Kruijt, B., Witte, J.-P.M., Jacobs, C.M.J. and Kroon, T. 2008. Effects of rising atmospheric CO2 on evapotranspiration and soil moisture: A practical approach for the Netherlands. Journal of Hydrology 349: 257-267.
The authors indicate that state-of-the-art climate models have been used to predict there will be "temperature-induced increases in evapotranspiration" with continued increases in anthropogenic CO2 emissions, and that this phenomenon will lead to reductions in soil water content. On the other hand, they report that experimental observations actually show that "elevated atmospheric CO2 concentrations tend to reduce stomatal opening in plants," and that this phenomenon "tends to lower transpiration rates," which should lead to increases in soil water content. The big question, therefore, is which of these two competing effects is stronger?
What was done
Kruijt et al. explore this important question via a coupled set of hydrological models -- which incorporate what is known about the effects of atmospheric CO2 enrichment on foliar stomatal impacts on plant transpiration -- and climatic scenarios pertaining to the Netherlands that were produced by climate models employed in the Third and Fourth Assessment Reports of the Intergovernmental Panel on Climate Change.
What was learned
The four Dutch researchers determined that the "direct effects of CO2 reducing evapotranspiration can be expected to be moderate, up to 5% in the coming 50 years and up to 15% by 2100, with relatively stronger effects in summer and in rougher, natural vegetation such as heath lands and (deciduous) forests." In addition, they say that these effects are "of similar order and opposite to the previously projected effects of [rising] temperature on evapotranspiration."
What it means
Kruijt et al. conclude that incorporating the direct effect of atmospheric CO2 enrichment on plant stomatal function in soil water balance calculations "is likely to result in larger ground water storage than expected in current climate scenarios," which bodes well for the Netherlands in the years and decades ahead.