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OPTIMIZATION OF THE CAPACITY OF CENTER PIVOT SYSTEMS BASED ON THE DRY BEAN MODEL CROPGRO

Alexandre B. Heinemann and Gerrit Hoogenboom Department of Biological and Agricultural Engineering. The University of Georgia, Griffin Campus, Griffin, GA 30223, USA. Introduction Dry bean (Phaseolus vulgaris. L.) is considered to be one of the main legume crops in . The expected dry bean production for December 1999 is around 2,888,968 (Instituto Brasileiro de Geografía, 1999). For the state of Parana, dry bean is the most important crop because of its economic value. The northern part of the state has a notable position due to the possibility to grow dry bean three times per year, e.g., during the spring, summer and winter seasons. The region is considered to be a high risk area for water deficits during the spring season due to the high values of potential évapotranspiration. Water deficit is one of the main factors that limits dry bean growth at the beginning of the spring growing season, i.e., September/October, in Londrina, located in the Southern Hemisphere (OHveira & Villa Nova^ 1996). Due to the high risk of , the use of irrigation has increased during the last decade! Most of the irrigation systems are center pivots that have been designed with a water application capacity ranging fi-om 5 to 7 mm/day for an average irrigated acreage of 60 ha. Many researchers have used the DSSAT (Tsuji et al., 1994) crop simulation models to identify the optimal for humid regions, including Faria et al. (1997a). However, there is no reference about the use of the DSSAT crop models simulation to determine the optimum center pivot capacity. The objective of this study was to determine the optimum center pivot capacity and the best irrigation management sti-ategy, using the CROPGRO simulation model (Boote et al., 1998; Hoogenboom et al., 1993) for a dry bean crop, grown in Londrina, in the state of Parana, Brazil, as a ñmction of net return. Materials and methods Three center pivots with different water application capacities, i.e., 5, 6 and 7 mm/day, were evaluated. 22 years of historical records for a weather station located in Londrina, Parana,' were used to simulate the individual bean growing season. Each center pivot system was simulated for six different irrigation thresholds, ranging fi-om 40 to 90 % of the remaining available soil water content (AWC) in the top 30 cm of the soil profile. Crop management inputs included a row spacing of 0.50 m, a plant density of 24 plant/m^ and a planting date of August 15. The cultivar used was L\PAR 57 and the specific cuhivar parameters were based on Faria et al. (1997b). The net return was based on an average commodity price often years, i.e. 0 72 $/kg (FNP Consultoria & Comercio, 1998). Results The three center pivot systems that were analyzed showed a variation in net return (fig. 1). The range in net returns is a result of a difference in water needs for the 22 years, caused by the variability in rainfall amount, e.g., a minimum of 260 mm and maximum of 563 mm for the growing season during the 22 years analyzed, as well as rainfall distribution and potential évapotranspiration. For the 5 mm/day water capacity center pivot, the best irrigation strategy was to irrigate when the irrigation threshold dropped below a level of 80% AWC. This system produced a mean net return of 1,627 $/ha and applied a total irrigation of 274 mm for the entire growing season. For the 6 and 7 mm/day center pivot systems, the best management regime was to irrigate when the irrigation threshold dropped below 70% AWC. Both systems resulted in a 133

similar net return of 1,631 $/ha. The difference between both center pivots was only in the mean amount of water applied per season, 234 mm and 242 mm respectively. This study showed that the best center pivot is a system that has a 6 mm/day capacity. It has a flow rate of 150 m^fh, which is 25 m'/h less than the 7 mm/day center pivot. Although the costs of irrigation in general are low in the state of Parmia, a famier that acquires a center pivot system with a 6 mm/'day water capacity, can save money by buying a smaller water pump and a smaller size diameter pipe from the pump to the pivot. He is gu^anteed to have at least the same or higher net retum as a center pivot with a capacity of 7 mm/day. Future research is needed to verify the performance of the 6 mm/day center pivot with other crops, such as maize. Also it has to be noted that growing dry bean three times per year is not environmentally viable for long- term rotations, due to the increase of pests and diseases, and a decrease of soil organic matter and erosion. Management practices that sustain the environment and improve production would include crop rotations of diy bean with maize, wheat, soybean, sunflower and other important crops of the state of Parana.

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Fig. 1. Net reUim, standard deviation and mean net retum (bars) as a function of irrigation depth and irrigation threshold for three center pivot water application capacities, i.e., 5 mm/day (1), 6 mm/day (2) and 7 mm/day (3). References Boote K.J., J.W Jones, G. Hoogenboom. 1998. The CROPGRO model for grain legumes. In Understanding options for agricultural production, ed. G.Y. Tsuji, G. Hoogenboom, P.K. Thornton, 99-128. Dordrecht, NE: Kluwer Academic Press Publishers Faria, R.T. de, D. de Oliveira, and M.V. Folegatti. 1997a. Determination of a long-term optimal irrigation strategy for dry beans in Parana, Brazil. Sei. Agrie. 54: 155-164. Faria, R.T. de, D. de Oliveira, and M.V. Folegatti. 1997b. Simulaçào da fenología e produçâo do feijoeiro pelo modelo BEANGRO. In X Congresso Brasileiro de Agrometeorologia 140- 142. Piracicaba,S.P.,Brazil. FNP Consultoria & Comercio. 1998. Agrinual:98 Anuario da Agrie. Bras.. Sao Paulo, Brazil. Hoogenboom,G., J.W Jones,.K.J Boote, W.T Bowen, N.B Pickering, W.D. Batchelor.'l993. Advancement in modeling grain legume crops. ASAE Paper 93-4511, 21 pag. Intituto Brasileiro de Geografía. 1999. http://www.ibge.gov.br/. Oliveira D., N.A. Villa Nova. 1996. Evapotranspiracao maxima e laminas de irrigacao necessarias para feijoeiro no Parana. Revista Bras. deAgromet., St. Maria, v.4, n.l, p.29-36. Tsuji, G.Y., G. Uehara and S. Balas. 1994. DSSAT. v3, vol. 1. 2 and 3. Honolulu, HI: University of Hawaii.