Sensitivity Study Of Stomatal Conductance Effect On Continental River Runoff For Past And Future Climates

Plants can regulate the opening and closing of stomata in response to changing environmental conditions; in a high CO2 atmosphere they are more efficient in their use of soil moisture. The stomata do not open as much or for as long, and less water is lost from leaves to the atmosphere. Continental evapotranspiration is then reduced, more moisture is left in the soil, and this additional surface water can lead to increased continental runoff. Recently, Gedney et al demonstrated the impact of an increase in atmospheric CO2 during the period of 1960 1994 on global runoff through stomatal conductance processes. The aim of this work is first to show that this important result is also found in the IPSL OAGCM combined with the ORCHIDEE biosphere model forced with CO2 measurements throughout the 20th century, and second to investigate whether this impact is still an important process in warm and cold climates changes, using simulations for the 21st century and the last glacial maximum (LGM, 21000 years ago). Our set of experiments shows that for both warm and cold climates, the stomatal conductance is an important player in explaining an increase (decrease) in runoff occurring in warm (cold) climate, but is not exceeding a third of the global variation. The changes in transpiration during the 21st century remain important to explain the runoff increase. For the LGM, the CO2 decrease and induced stomatal conductance impact on the runoff is much weaker than the change on hydrologic parameters due to the direct effect of Surface Temperature (SST lowering) and extend of the ice sheets in the Northern Hemisphere.