My land is steeply sloping and we changed our quad bike to a beasty Honda (500cc, think it's 35hp) because the smaller one wasn't up to the job of pulling a quad trailer up the fell.
Now admittedly the trailer often had in four sheep or a load of muck - but water isn't light either, if you think that 50 litres is going on for 8 stone, so a large teenager or small adult.