Monday, November 5, 2007
35-6
FAO model AquaCrop – Conservative Nature of Biomass Water Use Efficiency, a Key Model Fundamental.
The FAO AquaCrop model estimates yield as the product of biomass produced and harvest index. Biomass is calculated from the amount of water transpired, assuming a constant normalized biomass water use efficiency (WUEb) of the crop. The conservative nature of WUEb has received attention for decades, but was recently questioned in a publication. The conceptual basis and supporting data of the conservative behavior needs more elaboration, and the normalization requires further development. These are the focus of this presentation. The theoretical background and the experimental evidence for the near constancy of WUEb under variable environmental conditions are presented. Starting at the leaf scale of the hierarchical levels of biological organization, the basic gas exchange equations are outlined to demonstrate that normalized photosynthetic water use efficiency is proportional to the ambient CO2 concentration, supported by recent experimental evidence on several C3 and C4 crops. Additional factors are introduced to assess photosynthetic water use efficiency at the canopy scale, including the extent of radiation capture and role of respiration, again supported by data on canopies. The biomass chemical composition was then considered in the analysis of WUEb over a season. Highlighted are the needs to normalize WUEb for evaporative demand of the atmosphere to transfer WUEb between climatic zones, and for atmospheric CO2 concentration to deal with changes in CO2 in the past and into the future. Of the two procedures for normalization for evaporative demand, normalization by reference evapotranspiration is considered superior. A procedure to normalize for CO2 concentration is derived from the leaf scale and shown to be applicable to canopy scale. Some knowledge gaps and uncertainties are pointed out and their potential impacts on AquaCrop performance are discussed.
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