From the City of Tampa website FAQ:
"My water meter box is always full of dirt or debris. Does this mean my meter isn't being read?"
Dirt over the meter does not mean your meter is not being read. It is very common for meter boxes to become filled with dirt, leaves and other yard debris. There are many factors contributing to this. Meter boxes are generally slightly lower than ground level and are also typically at lower spots in your yard, so all run-off makes it into the meter box. It is our procedure to clean out the meter boxes so that the meter register is free from dirt and debris and easily read. It is common to clean out a meter box one month only to find the meter covered the following month. All it takes is a good rain or watering of the lawn for dirt from run-off to resettle over your meter. Dirt also can cover the meter as a result of insect or rodent activity or from slamming the meter box lid. Usually when we have reports of meter boxes being full of dirt and debris, the debris is easily cleared away and a meter reading can be taken.
To assist the reader, please keep the meter box free of garbage cans, boxes, piles of yard waste, or parked vehicles. It is against Tampa City Code to cause a meter to be inaccessible. Please do not install fences or plant trees or shrubs that restrict access to your meter or they may need to be removed.

- See more at: http://www.tampagov.net/city-of-tampa-utilities/how-do-i#dirt%20in%20meter%20box