Land Use Change Modeling in Brief
The land-use team simulated how land moves among agricultural, forest, and urban uses. These transitions are a function of economic returns to alternative uses, which are determined themselves by site characteristics such as farm rents, distances to cities, and population and income of cities. Returns to land and land-use transitions also are influenced by urban growth boundaries (UGBs), the land use planning lines that restrict development. Here we provide a brief explanation of land-use change modeling in Willamette Envision. For a more detailed explanation refer to Jaeger et al. (2016) and Bigelow (2015).
During a model run, Willamette Envision simulates land-use changes annually at the scale of parcels (referred to as Integrated Decision Units or IDUs within Willamette Envision). Growth in population and income increase the returns to developed uses relative to forest and farm uses, while agricultural land values are influenced by the availability of water for irrigation. We derived functions that estimate the economic returns to each land use from historical data for the Willamette Valley (Bigelow, 2015). In response to changes in the relative returns to different uses, IDUs may shift among agricultural, forest, and urban uses as the model runs. Land-use changes in the model are governed by probabilistic equations estimated with historical data from the National Resources Inventory (NRI).
To account for zoning rules under Oregon’s land-use planning system, the model treats land inside and outside of UGBs differently. Land outside of UGBs can move between undeveloped uses (i.e., ag-to-forest and forest-to-ag transitions are allowed), but transitions to developed use are not allowed. For IDUs inside of UGBs, all of the transitions are allowed, but development is treated as irreversible (i.e., once a forest or agricultural IDU is developed, it cannot return to its original use). Given the irreversibility of development, it follows that, over time, the share of developed land within each UGB will increase. To mimic the land-use planning process, we allowed for UGBs to expand once the developed share became sufficiently large. Once a specified threshold is exceeded, a UGB expansion will be triggered. UGB expansions occurred in a way that approximated rules under the land-use planning system. For example, land zoned for Exclusive Farm Use and Forest Conservation will be brought inside of UGBs only when other opportunities are exhausted. In the case of the Portland Metropolitan Area, priority is given to areas designated as urban reserves.
Population and Income Growth
Willamette Envision treats population and income growth as external drivers, forces that are determined outside of the model and can be varied for different modeling scenarios. The population and income projections adopted for the Reference Case and many other WW2100 modeling scenarios, were based on forecasts from the Oregon Office of Economic Analysis (2011; OEA) and Woods and Poole Economics, Inc. (2011). The OEA forecasts population for each county to 2050, while Woods and Poole forecast mean household total personal income to 2040 (in inflation-adjusted dollars). We used linear extrapolation to 2100, which implied diminishing growth rates over time for both variables (Fig.1). Projected county population was allocated to areas within UGBs and rural residential zones based on population percentages in the 2010 Census. Rural residential zones were allowed population growth until they reached a density of one household for every two acres of land. Once these areas were full, all new population was added within UGBs. All cities within a county were assumed to have the projected mean household income.