I want to make a water-level out of garden hose for levelling the yard
(approx 200' to 300'). I want to make one with a reservoir so as to
minimize the effect of the level jumping up and down as the hose is
moved and distorted.

Most instructions for a 'reservoir level' have you destroying a bucket
by putting a hole in it. Can anybody see a problem with simply letting
one end of the hose hang into the top of the bucket or a barrel? Of
course, all air must be out of the system, as is the case with any
water level. This would be no different than hauling a portion of the
hose over a fence etc, in which case I believe the water level still
works. Thanks.