Why P1=pgh/2

Let the width of the liquid stream be taken as 1 unit when seen from the right side. Therefore, Area of the left liquid column (A1) is given by A1=H1=H Here height of water column behind the jet is H. Area of the right liquid column (A2) is given by A2=h1=h Here height of flow in channel is h. Step 3/5 Static pressure on the left side of the jet is P1=2gH Static pressure on the right side of the jet is P2=2gh