I've been itching to calculate weighted densities using the 2010 Census data but, alas, I cannot yet. The Census Bureau has released the 2010 Census redistricting data, which includes census-tract data. But calculating a city's weighted density requires a list of all of the census tracts within the city's urbanized area. The Bureau has not yet released the data for urbanized areas (or, if it has, I cannot find it).
I'm left to speculate until the Bureau releases the full data files. My bet is that Austin's standard density (the population of the urbanized area divided by the land area) has increased but its weighted density has decreased. Just a guess.
We do have statistics for individual census tracts so there is no need to guess there. I'm struck by how uneven growth has been in the central core.
Take census tracts 6.03 and 6.04, which comprise West Campus. In 2000, tracts 6.03 and 6.04 had 5,273 and 5,199 inhabitants, respectively. In 2010, they had 7,793 and 6,496, respectively. This was a 36% increase in population. This really is remarkable growth. Austin's population grew by only 20.4% (656,562 to 790,390). Tracts 6.03 and 6.04 were already two of the city's densest in 2000 while I'll wager most of Austin's growth occurred in greenfields. It is harder to grow a built-out area's housing stock by 5% than it is to grow a cow pasture's housing stock by 5%. This shows that it is possible to accommodate population growth with infill development.
Downtown's population also boomed. Census tract 11 (which includes downtown and Rainey Street) more than doubled in population, from 2,386 to 5,512. This growth is less impressive to me than the West Campus growth, though. Downtown has (or had) lots of undeveloped land that easily could be developed as residential. West Campus did not.
The central core's growth was uneven, though. Take the Zilker (tract 13.03) and Bouldin Creek (tract 13.05) neighborhoods directly south of Downtown. The population of both neighborhoods actually declined. Zilker dropped from 3,022 to 2,871 and Bouldin Creek dropped from 5,659 to 5,488.
Part of the decline is surely due to the recession. Bouldin Creek added 232 housing units between 2000 and 2010 but it had only 88 more occupied units in 2010. The Zilker neighborhood added 334 housing units between 2000 and 2010, but it had only 56 more occupied units in 2010. There were a lot more vacancies in both neighborhoods in 2010.
But each still had more occupied units in 2010. The declining populations are due to declining household sizes. In 2000, the average Bouldin Creek household had 2.18 members; in 2010, 2.04. The average Zilker household shrank from 1.94 members to 1.78 members.
Zilker and Bouldin Creek illustrate that it takes a pretty signficant amount of new construction just to maintain the status quo in these central neighborhoods. It should be no surprise to anyone that singles and couples continue to displace households with children in these central neighborhoods. Given that trend, these neighborhoods must add a lot of new units just to maintain the same population. If Bouldin Creek had the same occupancy rate it had in booming 2000 but with 2010's average household size, it's 2010 population would have been 5,751, just 1.6% greater than 2000's total of 5,659. Those 232 new housing units were barely enough, in other words, to maintain its population. (Zilker would have seen a 10% population growth using 2000 occupancy rates and 2010 household sizes. Better, but not great.)
The numbers for Zilker and Bouldin Creek explain my prediction that Austin's weighted density declined between 2000 and 2010. I'm betting that most of the core neighborhoods -- the densest neighborhoods -- are like Zilker and Bouldin Creek rather than West Campus or Downtown