Improving irrigation schedules by using a biophysical and a decisional model

Abstract New irrigation scheduling approaches aimed at increasing efficient use of the allocated irrigation water, so as to give the highest crop production with the least water use, must be developed. Models can be used for such a purpose as they enable rapid and extensive condition testing. This study uses the management-oriented cropping system model MODERATO to improve irrigation schedules of a corn crop on a medium clay-silt soil of southwestern France, with no constraints regarding the amount of water for irrigation. Weather records from 1949 to 1997 are available of the study site. A ‘basic’ strategy resulting from discussions with the irrigation advisors is tested first. Then the starting, returning and ending rules for irrigation are optimised. The optimisation process is based on a step-by-step screening of the parameters for each rule. The calculated optimal strategy is the one which gives the highest average direct margin from irrigation over the 49 years of the weather records. Applying the ‘basic’ strategy gives a 10.30 Mg ha −1 average yield using 312 mm of irrigation water. The average direct margin is 454.76 € ha −1 . Optimising the starting and returning rules leads to a 555.68 € ha −1 average direct margin with a slightly lower yield (−0.11 Mg ha −1 ), but the amount of water used for irrigation is significantly reduced (−111 mm). Optimising the ending rule allows a further 7.62 € ha −1 , mainly due to an average 20 mm decrease in irrigation water used. The use of MODERATO as a training aid for irrigation problems is then discussed and considerations regarding the calculated optimal strategy are debated.