self priming can be a hairy little thing. a centrifuge pump, diaphragm pump, piston pump, roller pump. they all can partially create a vacuum to pull water up into them. to self prime them. though i honestly would not recommend it.
if your tractor is 12v and not like my old allis chalmers 1954 i want say tractor with 6 volt setup. you can buy a 12v plug in cigarette lighter pump. and then buy some generic cigarette lighter for a car or truck on the cheap. put a 15 amp fuse between the cigarette unit and battery. and have a quick easy way to connect the pump up.
and the 12v pump might be enough power to fill the barrels up fairly fast and if not. a way to easily prime the roller pump.
WARNING / NOTE: your tractor may not be setup or rather have a large enough alternator on it. to handle a 12v pump for long periods. and you may need to let tractor set there and run for say 15 minutes or more to let the battery on tractor recharge back up. but if your tractor already has say front work lights, rear work lights, actual front head lights. tail lights etc... most likely no need to worry. but if you have bare bones tractor. you may need to let tractor run a few minutes after each use to make sure battery has juice in it. so when you go to start it back up. you do not have a dead battery.
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-->strainer
-->foot valve, flapper check valve ((check valve that does not have a spring in it ))
-->hose
-->ball valve, knife valve, gate valve (any type of valve that you can easily open and close that is cheap)
-->hose
-->quick disconnect makes it easy to easily connect and un-connect hose from roller pump.
-->quick disconnect
-->roller pump
-->hose to barrels
doing above. you could trap water inside the hose that goes down into the water of the river. and might be a way to not need to prime things. though the hose would be rather heavy and very awkward.
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as far as a 12V pump. i went out and bought a very cheap sump pump like 40 bucks. only a trickle of water at 10 feet of lifting water but..... i plug in a 12v to 110 converter (most eletronic stores have them) into cigarette lighter on my S10 blazer. and have primed siphon lines to drain lakes, pump water to buckets for mixing concrete before. and it is awesome granted i dare not turn off my S10 blazer for about 15 minutes afterwards or i will need to jump it. but hey, it works. that is all i care. what i tear up errr blazer on other hand. i haven't yet. well i almost did. electronics started to get a little dim there a couple times. and had to turn off the pump.