LPJmL mechanistically represents biogeochemical land-surface processes at the global scale, simulating daily water fluxes in direct coupling with the establishment, growth, and productivity of major natural and agricultural plant types at 0.5° resolution (33). Spatially explicit data on cropland extent were obtained from the MIRCA2000 land-use dataset (27), the extent of areas equipped for irrigation from Siebert et al. (56), and the distribution of irrigation systems (surface, sprinkler, and drip—applies only to simulation LPJmL–WaterLimIrr and LPJmL–PHU; see Table 1) from Jägermeyr et al. (38). Land-use patterns were held constant at year 2005 levels, and sowing dates were fixed throughout the simulation period. Crop yields were calibrated to match average (1998–2002) reported national management intensities (8).

Photosynthesis is simulated in dynamic coupling to absorbed photosynthetically active radiation, temperature (optimum for photosynthesis, maize 21° to 26°C and wheat 12° to 17°C), day length, canopy conductance, and water stress (33). Daily carbon assimilated through photosynthesis is allocated to the crop organs roots, leaves, harvestable storage organ (for example, cereal grain), and stem. Allocation to each compartment is a function of the phenological development stage and biomass increment. LPJmL computes daily water stress as the ratio of soil water supply to the crop water demand. Increased vapor pressure deficit or water stress therefore affects the daily rate of carbon assimilation through reduced stomatal conductance. The daily increment of leaf area index is also down-regulated by water stress, with lasting effects over the growing season, affecting the harvest index and thus grain yield.

Model simulations followed a 900-year (no land use) and 120-year (land use) spin-up, recycling the first 20 years of input climatology (mentioned above). LPJmL–PHU outputs were corrected for unintended multiple harvests potentially occurring within a single calendar year when harvest dates oscillate around the end of the year. In cases where multiple harvest events are detected, yields were reported for the year in which most of the growing season occurred. The full irrigation setup (LPJmL–NoWaterStress) assumes unrestricted access to irrigation water to fulfill crops’ water demand (any soil water deficit is balanced the next day).

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