230gr and xoruss, I think it would really be worth the effort and expense to have everything ready to rig a trolley line to retrieve the water carts. Not that difficult, nor all that expensive to rig for 100 gallons or less. Some good, stout, long anchors stakes, couple of good pulleys with attachment rings, and appropriate rope. I figure both of you can figure out whether you need a gun, luff, or double tackle rig. Even a simple laundry line rig might work for you, xoruss, since you said it is pretty flat.
Either way, making a simple mechanical winch drum powered by hand, AC or DC electric motor, or gasoline/propane/diesel engine, could be used to run the water trolley back and forth.
Just my opinion.