The graph shows the amount of water that remains in a barrel after it begins to leak. The variable x represents the number of days that have passed since the barrel was filled, and y represents the number of gallons of water that remain in the barrel. A graph titled Water Supply with number of days on the x-axis and gallons of water left on the y-axis. A line goes through points (6, 33) and (15, 15).

What does the y-intercept represent?
the amount of water that is added to the barrel each day
the amount of water that remains in the water barrel
the amount of water that is initially in the water barrel
the amount that the water barrel decreases each day