Demanda hídrica de cacaueiro irrigado e conduzido a pleno sol em tabuleiro costeiro do Recôncavo da Bahia

Abstract

Efficient water management is extremely important for the development of cocoa, as the limiting in the soil reduces its productivity. Therefore, it is necessary to know the water demand of the crop. Therefore, the aim was to determine the water demand for the cocoa crop, using the weighing lysimeter as a measurement method based on four load cells, and to characterize the spatial distribution of the saturated hydraulic conductivity in an Oxisol of the Recôncavo da Bahia Coastal Tablelands. In the experimental area of 4,868.9 m2 of the Postgraduate Program in Agricultural Engineering of the Universidade Federal do Recôncavo da Bahia in Cruz das Almas, 530 plants of the variety CCN 51 were grown, spaced at three meters and irrigated by the micro sprinkler system. In the center of the area, one of the plants was grown in a 1.44 m surface lysimeter and the water consumption of the crop was monitored in the period from January, 1 of 2017 to December, 30 of 2017. Calibration of the lysimetric set was carried out in order to evaluate measurement errors attributed to this instrument and its adequacy to determine the water demand of the cocoa crop. The data were collected using a datalogger programmed to perform readings every 01 second, with storage of the averages of 15 minutes. Calibration was carried out between 3:00 and 5:00 am, with the addition / removal of known masses at 02-minute intervals; by means of the linear regression between the electrical signal in mV and the masses in kg, the calibration curve was obtained and subsequently the measurement errors and the operation of the lysimeter were calculated. The coefficients of determination of the calibration curves were greater than 0.9, and the absolute errors, uncertainty, hysteresis, repeatability and the accuracy of the instrument respectively 0.179 kg and -0.044 kg; 0.077 kg; 0.031 kg and 0.044 kg and 0.1 kg were found not to interfere in the calculation of water demand for cocoa, proving that the measurement instrument accurately measures the crop evapotranspiration (ETc). After this analysis, and with the objective of evaluating the water demand of young cocoa plants grown in the field, determining ETc and the cultivation coefficient (Kc) during the growth and production cycle; measurements were performed daily, with ETc determined by mass balance and Kc by ETc ratio and reference evapotranspiration (ETo) calculated with the Penman-Monteith equation. The annual rate of ETc was 800.68 mm with a daily average of 3.8 mm and the average Kc of 0.92. The water demand of young cocoa plants showed how demanding the crop is in water, and in the phases of growth and physiological maturation of the fruit, ETc was greater in relation to the phases of flowering, development and fruits filling. Studies were also carried out to evaluate the physical-hydraulic attributes, with the objective of characterizing and modeling the spatial variability of the saturated hydraulic conductivity (K0). In this study, soil sampling was performed on the undisturbed structure at 50 points spaced from 8.0 to 8.0 m, in three depths 0.0-0.15 m, 0.15-0.30 m and 0.30-0, 45 m. In the laboratory, the K0 was determined using a constant head permeameter method, pore distribution using a tension table and soil density. The average K0 values at the depths evaluated were 40.41 mm h-1, 26.49 mm h-1 and 37.82 mm h-, respectively, and the Gaussian model was the one that best fitted the data set. K0 showed strong spatial dependence, being an important attribute in the delimitation of homogeneous zones for specific site management purposes.