Productivity, evapotranspiration, and water use efficiency of corn and tomato crops simulated by AquaCrop under contrasting water stress conditions in the Mediterranean region

The FAO AquaCrop model has been conceived as a tool for simulating, on a daily scale, the canopy cover(CC), biomass and the actual evapotranspiration and for simulating, on a seasonal scale, the final biomass,the harvested yield, the cumulate actual evapotranspiration, and the crop water use efficiency. Thisperformance was analysed after a series of tests carried on 12 crop cycles, concerning corn and tomatogrown in the Mediterranean region, and having three levels of plant water stress: absence of plant waterstress (control), moderately stressed and severely stressed. The results highlight the effects of threefactors affecting the AquaCrop performances: the species, the level of plant water stress during the cropcycle and the output variable to simulate.The AquaCrop adequately simulates the daily canopy cover (CC) in control treatments of tomato andcorn, and in moderate stress treatment of corn. In the severe stressed treatment of corn, the simulatedvalues of CC were close to the measured values only from sowing to 60 days after sowing, after that thesimulated values do not fit the measurements.The AquaCrop model adequately simulates the daily biomass accumulation under all treatments intomato and under non-stressed and moderate stressed treatments in corn. However, the simulatedbiomass outputs were generally overestimated during the late stages of the crop cycles and, conse-quently, the yield also exhibited a tendency to be overestimated. Nevertheless, the yield overestimationcan be retained as acceptable because the normalised differences (D) between the simulations and mea-sured values were less than 15% on average. An exception was the tomato yield simulated in the severelystressed treatment, for which D was greater than 30%. In contrast, in the case of the severely stressedtreatment in corn, AquaCrop did not exhibit any aptitude for simulating the biomass or the grain yield.In fact, the model predicts the absence of any yield production, while 5 t ha−1of grain were actuallymeasured in the severely stressed treatment.The daily actual evapotranspiration simulated by AquaCrop was consistent with the observations onlyin the case of the control treatments of tomato, in all the three seasons. In contrast, for the other treatments(all treatments in corn and all stressed treatments in tomato), the quality of the evapotranspirationsimulation was poor.In general, AquaCrop underestimated the seasonal values of evapotranspiration. The normalised dif-ferences between the seasonal values of the observations and simulations are acceptable in the case ofthe tomato evapotranspiration (D = −7%). However, in the case of corn, the differences are related to thelevel of plant water stress, and they become unacceptable (D = −36%) in the severely stressed treatments.The overestimation of the yield and the underestimation of the seasonal evapotranspiration cause thesimulations of the water use efficiency to be overestimated. In the specific case of corn, due to the unac-ceptable performance of the model under severely stressed treatments, the linear regression betweenthe observations and measurements of water use efficiency is unsatisfactory.The potential uses of the AquaCrop model as a tool for research purposes aimed to enhance the waterefficiency and as a tool for managing irrigation have been deeply discussed. The paths that should befollowed in the future to improve the model simulations have also been suggested