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AquaCrop results from the revision of the FAO Irrigation and Drainage
Paper No. 33 “Yield Response to Water” (Doorenbos
and Kassam, 1979), a key reference for estimating the yield response
to water. AquaCrop evolves from the fundamental equation of Paper No.
33, where relative yield (Y) loss is proportional to relative evapotranspiration
(ET) decline, with Ky as the yield response proportional factor.
AquaCrop advances from the Ky approach by:
(i) dividing ET in soil evaporation (E) and crop transpiration (Tr), to avoid
the confounding effect of the non-productive consumptive use of water (E),
(ii) obtaining biomass (B) from the product of water productivity (WP) and cumulated
crop transpiration,
(iii) expressing the final yield (Y) as the product of B and Harvest Index (HI),
(iv) normalizing Tr with reference evapotranspiration (ETo), to make the B-Tr
relationship applicable to different climatic regimes, and
(v) running with daily time steps (either calendar or growing degree days), to
more realistically account for the dynamic nature of water stress effects and
crop responses.
As the Ky approach, AquaCrop is water-driven, meaning that the crop growth and
production are driven by the amount of water transpired (Tr). AquaCrop focuses
on the fundamental relation between B and Tr rather than Y and ET (as in the
Ky approach), relying on the conservative behaviour of WP. A schematic representation
of these evolutionary steps is reported in the figure below.
AquaCrop includes the following sub-model components: the soil, with its water
balance; the crop, with its development, growth and yield; the atmosphere,
with its thermal regime, rainfall, evaporative demand and CO2 concentration;
and the management, with its major agronomic practice such as irrigation and
fertilization. AquaCrop flowchart is shown below.
Soil salinity, capillary rise from shallow water tables, and crop rotation
are not yet implemented in AquaCrop 3.0
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