Isotope studies have shown that almost all oceanic moisture falls as rain within the first 150 miles from any coast. All the rest of the rainfall on land is recycled water, evaporated from the land and the vegetation on it. Bare land such as sand or rock desert can only evaporate a small amount of the water before it runs off back to the sea. Farmland will hold a little, but the best reservoir is a forest.
A single oak tree may have ten to thirty acres of leaf surface, so forests are the best thing for ensuring that inland areas get rain. In fact, to destroy the Amazon forest, all we need to do is to chop down the first 200 miles, and we’re busy doing that now.
Until relatively recently, most scientists thought rain was caused by mineral particles in the air which were just the right size for water (or ice) to condense around them. Research is beginning to show that a major factor in rain creation could be bacteria associated with plants. http://en.wikipedia.org/wiki/Bioprecipitation
If this is true then it is yet another way in which forests create rain. A forest has a far greater plant surface area than farmland or desert.
The image shows two views of the rabbit-proof fence, stretching aobut 2,000 miles across South-Western Australia. It separates off native land from farmland with the idea of keeping rabbits out.
Clouds form a lot more on the native side than the other, showing a correlation between farmland and decreased rainfall. That’s it really, but there are some speculations and other stuff in a NY Times article about it.
There is a research paper by Tom Lyons of Murdoch University, impact_of_clearing (pdf) which goes into detail, and an abstract of another paper: The role of land use change on the development and evolution of the west coast trough, convective clouds, and precipitation in southwest Australia on the JGR site
(And the fence, it doesn’t work, obviously — what kind of idiot would try to build a 2,000 mile long version of something they haven’t invented a one mile version of yet? And more to the point, what kind of idiot would pay them to do it? Oh, hang on, the kind of idiot that would chop all the trees down and expect to just keep on farming without them forever. The fence builders saw them coming didn’t they?!