The effects global warming might have are hard to predict because the global climate system is so complicated. One effect may cause another, and a cascade of problems might result. In the same way, some effects may cancel out other potential impacts.
In these pictures you can see predictions of temperature increase and rainfall by 2080. At the global level the evapo-transpiration cycle is expected to speed up. This means it would generally rain more, but also evaporate fast - leaving soils drier during critical parts of the growth season.