Local impacts associated with elevated atmospheric CO₂ on water productivity of corn in Drome, France | Impacts locaux attendus de l'augmentation du CO₂ dans l'atmosphère sur la productivité de l'eau du maïs dans la Drôme, France

This study was conducted to assess local impacts of doubling of the rate of atmospheric CO₂ (660 ppm) on irrigation demand and maize crop water productivity in the Drôme valley (SE France). This region is characterised by severe water shortages in the summer season. A comparison of historical records, between the periods of 1990-2001 and 1960-89, shows that in July, the mean temperature increased by 2 °C and the river stream flow diminished by 16%. At peak periods, the maize irrigation requirements represent 80% of the seasonal irrigation demand. The ARPEGE-CLIMAT atmospheric model, regionalised at high resolution on the Rhône catchment, was used to calculate monthly climate variations. For a doubled rate of CO₂, in July, an increase of 4 °C in average temperature and a decrease of 30% of rainfall are expected. Monthly climate variations were introduced into the stochastic weather generator LARS-WG to simulate local daily climate scenarios for present (1 x CO₂) and future (2 x CO₂) climates. Then, the STICS crop model was applied to simulate the direct and indirect effects of increasing atmospheric CO₂ on irrigation demand and maize crop production. Several simulation conditions, such as soil types, sowing dates and water stress levels, were studied for current climate and future climate scenarios. Principal results show that global warming increases water irrigation demand by 14%; lowers the maize cultivation cycle by 20%; and reduces the yield by 15%. On the other hand, a direct increase in CO₂ concentration induces an average yield increase of 18%; these opposite effects balance the maize yield. The maize crop water productivity increases by 13% but it depends greatly on the water stress level. Similarly, earlier sowing dates allow earlier irrigation and a reduction of the water application depth by 40 mm in peak water demand periods and low water availability.