I would suggest thinking about it like this.
Start with an integrator to represent the tank. The tank level is the integral of the in flow minus the out flow. So make an integrator that integrates the difference between those two signal. The inflow (if any) is probably an independent input and could come from some kind of source block. Then work backward to get the out flow. The out flow is the sum of the flows through each spray nozzle, so you need a summer to sum up the individual flows. Each flow can be found knowing the pressure differential across the nozzle. So for each nozzle make a block (subsystem) that can convert an input pressure differential to a flow (you'll need to figure out the details of what's in there but it's just an algebraic relationship, no integrators). Now you have to keep working backward to figure out the upstream pressure. This is probably the tank pressure minus any piping losses. These losses are a function of the spray flow rate. So make a block (subsystem) that captures this relationship. The input to this block is the flow rate. You have the flow rate as the output to one of your previous blocks so you can wire that back. Then use a summer with a plus and minus to subtract the pressure loss from the tank pressure. It will have as inputs the tank pressure, and the pressure drop you calculated in your previous block. Finally you need the tank pressure. This is just a function of the tank level. So make a little subsystem to compute tank pressure as a function of tank level (again a relatively simple algebraic function, e.g. p = rho*g*h), The output of this block gets wired to your piping loss block and you are done.
So the idea is you start somewhere in your system and just make a block that computes some output that you will need in terms of some inputs. If you don't have all of the inputs yet don't worry, just leave it disconnected. Continue to work your way backwards to calcuate the unknow signals ,until you have values for all of the signals you need and they are all connected up. I like to start with integrators as the outputs of these are the system states, In this case there was just the tank level, so I started there.
