Wyoming State Water Plan, Wyoming Water Development Office
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Green River Basin Water Plan
Technical Memoranda

SUBJECT: Green River Basin Plan
Population Projections

PREPARED BY: Gary Watts, Watts & Associates, Inc.


Introduction

This memorandum presents population projections for the Green River Basin and its communities and rural areas for the time period from 2000 through 2030. The projections provide a basis for estimating municipal and domestic water needs in the Basin over the next 30 years. They also provide a basis for assessing water-based recreational resource needs.

The river basin planning process developed by the Wyoming Water Development Commission (WWDC) specifies that population projections be developed 30 years into the future for each river basin for each of three planning scenarios:

  1. Low Growth

  2. Moderate Growth

  3. High Growth

The rationale and approach employed in developing population projections for the Green River Basin are described below.

Rationale and Approach

Generally, there are three commonly accepted methods for producing population projections: 1) time series analyses, 2) cohort survival analyses, and 3) employment-driven approaches. Each approach has advantages and disadvantages that must be evaluated in specific planning applications. Some methods combine more than one of the three approaches to produce projections.

Time series analyses typically involve assessing past growth trends and using those trends to project population into the future. Analytical methods used with this approach include computing average annual historical growth rates and fitting linear or non-linear equations to historical population data or indicators of population growth such as school enrollments, sales tax revenues, or other variables. Projections are usually based upon the assumption that future population changes will mimic past average annual growth rates or trends. The time series approach is relatively easy to implement, and its results are easily understandable. Its main disadvantage is that it does not explicitly consider the three primary determinants of regional population change: mortality rates, fertility rates, and migration patterns. Instead, the method implicitly assumes that past trends in relationships among these variables will continue into the future.

The cohort survival approach explicitly recognizes the three primary determinants of population change and requires forecasts of future mortality and fertility rates as well as net migration patterns. Projections of mortality and fertility rates by age and sex group (cohort) are available from the U.S. Census Bureau (USCB), as are projections of net migration patterns for different regions of the country. Projections of change for each cohort can be combined to produce forecasts of future population for a given geographic area. The advantage of the cohort survival approach is that it explicitly recognizes the determinants of population change and takes them into account in developing projections. Its primary disadvantage is that in rural areas with relatively small population bases, the effects of changing migration patterns over time can overwhelm the effects of changes in mortality and fertility rates, thus making the projections extremely sensitive to assumptions about future net migration patterns.

Employment-driven approaches to population projections are based upon the assumption that net migration patterns are primarily determined by job availability, and that job availability can be projected into the future with reasonable accuracy. Thus, if we could accurately project how many jobs would be available in the Green River Basin in the year 2030, we could estimate population in that year based upon estimates of how many people would be associated with each job. Estimates of the number of persons associated with each job may or may not explicitly take into account such factors as labor force participation rates, fertility rates, and mortality rates. If these factors are taken into account, the employment-driven approach becomes a special case of the cohort survival approach where net migration estimates are based upon employment forecasts.

The employment-driven approach incorporating labor force participation, fertility, and mortality rate changes is the most sophisticated approach commonly used for projecting regional population changes and can provide relatively accurate forecasts over relatively short time frames. Its primary disadvantage lies in the difficulty of forecasting economic activity and associated labor requirements by economic sector over long time periods. Even if future economic activity by sector could be forecast reliably 30 years into the future (a doubtful proposition), technological changes that affect the amount of labor needed by each sector often cannot be forecast reliably. For example, Wyoming's Powder River Basin has become the world's leading coal producer in recent decades with little growth in employment opportunities because of productivity gains associated with mechanized mining equipment. Accurately forecasting current levels of mining employment in the Powder River Basin 30 years ago would have been very difficult because the technology that determines current employment levels did not exist then.

Another disadvantage of the employment-driven approach is that migration patterns in the United States in recent decades have become less dependent upon local job availability and more dependent upon amenities and lifestyle variables such as climate, local infrastructure, crime rates, and scenic attributes. Many industries and jobs in the U.S. economy are now "portable" in the sense that they can be shifted from one locale to another based upon amenities or lifestyle variables with little change in competitive advantage.

Given the inherent shortcomings of all three population projection methodologies, there is no compelling reason to generate population projections for the Green River Basin using one method to the exclusion of others. Instead, the approach taken for this study involves using three different methods to create a range of projections representative of low, moderate, and high growth scenarios.

One of the methods involves refining and extending population forecasts for Wyoming cities, towns, and counties prepared by the Wyoming Department of Administration and Information (WDAI). The WDAI methodology incorporates elements of all three approaches described above. The second method involves refining and allocating to the Basin the population projections for the state of Wyoming prepared by the USCB. These projections were developed using the cohort survival approach with net migration pattern based upon address changes reported on federal tax returns. The third method involves a time series analysis of past population trends in the Basin to estimate a likely maximum rate of future population growth.

A primary objective of incorporating the state and federal projections into the analysis is to provide a basis for comparability with population projections for other river basins in the state. To the extent that the state and federal projections can be incorporated into the planning scenarios for other basins, there will be a consistent, comparable set of assumptions for population projections from basin to basin.

Current Population Estimates

The first step in developing population projections for the Basin was to estimate its current population. Estimates are necessary because the results of the 2000 census are not available as of the date of this report. Furthermore, it would be useful to have a consistent data source for the current population of all river basins in the state, and the results of the 2000 census may not be current when other basin plans are developed.

The Division of Economic Analysis of the WDAI produces estimates of the population of Wyoming's counties, cities, and towns on an annual basis, and projects those estimates 10 years into the future. These estimates and forecasts are available from a WDAI web site (WDAI, 1999). The WDAI forecasts for the year 2000 were used as current population estimates for this report and could be used to develop comparable current population baselines in other basins.

Because the geographical boundaries of the Green River Basin do not adhere to county lines, it was necessary to adjust the WADI county population estimates by to reflect only the proportion of each county that lies within the Basin's boundaries. That adjustment process was as follows: