Estimativa das necessidades de irrigação e avaliação do impacto da cobrança pelo uso de recursos hídricos na bacia do Baixo Jaguaribe-CE

Abstract

The goal of this research was to estimate the irrigation requirements for crops grown in the Low Jaguaribe Basin, Ceará State, Brazil, providing essential information for an adequate irrigation scheduling and the establishment of the water rights for each farm. In addition, an analysis on the impact of the price of water on the farm income was conducted. Based of the Ceara Department of Water Resources database, 36 different crops were identified in the basin, all crops irrigated from water pumped from the Jaguaribe river, amounting to 7,570 ha annual acreage. Interpolation techniques were applied on weather data from four stations to generate individualized water budget for each irrigated field for every county within the basin, leading to a more precise estimation of the volumes and flow rates to be assigned as the water right for that individual farm. Furthermore, planting date, values of crop coefficient for every growth stage, and the efficiency of the irrigation system were applied to estimate the gross weekly water depth and flow rate requirements for every plot in the basin. Total annual water demand for the Low Jaguaribe basin was estimated in 103.8 million cubic meters, ranging from 73 thousand m3 per week during the months of March and April (due to the peak on the rainy season), to 5 million m3 per week in October, when the irrigation water requirement peaks. Clearly the unitary flow rate needed to supply irrigation requirement peaks at a value of 8.4 m3.s-1 in October. Simulations in the demands and on flow rate have indicated volume potentially applied to the field can be reduced in 13.2% to 18.8%, by increasing irrigation efficiency to values compatible with the potential of the irrigation system installed in the field, by replacing the surface irrigation by other more efficient technology, and by replacing rice by another crop. By analyzing the water requirements from rice, cowpea and banana fields, which amounted to 44% of the irrigated acreage in the basin, and taking into account the area of each farm in connection with the water fee to be charged in the state of Ceará, it was found an average value of R$ 7.04 per 1,000 m3 of water supplied to the field. Therefore, resulting in an estimated early gross income of R$ 730,900 from water sale to irrigation in the Low Jaguaribe basin. Three alternative parameters were applied in order to analyzed the effect of charging for water: a percentage of the total production cost, a percentage over the gross income, and a percentage over the net benefit. It was found the last parameter produced more adequate results. In addition, it was found most of the crops evaluated, except sugar cane, rice and corn, provided income to pay even for the for the highest values of water fee posted by the state of Ceará.