Possible ways to take the slope over a given month only in segments where slope/derivative are negative

I have a tag that pulls tank level (measured in "feet").  Each foot is equivalent to 9,400 gallons, so over a given day, week, month, etc. I can measure the # of gallons of material pulled from the tank.  Over a month we had several trucks to this tank, so simply taking End Inventory-Start Inventory will not give total volume consumed.
 
If there was a way to measure the portions of the curve/trend that are negative I could measure the ft/min or gal/min and multiply by the time period to get the total volume consumed.  Does anyone know of a possible Performance Equation setup to measure the negative portions of the a curve's slope?