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7.0.1.1 PS‑2: WATER-LIMITED PRODUCTION POTENTIAL
Learning objectives and requirements
Objectives
  • Defining Water-limited Production Situation (PS‑2)
  • Understanding input data requirements
  • Understanding drought impacts on production
  • Towards simulating the Actual Production Situation (PS‑n)
Stomata Production Stiuation

Production situation PS‑2 represents a land‑use system in which production possibilities are determined by irradiance of photosynthetically active radiation (PAR), temperature, and availability of water. The land use requirements 'optimum availability of PAR', 'optimum temperature' and 'optimum availability of water' are matched against the land qualities 'actual PAR', 'actual temperature' and 'actual availability of water' to determine the water limited production potential.

Production situation PS‑2 is already a much more complex situation than PS‑1 but still less complex than the production environment of many farmers in developing countries. Advanced farmers may examine alternative PS‑2 scenarios to evaluate water management options, identify optimum planting or sowing dates, select physically suitable areas for agricultural expansion in critically dry regions, and much more.

Analysis of production situation PS‑2 is based on the same principles and calculations of a PS-1 system, but the procedure also incorpotates the effects of drought. Recall that synthesis of plant matter involves uptake of CO2 from the atmosphere through stomatal openings in the leaves.

Leaves have microscopic mouths <i>aka</i> stomata.
Image 1: Leaves have microscopic mouths aka stomata.
Intake of CO2 is almost always accompanied by transpiration, which requires water.
Photosynthesis at leaf-level during which a major portion of incoming solar energy (light) is used in vaporizing water, and only a small portion (3%) is used in the actual photo-chemical reactions in carbon-hydrate (glucose) production.
Image 2: Photosynthesis at leaf-level during which a major portion of incoming solar energy (light) is used in vaporizing water, and only a small portion (3%) is used in the actual photo-chemical reactions in carbon-hydrate (glucose) production.
The water lost in transpiration must be replenished by uptake from the soil. If the possibility of lateral water flow through the soil is ignored, availability of water for uptake is determined by:
  • water (vapour) flow through the upper boundary of the rooting zone
  • water flow through the lower boundary of the rooting zone
  • uptake of water by the roots (equal to transpiration losses).
coefficient of water sufficiency
Drought in CO2 assimilation
Pathway of calculations

The flow diagram below presents a routine to extend the analysis of production situation PS-1 to an analysis of situation PS-2. The routine bypasses the operation 'cf(water) = 1' in the case PS-2 are desired. Instead, it here matches the momentary water needs of the crop against the momentary availability of soil moisture.

Flow diagram of a routine to calculate PS-1/PS-2. Substitution of cf(water) with a dynamically calculated variable extends the analysis of production situation PS-1 to an analysis of situation PS-2.
Image 3: Flow diagram of a routine to calculate PS-1/PS-2. Substitution of cf(water) with a dynamically calculated variable extends the analysis of production situation PS-1 to an analysis of situation PS-2.

The calculated sufficiency of moisture supply, i.e. cf(water) with a value between 0 and 1, is used in the calculations as follows.


Water-limited production of sugars


The potential gross and water-limited production of assimilates by a field crop can be calculated from Equation 8.10.

       Fgass = Fgc * 30/44 * cf(water)                                                                         (8.10)

where:

Fgass

is gross rate of assimilate production by a field crop (kg ha-1 d-1)

Fgc

is gross rate of CO2 reduction by a closed reference crop (kg ha-1 d-1)

30/44

is ratio of molecule masses of water and CO2

cf(water)

is correction factor for suboptimum availability of water (= 1.0 in PS 1).

Note that production situation PS-1 is, by definition, free from water stress. Hence the correction factor for availability of water aka cf(H20) then assumes a constant value of 1.0. In calculations for other production situations, cf(water) can be less than 1.0 and is re-calcualted on a daily interval hereby expressing the effect of water stress on assimilation.

  • Assume we have a gross rate of CO2 reduction for a closed reference crop (Fgc) given the following day length, AMAX, LAI, ke, PARCAN, and extinction coefficient (ke). Ke describes the efficiency of canopies light use at low light intensity.
    Gross Rate of CO2 Reduction Algorithm Interface.
    Image 4: Gross Rate of CO2 Reduction Algorithm Interface.
  • Run the potential gross rate of assimilate production algorithm through the following JAVA Webstart link GrossRateAssimilateProductionAlgorithm. This interface should result:
    Gross Rate Assimilate Production Algorithm Interface.
    Image 5: Gross Rate Assimilate Production Algorithm Interface.
    The use of the algorithm is detailed in the following Javadoc Class GrossRateAssimilateProductionAlgorithm

  • Vary cf(water), the ratio of 'actual transpiration' over 'potential transpiration', between 0-1, and plot the results.
  • Question 1
    Is the effect of drought on CO2 assimilation in crops linear or exponential?

     

     


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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