A question from Yahoo! Answers:
Why didn’t Africans start irrigation through canals hundreds or thousands of years ago?
There are so many people throughout history just sitting around in Africa unemployed, why couldn’t they just get together and start digging canals for their descendants? They would have plenty of crops and shade by now instead of still having nothing. Is it laziness or some actual reason?
Africa is a large continent. In many places (such as equatorial Africa with its rainforests), the problem is not the lack of water, but the excess thereof. You cut down trees and clear away the brush, you plant something on the resulting patch, and three to five years later, the topsoil is all washed away by rain that now hits the ground directly rather than gently trickling down through the foliage. Now you need to clear a new patch and let the jungle to reclaim the old one. Oh, and did I mention malaria?
Where you do have deserts, you have another problem. There is no way to build canals in the sand, unless you have 20th century technology (concrete and plastic) to prevent seepage. Egyptians, for example, were well aware of Sumerians’ irrigation practices, but couldn’t use them for this very reason and had to live close to the Nile instead…