Qualidade da água de irrigação e evolução da salinidade nas propriedades assistidas pelo "GAT" nos Estados do RN, PB e CE.

Abstract

The small irrigation in Northeast Brazil has been stimulated by several governmental programs. In the case of the Sub-Program GAT-PDCT / NE, which had placed irrigation as the flagship among the technologies implemented by it, among its concerns, it was also to assess the consequences of using such technologies. The objective of this work was to study the quality of irrigation water in time of the different water sources used to irrigate the properties assisted by the subprogram in the states of Rio Grande do Norte (RN), Paraíba (PB) and Ceará (CE ) to perform a correlation study between the different chemical characteristics of the waters to simplify the laboratory analyzes, besides evaluating the consequences of the different qualities of irrigation water in the chemical properties of the soils. The study was based on water analysis results collected monthly from the beginning of 1988 for 34 months for the sources located in the state of PB for 19 months for the State of the NB and for 11 months for the State of the EC, being performed complete chemical analyzes for water collected in the first year and only the electrical conductivity for those collected in the following years. In November 1990, soil samples were collected at depths 0 - 20, 20 - 10 and 40 - 60 cm, in both non - irrigated and irrigated plots of 25 properties (12 in RN and 13 in PB), among those where the water used in the irrigation was studied, and the chemical analyzes were completed. Water and soil analyzes were performed according to the usual procedures recommended by RICHARDS (1954) and EMBRAPA (1979). From these data, descriptive statistics and regression analysis were performed. The results of the water analysis revealed that the CEa of the studied waters in the year of 1988 ranged from 0.10 to 13.00 dS / m, with 75% of the analyzes being in the range of 0.25 to 1.50 d5 / m only 5 samples in one property showed values ​​higher than 5.00 dS / m. The waters of the three States presented similar chemical composition, being predominantly chlorinated and sodium, except waters of low salinity, that had relatively higher concentrations of bicarbonates. The CEa other chemical characteristics presented considerable variations during a year of study, except the pH. EC annual variations were also verified and, in the case of a large reservoir, spatial variations were observed, and these fluctuations were related to rainfall distribution and quantity. According to RiCHARDS (1954) classification, the waters used for irrigation in the properties assisted by the GAT have high to very high salinity in approximately 2/3 of the sources, while only presenting sodium risks in 8% of them. On the other hand, the classification of AYERS & WESTC0T (1985) showed that approximately 2/3 of the agaves have a slight to moderate use restriction regarding salinity, infiltration and toxicities of Na and CL, some sources having severe restrictions. chloride toxicity, the NR and EC states have more than 25% of their sources. The regression analysis study revealed that CEa can be used to estimate SCAT, RES, Cl, Na, Ca + Mq, Ca and RAS, as well as RAS can be estimated from RAS with good precision. In all the studied properties (25), with the exception of one that already had heavily sodified soil before irrigation, there was an increase in soil EC and soil PST in relation to the initial analyzes (non-irrigated area), however, in only 5 plots of 31 a shown in these properties was found a problem of saline and / or sodium character. It was also verified that in plots that were still under irrigation to CEs ranged from 0.6 to 3.4 times CEa, and in most of them, the ratio was between 0.8 and 1.2. On the other hand, it was observed only correlation between RAS of irrigation water and soil PST, however, the relation between RST and RAS of the saturation extract showed high r2 only for the superficial layer (0-20 cm), evidencing that the soil in every root zone had not yet reached equilibrium with water.