The Romans built aqueducts that spanned hundreds of miles and they did it thousands of years ago. Why do you assume that today water has to be 'driven' with trucks or something?
Whatever you do, if the target is reducing CO2 emission, you'll have to account for the CO2 investment. All I am saying is it may be more cost-effective to leave the desert alone and put our Trillion trees where conditions are already great for large trees (Alaska, NZ), instead of pipe-dreaming about Roman aqueducts delivering water to a green Sahara or Gobi.
