The strategy of the study required a model of urbanization that could be used to predict how the region would change with the existing trends of development and as a result of alternative conservation and development strategies. The model of urbanization was formulated conceptually and logically, then organized as a GIS-based allocation algorithm.

As described in the flow chart in Figure 11, the model begins with a map representing the state of urbanization at time 1 (1990+), and results in output maps of future urbanization at future times (in this case, 2010 and 2030).

The exogenous changes that trigger the model are new population and the resulting demand for land for new urban uses. New development is allocated according to the four primary categories of urban development recognized by California regional planning agencies: commerce and industry, multi-family residential, single-family residential, and rural-residential development. Multi-family residential occurs at about 20 units per hectare; single-family residential is considered to be 10 units per hectare; and rural residential is considered to be an average of two hectares per unit.

The algorithm begins with an assessment of the land available for future development. Developable land is unbuilt and privately owned.

The model then considers existing local plans, zoning, and other constraining regulations. In the trend, all future allocation will conform to these constraints.

Next, the model identifies zones of development priority based upon two assumptions: that new development occurs on or near existing, accessible utility infrastructure; and it occurs close to existing transportation networks (Figure 12). Three priority levels are defined, the highest of which has existing utility infrastructure and is within 100 meters of major roads and intersections.

In addition, allocation of urban development reflects regional differences in land desirability from the point of view of potential developers. As seen in Figure 13, the land most desirable for development is close to the coast, and intensity of new construction tends to decrease with increasing distance from the coast.

The combination of these attractive factors–available land, zoning, infrastructure, and regional desirability–results in a map of ranked priority zones for each land use group.

While recognizing that these variables are important, they are by no means the only factors governing new development. Therefore, the model incorporates an element of randomness into the new urban development allocation. The randomness does not violate any of the previously described constraints or demands.

In each time period (1990-2010, 2010-2030), locations for each of the four major groups of new development are determined in an iterative four-stage process based upon the assumption that priority among land uses is determined by ability to pay. This results in an allocation sequence of commerce and industry first; second, multi-family; third, single-family; and last, rural residential development. The land uses are allocated serially, following traditional pyramidal zoning. That is to say, purely residential zones do not contain intensive activities such as commerce and industry, whereas residential development can and does occur in commercial and industrial zones.

The first use of the model of urbanization projected the trend of future development based on an extrapolation of population forecasts by the State of California.

As can be seen in Figure 14, the population of the study area is currently about 1.1 million. It is projected to grow to about 1.6 million by 2010 and 2.1 million by 2030. Of the counties included in the region–Orange, San Diego and Riverside–most of the increase is expected to take place in Riverside County because it has the most unbuilt land and lower costs of land and construction. Based on current patterns, population growth will cause shifts in lands use as shown in Figure 15.