Diagnóstico de necessidade de drenagem no perímetro irrigado de São Gonçalo.

Abstract

The present research had the objective of accomplishing field studies that identifies the drainage need, seeking to establish a maintainable agriculture in the farm belonging to the Escola Agrotécnica Federal of Souza sited at the irrigated district of São Gonçalo. 90 observation wells were installed for monitoring the water table depth after the discharge from five irrigation events and from occasional rainfall. Punctual readings of the water table depth were made at 6, 12, 18, 24 and 48 hours after each event, and the hydraulic conductivity and electrical conductivity of the water were determined following a systematic sampling scheme in a grid of 30x30 m. The data were interpreted through the descriptive statistics and the geostatistics for general and spatial descriptive analysis, respectively. The krigagem interpolation process drew up Semivariograms and isolines maps. A medium variability was verified both for the water table depth (30,59 < CV <56,64) and for the water electrical conductivity (39,99% < CV <86,90%), while the hydraulic conductivity presented a high discharge variability (153,55 < CV < 228,92). Regarding the structure of dependence, the studied variables presented moderate spatial dependence, except the hydraulic conductivity that did not present a spatial structure of "dependence well defined. When analyzed with rainfall and irrigation events, the water table depth presented a dependence reach that varied from 99 to 122 m (sub-area I), from 106 to 122m (sub-area II), with irrigation from 65 to 122m (sub-area II), and with the water electrical conductivity from 53 to 93m. The isolines map drew up for the water table depth and water electrical conductivity variables, provided, from values estimated by the krigagem process, a spatial view of the variability* pattern of these variables, becoming an important tool for indication of the drainage behavior in the area. The selection of the areas taking into consideration the variability, considers the spatial position of the observations and allows the drainage project not to be overestimated in some areas and underestimated in others, implicating in high costs. The obtained values of the water electrical conductivity and of the water table depth indicate the need of a drainage system to lower the water table and to remove the excess of salts from the studied soil profile.