is made in succeeding years from 1980 to 2000, the discounted cost of
photovoltaic systems are compared with the discounted energy cost of a
conventional electric powered irrigation system. Economic feasibility
is said to be established in the first year in which an investment in a
photovoltaic system has a discounted cost less than the corresponding
cost of a conventional electric powered system.
 From the above discussion it should be apparent that most of the
technical variables entered into equation (9) such as the number of
irrigations per year, and the overall irrigation strategy remain fixed
over time. Given the variation in weather and other factors over time,
it is highly unlikely that any of these technical variables are in fact
constant. Similarly, escalation rates in electricity prices almost
certainly vary from year to year rather than exhibiting some constant
real rate of increase. However, when evaluating an investment over
a considerable length of time, detailed information on the behavior of
technical and economic variables can seldom be known with a high degree
of certainty. In such circumstances, the decision maker must formulate
expectations concernin g how factors affecting the investment decision
will change over time. Within the present context, the fixed nature of
the technical variables and rates of change in the economic variables
can be construed as representing a given set of expectations. Thus,
the economic feasibility criterion estimated in this component can be
considered as based on various technical and economic expectations
specified in terms of the simulation program's input variables.

 AN APPLICATION TO CITRUS IRRIGATION

 The simulation model was utilized to examine the economic feasibility
of utilitizing photovoltaic arrays to power permanent overhead citrus
irrigation systems in the Ridge area of central Florida.1 The initial
investment year was varied from 1980 to 2000 assuming an investment life
of 20 years. The choice of a permanent overhead type system rests mainly
on the fact that about 90 percent of the acreage utilizing these systems


 11Counties located in the Ridge area of Florida can be found
in Stanley et al. [1980, p. 4].