significant negative term in equation (9) is the PC. term. Given that
the cost of photovoltaic systems in the future is expected to decline,
equation (9) demonstrates that the rate of decline will be a significant
factor in determining when photovoltaic irrigation systems will be eco-
nomically feasible. In addition, VSEC and VCEC enter equation (9)
with a positive sign (note VSEC < 0 which implies -VSEC > 0). Thus,
increases in conventional fuel prices or the value of electricity sold
to the utility also offset the fixed array cost.
 Examination of equation (9) also reveals that the fixed cost of
water dispersal system structures (SC.) drops out of the equation when
similar irrigation systems are being compared. This could also occur
if different system types have similar structure costs. In addition,
when the pump-power units being compared are both electric, ;he cost of
these units (PPUe) also drop out of equation (9). The economic feasi-
bility of photovoltaic systems then rests primarily on a comparison of
the discounted energy costs of irrigation over a fixed time horizon.

 SIMULATION MODEL FOR ASSESSING THE ECONOMIC FEASIBILITY OF
 PHOTOVOLTAIC POWERED IRRIGATION SYSTEMS

 This section describes the computer simulation model used to deter-
mine the first year in which photovoltaic powered irrigation systems
will be economically feasible. Because the economic feasibility of these
systems depends on a number of assumptions regarding energy price increases,
photovoltaic system costs, irrigation levels, and other technical and
economic factors, the simulation model is designed to facilitate a wide
variety of assumptions via user specified input variables.
 The model can be used to simulate a number of different types of
 irrigation systems by appropriately specifying a set of technical input
 variables. However, the model can only compare photovoltaic powered
 systems with conventional electric powered systems. The simulation
 model is composed of four basic components (Figure 1),whose functions


 The term (PPUk PPUe) k = g,e,d,l is probably positive. However,
 even if it is negative, the -PC. term would dominate this term in equa-
 tion (8).