| Learning objectives and
|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.
|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.
The calculated sufficiency of moisture supply, i.e. cf(water) with a value between 0 and 1, is used in the calculations as follows.
Fgass = Fgc * 30/44 * cf(water) (8.10)
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.
Pathways in calculating cf(water)|
Availability of water in soil
As introduced above, synthesis of plant matter involves uptake of CO2 from the atmosphere through stomatal openings in the leaves, a process always accompanied by transpiration of water.
Lost water must be replenished by uptake from the soil. Ignoring lateral water flow through the soil the rate at which the volume fraction of moisture in the rooting zone changes follows from a simple water budget equation. RSM = [UPFLUX + (CR + D) TR] / RD (9.1) where RSMis rate of change of volume fraction of moisture in the rooting zone (d-1) UPFLUX is net rate of water (vapour) flow through the upper boundary of the rooting zone (cm d-1) (CR + D)is net rate of water flow through the lower boundary of the rooting zone (cm d-1) TRis actual rate of transpiration (cm d-1) RDis equivalent depth of the rooting zone (cm).
Figure 9.1 shows the water fluxes that condition the volume fraction of moisture in the rooting zone and the availability of water for uptake by roots.
Fig 9.1. Water fluxes conditioning the volume fraction of moisture in the rooting zone.
Recall that the actual rate of transpiration (TR) is less than the theoretical maximum rate (TRM) when a crop senses moisture stress. Under conditions of steady state, assimilation decreases proportionally to transpiration. The water uptake correction factor (cf(water)) is the relative rate of gross assimilation and represents the sufficiency of the land quality 'water availability' (see also INTSUFF; Equation 6.11). cf(water) = TR / TRM (9.2) where cf(water)is relative rate of transpiration by plants exposed to water stress TRis actual rate of transpiration (cm d-1) TRMis maximum rate of transpiration (cm d-1). The difference between analyses of production situation PS 1 and production situation PS 2 is that cf(water) in Equations 8.10 (Fgass) and 8.14 (MRR(org)) is 1.0 in production situation PS 1, and between 0 and 1.0 in production situation PS 2 (calculated as a function of, inter alia, all water fluxes to and from the rooting zone). Traditionally, availability of water is modelled by so Table 9.4 lists the additional data needs for analyses of production situation PS 2. The data items are grouped in five categories: General data, Management data, Crop data, Weather data and Soil/terrain data. Note that the data in Table 9.4 have to be collected in addition to the data listed in Table 8.3. Note further that tabulated (default) values for crop and soil parameters are mentioned in Table 9.4 to help you fill data gaps. Default values are to be used with caution; they are no substitute for measured values.
By default, only the world outlines are displayed when you first start the IDV. You can change the default to use other system maps, or add in your own. The IDV can use McIDAS map files and ESRI shapefiles for map backgrounds.
Retrieve the map with provincial boundaries of Zimbabwe and superimpose it on the image. By default, IDV does not have a World Administrative Units map. There is a plugin that provides first-level administrative unit boundaries for the world (last updated: 2002). For more details on adding/removing plugins see Text Point Data Once you have succesfully added the plugin (and restarted the IDV), enable the provincial boundaries map as follows:
|Well done, continue with the next session.|
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