Simulação da necessidade de irrigação no estado do Rio Grande do Sul para compensar perdas por deficiência hídrica de milho, soja e feijão

Abstract

The temporal and spatial variability of the rainfall amount and its distribution is usually the major cause of agricultural yield losses. These variables are responsible for the oscillations in yields of Spring and Summer crops in Rio Grande do Sul State. Irrigation practices are recommended by researchers to minimize grain yield reduction due to soil water deficit. However, to establish an adequate irrigation management, it is necessary to know the rainfall amount and its distribution, as well as the crop water requirements in different environmental conditions, soils and crop sowing dates. The objectives of this work were: to determine irrigation needs for maize, dry beans and soybean crops for Depressão Central, Planalto Inferior, Planalto Médio, Missões and Baixo Vale do Uruguai regions, to estimate reference vapotranspiration (ETo) and cumulative irrigation water depth for maize, dry beans and soybean in different sowing dates; to calculate maize, dry beans and soybean crop production cost, considering different grain yield expectations and product prices for nonirrigated and irrigated areas and, to calculate the minimum irrigated area necessary to equalize 0, 20, 40, 60 and 80% of harvest frustration levels in non-irrigated areas. This work was developed based on a historical series of meteorological data (13 years data series) from Santa Maria, Cruz Alta, Passo Fundo, São Luiz Gonzaga and São Borja meteorological stations. The daily and cumulative soil water balance was estimate during the crop cycle in all sowing dates. Sowing dates were fixed on the 1st and 15th days of month within each crop promoted sowing dates, based on a specific agroclimatic zoning. Cumulative irrigation water depth for each crop, sowing date and year were estimated with the Sistema Irriga http://www.irriga.proj.ufsm.br). Crop production cost for irrigated and nonirrigated areas was determined. Over net incomes of non-irrigated areas were applied 0, 20, 40, 60 and 80% of harvest frustration levels. From the relationship among net incomes and irrigated areas, the minimum area to be irrigated in order to compensate the different frustration levels in nonirrigated areas was determined. Results indicated a higher irrigation needs for maize, dry beans and soybean in the Planalto Inferior region. A lower irrigation water depth for soybean were verified for the Planalto Médio region. Similar results of supplementary irrigation water depths were simulated for Planalto Inferior and Depressão Central Regions. The irrigation water depth for drybeans were 125, 133, 93, 90 and 144 mm for Depressão Central, Planalto Inferior, Planalto Médio, Missões and Baixo Vale do Uruguai regions, respectively. Considering an yield reduction of 20% on the maize yield of 4.02 Mg ha-1 the necessary irrigation area were 3.74, 13.26, 32.30, 70.38, 146.53 and 298.85 ha for non-irrigated areas of 50, 100, 200, 400, 800 and 1600 ha, respectively for irrigated yield of 6.0 Mg ha-1 and marcket price of R$ 16.00. Crop production costs decreased with the increase of grain yield expectation and cultivated area. The decrease in necessary irrigated area to compensate 0, 20, 40, 60 and 80% of harvest frustration levels was more significant for the increase of product prices than the increase in grain yield expectation and cultivated area.