Método empírico para avaliação da qualidade da água de irrigação.

Abstract

Studying the quality of irrigation water from different sources and its variation over time in 54 properties assisted by the Subprogram Generation and Adaptation of Technology (GAT), linked to the Scientific and Technological Development Program for the Northeast (PDCT / NE) in the states of Rio Grande do Norte (RN), Paraíba (PB) and Ceará (CE), MEDEIROS (1992) found highly significant empirical relationships between the electrical conductivity of irrigation water (CEa) and different chemical characteristics used in different water quality classification schemes. The present study aimed to study the feasibility of using CEa for irrigation, for determination of other characteristics considered indispensable, aiming to streamline, cheapen and simplify the water quality assessment. The work was developed using the same results of chemical analyzes, mentioned in the studies of MEDEIROS, serving to assess the degree of reliability of the equations found with which complementary studies were also carried out. In a second step, to evaluate the applicability of the equations, another database was used, this one extracted from the files of the Laboratory of Irrigation and Salinity (LIS) of UFPB. Starting only with the results of CEa and using the developed equations, some essential chemical characteristics were estimated in the evaluation of water quality. The results were subjected to classification, according to proposals by RICHARDS (1954) and FAO cited by AYERS & WESTCOT (1991), and compared with the original data. For the GAT data, the estimate corresponded to the expected, obtaining about 6% of the samples outside the 95% confidence interval, calculated for the respective equations. The LIS data, however, showed an index of 11%, therefore above expectations. Regarding the classifications, both databases behaved similarly, being around 75% of the classes with identical estimates to the data when considering the criteria of AYERS & WESTCOT (1991). According to the classification proposals of RICHARDS (1954), it was found that 88.9% and 86.1%, of the data from the GAT and LIS, respectively, had an estimate of the correct classes. For low CEa levels (<0.75dS / m) the hit rate was 100%. The behavior of the salts showed that, in average terms, in waters with very low concentrations (EC <0.60dS / m) the HC03 content predominates over the CI and the contents of Na, Ca and Mg are at relatively close levels. However, for waters with high CEa, there is a predominance of Cl over HC03 and Na over Ca and Mg. The equations showed that they can be used for exploratory survey purposes, however, the application in the field, in research and in judicious evaluations of water quality, the methodology presented limitations, requiring complementary studies.