The picture above is of a stoma. Stomata are small (microscopic) structures in the leaf of a plant that can open and close. The plant uses the stomata for gas exchange, to take in CO2 and let out O2 during normal photosynthesis. The subtle yet important bit to recognize here is that when the stoma opens to take in CO2 it also loses water vapor which has to be replaced through the roots of the plant.
According to an article written by Richard A. Betts et al. and published in this weeks Nature, the billions and trillions of stoma the world over have a large impact on continental water runoff. The new model predicts that as the CO2 concentration increases in our atmosphere plants don't need to open their stomata as often. As a result, plants need less water, and so less water is absorbed. Multiply this several more million billion times for an ecosystem and the effect is like squeezing a water-logged sponge, but on an entire ecosystem level.
We find that the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by 6 per cent relative to pre-industrial levels...The implications of this finding are important for climate forecasting, city planning, water treatment and disaster preparation. While more available water sounds like a good thing, the ability of ecosystems to absorb large floods of water, and then slowly release that water over time is critical to many cities water supplies, or for protection from severe weather events.