Modelo de otimização para o dimensionamento e operação de sistemas de microirrigação.

Abstract

An optimization model has been developed for minimizing the total costs (investment and operational) of microirrigation systems for leveled and unleveled areas, considering the influence of the water demands, the division of the system in subunits, the number of operational units, the shape of the irrigated area and the operation with changeable tariffs of energy with the irrigation schedule effects- The model was applied in microsprinkler system, for a referencial hypothetical area of 34 ha (340000 m2) with the culture of the mango (Mangifera indica L.), The water demands were computed by the annual climatic water balance and monthly soil water balance, considering the annua! and monthly average precipitation with 75% of occurrence probability and the reference évapotranspiration for two localities in the Northeasteam region of Brazil with climatic diversities, Iguatu (Ceará State) and João Pessoa (Paraíba State). The otimizatiou technique was based on nonlinear programming through the Matlab software. The water demand computed by the annual water balance underestimated the necessities of irrigation in relation to the monthly water balance, leading to a shorter time of functioning of the system and, consequently, smaller operational energy cost. However, the investment costs did not suffer important alterations with the two water balances for the same number of operational units, thus evidencing that there was no modification in the design for the two situations. In João Pessoa it was possible to increase the number of operational units, leading to a higher reduction of the investiment costs and the total costs. As for the application of the energy tariffs, the model allocated the functioning period to the lower tariffs time, promoting significant saving in the operational costs. The changeable tariffs did not affect the sizing of the system. Thus, for the currently operation systems measures of energy saving can be adopted, through modifications in the daily periods of operations. The investiment costs represented almost the total amount of the costs. The main costs was those of the lateral polyethylene lines, the emitters, the PVC tubing and the water treatment system. The factors that most affected the total costs of the system was the number of subunits and operational units, having lesser effect the shape of the area. The increase in the number of subunits and operational units of the system reduced the investiment costs significantly. However, the subunit of minimum cost and bigger number of operational units did not guarantee the attainment of the minimum cost system. The subunit of minimum cost for one determined service pressure was that one that uses all the head loss available. The preestablishment of percentages of head loss in the lateral lines and manifold lines did not guarantee the attainment of lhe subunit of minimum cost. The system of minimum cost was achieved with the minimum service pressure of the emitter, lower emission unifonnity and lower irrigation frequency. The system design without considering the energy cost was inefficient under the energetic and economic aspects.