Trigger output with flow meter

Hello.

I tought about controlling the watering of the plants in a solar and shutting down the pump after a certain number of liters have flown.

In my application I measure the input from the flow meter so that I can add up the volume.

The addition needs to be done every 1s and continuously. I managed to simulate this bit by using a mathematical function block. The only problem I had was that I couldn't reset the mathematical block ( I had the "last value" option checked).

I was thinking that there could be another solution to this problem. 

Can anyone help me out?

Best regards,

    Edward.