Simulation of Projected Water
Demand and Ground-Water Levels in the Coffee Sand and Eutaw-McShan Aquifers
in Union County, Mississippi, 2010 through 2050
APPENDIX A. Derivation and methodology of the projections for population, employment, and median household income for Union County, Mississippi
Projections of population, employment, and median household income to the year 2050 were prepared for this study by Dr. James H. Eblen, Tennessee Valley Authority (written commun., 1999). The methodology and results were reviewed by Dr. Charles Campbell, Economist, Mississippi State University (written commun., 1999). The following counties are included in the Tennessee Valley Authority (TVA) area of northeastern Mississippi: Alcorn, Atalla, Benton, Calhoun, Chickasaw, Choctaw, Clay, Itawamba, Kemper, Lafayette, Leake, Lee, Lowndes, Marshall, Monroe, Neshoba, Noxubee, Oktibbeha, Panola, Pontotoc, Prentiss, Tallahatchie, Scott (south of Leake County), Tate, Tippah, Tishomingo, Union, Webster, Winston, and Yalobusha (fig. 1).
The economy of any small area, such as a county, depends on the economy of a larger area. Union County is a part of the economy of northeastern Mississippi and of the Nation. Therefore, forecasts and projections for these larger areas were utilized, to the extent available, to prepare the projections for Union County.
Specific forecasts and projections include TVA economic forecasts for northeastern Mississippi and corresponding national forecasts to the year 2021, along with U.S. Census Bureau projections of the population of the United States to 2050. The northeastern Mississippi forecasts were generated by TVA's Economic Forecasting system, whereas national forecasts are from Standard and Poor, Data Resources Incorporated (DRI). A linear-trend model was used to prepare the Union County projections. Trends were applied either to the data series itself or to a derived data series that shows the relation between the data series and a related series for northeastern Mississippi or for the Nation. The derived series are the "shares" referred to later in this Appendix.
A projection extends the past into the future and is an accurate predictor of the future only if the future events follow the pattern established by that segment of the past. The projections provide a range of values for population, employment, and income. The normal-growth projections in this study show what would happen if the growth pattern of the last two decades continues. For the high end, alternative projections were developed by various methodologies designed to capture the likely impacts of continued development at the higher rates seen during the last few years. Together, the normal- and high-growth projections provide a range within which growth can reasonably be expected to occur.
The normal-growth projections of population, total employment, and manufacturing employment growth for Union County since 1989 were calculated to 2021. The county shares of the northeastern Mississippi area are projected to continue to change at a reduced or dampened rate in the same direction as indicated by the trend since 1989. The dampening effect assumes that by 2021, the county will be growing at the same rate that northeastern Mississippi is growing. Projections were extended from 2021 to 2050 by extending the linear trend of the relation of each variable to changes in the population of the United States (table A1).
For normal-growth projections of median household income, a linear trend was used to calculate the United States median household income to 2050 by using data from 1979 to 1997. Then for Union County, the linear trend of median household income as a percentage of the national median household income was extended to 2010. From 2010 to 2050, the assumption was made that the median household income would increase at the national rate.
High-growth projections of population for Union County were calculated by a linear trend line from 1995. The decision to begin the historical period with 1995 assumed that the relatively high-growth rate from 1995 to 1998 would continue.
High-growth projections of total employment to the year 2021 for Union County were calculated similarly to the high-growth projections of population. These shares were then extended to 2050 by using the relation of total employment to the United States population. The employment projection for 1998 was adjusted to account for the recent job increases associated with a new major facility. A linear trend based on 1995 to 1998 employee counts was then used to project total employment to 2050. High-growth projections of manufacturing employment were determined by a linear extension for Union County trends. Nonmanufacturing employment is the difference between the total and the manufacturing employment (table A2).
High-growth projections of median household income were prepared by adjusting the normal-growth projections upward by the difference in the DRI control and high forecasts for the Nation. Ratios were held constant after 2021, the last year of the DRI forecast.