"Loss of Head" is:

RAPID SAND OR MECHANICAL FILTERS Filtration removes the remaining alum not removed by sedimentation. As filtration proceeds, the suspended impurities and bacteria clog the filters resulting in their reduced efficiency called "loss of head”. Backwashing Rapid sand filters need frequent washing daily or weekly, depending upon the loss of head. Washing is accomplished by reversing the flow of water through the sand bed, which is called "backwashing". It helps in dislodging the impurities and cleaning up the sand bed.  Essentials of preventive and community dentistry  Soben Peter  5th edition