Hi again, thanks for the comments.
Countryboy-The goal is to use a RAM pump to pump water since we are connecting it to existing water pipes in grazing areas. Still, the idea of the waterwheel is very interesting, and the creating a wheel similar to the Mazowe wheel (from the article you linked to) is something which I think could be very useful in more "clustered" agriculture areas, where a number of people could benefit from the waterwheel pump.
Nando-We will be dealing with relatively low head in our system, which is why we are looking at the possibility of using alternative materials. We are going to install PVC pumps and piping in a few farms, however, but that is possible mainly because we are getting funding do so from a Panama based NGO. To install pumps on a large scale would, of course, be more costly and would not be supported by most NGOs. I think you're right -that it would be quite inefficient to build the entire structure out of wood (though as elvin suggested, using bamboo may be a possibility-I have also read about some instances where bamboo was used, at least for the piping).
Are there any materials that people have in mind?
Thanks for the replies!
-Ankur