If you want, you can incorporate a built-in internal pocket for your water bladder (i.e. hydration system). The given dimensions in figure 10 can accommodate a 2.5 L bladder. The pocket itself is pleated at the bottom and has a top flap secured down with hook and loop fasteners to keep the water container in place. It is then sewn onto 1.0 oz nylon fabric with the same cut as the back/bottom panel. This entire assembly is then sewn at the wrong side (inside) of the back/bottom panel (photo 14).
To provide an exit port for the drinking tube,
cut a divot at the middle top edge of the back panel as shown
in figure 11. Next, fold divot flaps inwards and sew flat at the
folded edges to form a sort of rectangular notch. Refer to figure
12 for athe graphic representation. Then cut a 3" x 1 3/4" piece
of fabric similar to the back panel and sew the short sides and
a long side 1/4" inwards. These are the so-called "finished edges".
This will then be sewn over the notch to form a shingle-like cover.
Sew at the two short sides and at the long, unfinished edge that
coincides with the edge of the back panel. The unsewn long side
(finished edge) is where the hose exits. Photo 14 shows the water
bladder pocket assembly already sewn to the wrong side of the
back/bottom panel. The yellow stitch line (can you see it?) denotes
the outline of the water bladder pocket. Note the notch at the
top middle edge where the hose exits.