Assessment of the FAO AquaCrop model in the simulation of rainfed and supplementally irrigated maize, sugar beet and sunflower

Farming in Serbia is traditionally rainfed. Analyses show that drought events of varying severity are frequent in this region, although there is no specific pattern. There is a distinct need for an objective assessment of the impact of drought on strategic field crops, to solve the dilemma whether irrigation is required or not. For this reason, and based on available field data, the FAO AquaCrop water driven model was selected to simulate yield and irrigation water use efficiency (IWUE) for three major field crops (maize, sunflower, and sugar beet), under two scenarios: (1) natural water supply and adequate supply of nutrients, and (2) supplementary irrigation and adequate supply of nutrients. The experiments presented here were conducted between 2000 and 2007 in northern Serbia, where chernozem soil is prevalent. Data of 2003 cropping seasons were used for local calibration, whereas the remaining years for validation. Results were such that local calibration resulted in very minor changes of AquaCrop coefficients (e.g., maize basal crop coefficient, sunflower harvest index, etc.). Simulated maize yield levels exhibited the greatest departure from measured data under irrigation conditions (-3.6 and 3.3% during an extremely dry and an extremely wet year, respectively). Simulated sunflower yield levels varied by less than 10% in 8 out of 10 comparisons. The most extreme variation was noted during the extremely wet year. The difference between simulated and measured values in the case of sugar beet was from -10.2 to 12.2%. Large differences were noted only in two or three cases, under extreme climatic conditions. Statistical indicators - root mean square error (RMSE) and index of agreement (d) - for all three crops suggested that the model can be used to highly reliably assess yield and IWUE. This conclusion was derived based on low values of RMSE and high values of d (in the case of maize and sugar beet 0.999 for both yield and IWUE, and in the case of sunflower 0.999 for yield and 0.884 for IWUE). It is noteworthy that under wet conditions, the model suggested that sunflower and sugar beet do not require irrigation, as confirmed by experimental research. These data are significant because they show that the AquaCrop model can be used in impartial decision-making and in the selection of crops to be given irrigation priority in areas where water resources are limited.