Al:
Dankoff pumps makes various pump lines, and a number of models with in each pump line. Based on what you have said, I would look at their Suncentric pumps. An Example is:
http://store.altenergystore.com/Solar-Water-Pumps/Surface-Pumps/Suncentric-Centrifugal-Pump/SUNCENTRIC-7426-DC-SURFACE-PUMP/p387/I would attempt to do it with a direct connection to a PV array. That will be the least expensive.
However, because your system works on emitters at x gph your 8 hour pumping time may be fixed and it may mandate batteries. With batteries, you could also build in a reserve of 2-3 days pumping, if desired.
Another key point would be the minimum psi required by the emitters and the height of the highest emitter above the water.
Also, what months do you need to irrigate ? If you have to irrigate in the October-Feb period, you may have to upsize the PV array.
With that data, they can start sizing a system out for you.
From the looks of it, you might get by with one pump directly from the river.
If you ran new piping for the solar pump and Tee it in some place just down stream of the gas driven pump from the tank (installing valves in both lines, just in front of the Tee), you could keep the gas driven system in place for back up on cloudy days. Or, If you can only get 6 hours of pumping from the solar system, you might be able to switch over for the final two hours of pumping.
I would write an e-mail to the sales staff here at the Alt-E-Store, giving them the info you told us, the info that I pointed out, and ask for their recommendation and cost for a system.
Ken
