Hydrology

CO2 is what plants metabolize as food. More rainfall and CO2 allows greater and longer growth cycles. Sunlight (Temperature variant) is the other factor. Too much of any of these can be detrimental to flora. An increase in plant growth allows greater penetration of rainfall, hence less runoff. Note California hillsides after a brush fire and then Winter rains...mudflow due to mass runoff and unstable soils. There is a "best condition" balance of all these based on the terrain, soil composition , Latitude, etc. Look at the Amazon forrest versus the Olympic rainforrest. Or desert thunderstorms versus Iowa cornfield thunderstorm runoff.
http://www.co2science.org/subject/r/summaries/runoff.php