1/J m-2 s-1 (de Wit et al., 1978).

Light response curves of maize leaves at several temperatures (de Wit et
al., 1978).
Image 1: Light response curves of maize leaves at several temperatures (de Wit et al., 1978).

The above figure presents light response curves of maize leaves at several tempera­tures. Observe that ambient temperature has a much more pronounced effect on AMAX (the plateau) than on EFF (the initial angle of the curve).

It is unfortunate that curves like those Figure 1 cannot be used to describe the assimilatory potential of field‑grown crops. It appears that the photo­synthetic activity of plant leaves is influenced by the radiation and temperature to which the leaves were exposed in the past. It is for this reason that the AEZ team defined crop‑adaptability groups with different AMAX‑to‑tempera­ture relations. The response curves in Figure 2 resemble those used by the AEZ team (FAO, 1978).

Light response curves of maize leaves at several temperatures (de Wit et
al., 1978). <br>I = C3‑crops in cool and temperate climates; <br>II = C3‑crops in warm climates; <br>III = C4‑crops in warm climates; <br>IV = C4‑crops in cool climates.
Image 2: Light response curves of maize leaves at several temperatures (de Wit et al., 1978).
I = C3‑crops in cool and temperate climates;
II = C3‑crops in warm climates;
III = C4‑crops in warm climates;
IV = C4‑crops in cool climates.

Note that Figure 2 is a simplification; the optimum temperature for assimilation by a C3‑crop cannot be a steady 18 oC in cool climates and 27 oC in the tropics if it is co‑determined by the temperatures to which the crop was actually exposed.

       Therefore actual assimilation will be calculated as a fraction of assimilation at a reference temperature (Tref). Tref is the temperature to which the assi­mi­lating plant 'got used'; it is tentatively defined as the weighted average of the daytime tem­peratures (Tday) over the past 10 days, with a mini­mum of 15 oC and a maximum of 30 oC.

 

Curves I and II in Figure 2 suggest the following AMAX‑to‑temperature relation for C3‑crops.

 

       AMAX = 1.8 * Tref - 0.15 * (Tref - Tday)2                                                                         (8.2a)

 

Approximate AMAX‑to‑temperature relations for C4‑crops are obtained by dividing response curves III and IV in Figure 2 in three linear trajecta.

 

if Tday <= Tref then

AMAX = 110 - 10 * (Tref - Tday)                                                                         (8.2b)

if Tday > Tref then

       AMAX = 110 - 2 * (Tday - Tref)                                                                           (8.2c)

 

if AMAX > 88 then AMAX = 88                                                                        (8.2d)

where

AMAX       is maximum rate of assimilation at actual temperature (kg ha-1 h-1)

Tref       is reference temperature (oC)

Tday         is daytime temperature (oC).

 

Implementation To calculate AMAX, you can run the MaximumAssimilationAlgorithm, by clicking the following JAVA Webstart link.

The following interface should appear:

Maximum Assimilation Algorithm Interface.
Image 3: Maximum Assimilation Algorithm Interface.
  • Programmers may wish to read the Java documentation of the MaximumAssimilationAlgorithm class, see Class MaximumAssimilationAlgorithm Javadoc. Code is available on request.
  • The MaximumAssimilationAlgorithm is just one of the algorithms out of a whole group of algorithms that were developed for PS-1 and PS-2. For an overview of all implemented algorithms, see nl.itc.cropspecificproductionlevels package-summary.
  • Some algorithms were grouped e.g. SunlightAlgorithm, CropGrowthAlgorithm and OrganGrowthAlgorithm. One algorithm may call another algorithm, etc.
  • To run the whole simulation for a single point for a complete growing season, you can use ProductionAlgorithm.

AMAX‑to‑temperature response curves relate AMAX to the equivalent daytime temperature (Tday), not the average daily temperature (T24h).

 

Additional reading

Average daily temperature (T24h) is a function of equivalent daytime tem­pe­ra­ture (Tday), equivalent night temperature (Tnight) and daylength (DL).

The equivalent daytime temperature (Tday) is found by integrating the temperature curve between sunrise and sunset (M. v.d. Berg, pers. comm.). It is assumed that the maximum tempera­ture occurs at 14.00 hrs and the lowest temperature at sunrise.

 

       Tday = Tmid + (SUNSET - 14) * AMPL * sin(AUX) / (DL * AUX)                 (8.3.1)

with

       Tmid = (Tmax + Tmin) / 2                                                                                (8.3.2)

       AMPL = (Tmax - Tmin) / 2                                                                               (8.3.3)

       SUNRISE = 12 - DL / 2                                                                                     (8.3.4)

       SUNSET = 12 + DL / 2                                                                                     (8.3.5)

       AUX = PI * (SUNSET - 14) / (SUNRISE + 10)                                                (8.3.6)

 

where

Tmax     is maximum daily temperature (oC)

Tmin      is minimum daily temperature (oC)

DL         is daylength (h d-1)

PI           is a constant (PI = 3.14159).

 

The equivalent night temperature (Tnight) is found by integrating the temperature curve between sunset and sunrise.

 

       Tnight = Tmid - AMPL * sin(AUX) / (PI - AUX)                                                 (8.3.7)

 

The daylength (DL) is a function of the day in the year and the latitude of the site (de Wit et al., 1978).

 

       DL = 12 * (PI + 2 * asin(SSCC)) / PI                                                                   (8.4)

with

       SSCC = SSIN / CCOS                                                                                       (8.4.1)

       SSIN = sin(LAT * RAD) * sin(DEC * RAD)                                                    (8.4.2)

       CCOS = cos(LAT * RAD) * cos(DEC * RAD)                                                (8.4.3)

       DEC = -23.45 * cos(2 * PI * (DAY+10) / 365)                                                 (8.4.4)

 

where

RADis a conversion factor (degree to radian; RAD = PI / 180)

LAT is latitude of the site (degree)

DEC is declination of the sun (degree)

DAY is Julian day number on the northern hemisphere, or Julian day number plus or minus 182 on the southern hemisphere.

Note that Equations 8.2, 8.3 and 8.4 relate AMAX to a few readily available data, viz. latitude of the site (LAT, in degree), Julian day number (DAY), and daily maximum and minimum temperatures (Tmax and Tmin).

Sunflower case
Question 1
Check the information on crop photosynthetic mechanism. What is the crop type for sunflower: C3 or C4?
C3
C4

7.0.1.0.0  Data requirements

 


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U09-NRM-127: The role of Distributed Data Access Technologies in NRM - for ITC-IDV version 2.7 > Thematic Expert Models > Food security > Biophysical Production Simulations