For nearly 30 years, city officials have used special tax districts to funnel public money into redeveloping some of Tampa’s poorest neighborhoods. That strategy relies on a steady rise in property values; every time values rise above a set base value, the resulting taxes go back into the area to pave roads, build sidewalks and support local businesses. But the system has unraveled during the past few years as the city and the region lost a decade or more of property value growth. The results are obvious in Mayor Bob Buckhorn’s 2013 city budget, which goes to the city council for its first public hearing Sept. 13. The city’s total revenue from the six special districts, known as community redevelopment areas, or CRAs, has fallen by nearly half since 2010 to $17 million. The decline has hit all the CRAs, some of them harder than others. Ultimately, it leaves them all less self-supporting and potentially more dependent on aid from the city’s operations budget.