HISTORIC POPULATION PROJECTION MODELS FOR LARGE URBAN CENTERS: ARE THEY MISSING AN IMPORTANT FACTOR?

Population projections provide the base data used in models that analyze future social, economic and natural resource needs. These models are used by all levels of leadership to develop plans that determine future spending and resource allocations. The accuracy of the future-needs projections depends on the accuracy of the population projections and therefore a rigorous analysis of the factors that impact the future projections is beneficial. Most projections involve a cohort-component type of analysis with birth, death and mobility factors applied to current population measurements. The Demographic Transition Model recognizes that with increased industrialization birth and death rates become more stable and typically level off at a value near replacement; therefore, the third factor of mobility, which includes immigration and relocation, becomes more important in developed countries like the United States. This paper evaluates the mobility factor growth relationship of the eleven largest United States urban centers to their surrounding adjacent and second tier counties. The results indicate that a strong positive correlation with a relatively constant relationship is evident between the large-city county and the adjacent counties. The correlation also indicates that over the past forty years the adjacent counties have exhibited a growth rate that is on average six percent higher than the urban center county.

Three of the large urban centers in the study are located in the state of Texas and these study results are compared with published population projection numbers as prepared by the State of Texas. A comparison of the projections with historical relationships indicates that the projections do not always accurately reflect the mobility factor relationship between the large cities and their adjacent counties. State agencies like the Texas Water Development Board use these projections to determine water demand projections for future needs and incorrect population projections may misdirect water resource planners. In order to avoid a misallocation of future resources, the mobility factor used in population projections should reflect the strong positive growth relationship between large urban centers and adjacent counties.