I have been using the AquaCrop model since its introduction with version 3.1. I have recently modeled some potato cultivars subject to deficit and full irrigation in a dry and hot climate in south of Iran. I used the latest version 6. Based on my several experiences in calibration and validation of the AquaCrop model (trial and error approach to calibrate the model), I have some suggestions for the model developers in order to improve the model performance in its the future versions.

We know that the AquaCrop model is basically developed for simulating irrigation managements, and it is a water-driven model for simulating yield and biomass of field crops. Incorporating more parameterization options that consider the physiology of crop would help for more accurate simulation. Below I would like to share my thoughts with those who are interested in modeling with AquaCrop and have experiences with it or are going to use it in future. I would be happy to learn from you and your experiences in what has been reflected below, or other options based on your experiences.

1) I think that the latest version is good for no water stress but still it needs more improvements for simulation under water stress, especially in hot and dry climates with high VPD (>3-4 kPa). Maybe some deficit irrigation in moderate and temperate climates (low to moderate VPD)  would still provide good conditions for simulation of deficit irrigation. I think that VPD plays a major role when we simulate deficit irrigation levels (mild, moderate, severe). 

2) The model still has shortcomings in modeling senescence and it is not able to physiologically simulate senescence particularly under high water deficit. The model cannot match well the biomass and yield accordingly. In such conditions, soil water is not simulated so good as the ones in earlier times of growth period.

3) It is better to allow the user to assign multiple values  of calibrated WP* during the growing stages instead of just one WP*. This is most important for the later stages for biomass simulation, and assigning a single calibrated WP* may not allow full performance of the model when crop mature. At least 2-3 WP* values should be considered depending on the length of growing period.

4) The interaction of water stress and heat stress is very important particularly in the arid and semi-arid areas where climate change may have higher sever effect on crop physiology. For example, for tuber crops such as potato, secondary growth may happen in later time of the season for which biomass may jump after the crops have experiences high heat and water stress during the middle of growth stages.   

5) The recommended WP* ranges for the C3 and C4 crops as outlined in the manual andthe model may not work well in hot and dry climates and deficit irrigation practices. For potatoes we chose calibrated values below the recommended one, and also in a very recent publication (Agricultural Water Management 203 (2018) 438–450), the authors have assigned WP*=21 for seed production of maize (C4) under full and deficit irrigation that is far below the recommended one. Therefore, it seems that in hot and dry climates WP* is lower than the recommended values for both C3 and C4 crops specially if crops are subject to deficit irrigation. 

Thank you and look froward to hearing more from the other AquaCrop model users!