Abstract
One way of classifying water quality is by means of indices, in which a series of parameters analyzed are joined a single value, facilitating the interpretation of extensive lists of variables or indicators, underlying the classification of water quality. The objective of this study was to develop a statistically based index to classify water according to the Irrigation Water Quality Index (IWQI), to evaluate the ionic composition of water for use in irrigation and classify it by its source. For this purpose, the database generated during the Technology Generation and Adaptation (GAT) program was used, in which, as of 1988, water samples were collected monthly from water sources in the states of Paraíba, Rio Grande do Norte and Ceará. To evaluate water quality, the electrical conductivity (EC) of irrigation water was taken as a reference, with values corresponding to 0.7 dS m-1. The chemical variables used in this study were: pH, EC, Ca, Mg, Na, K, Cl, HCO3, CO3, and SO4. The data of all characteristics evaluated were standardized and data normality was confirmed by Lilliefors test. Then the irrigation water quality index was determined by an equation that relates the standardized value of the variable with the number of characteristics evaluated. Thus, the IWQI was classified based on indices, considering normal distribution. Finally, these indices were subjected to regression analysis. The method proposed for the IWQI allowed a satisfactory classification of the irrigation water quality, being able to estimate it as a function of EC for the three water sources. Variation in the ionic composition was observed among the three sources and within a single source. Although the water quality differed, it was good in most cases, with the classification IWQI II.
Highlights
Water quality is understood as the physical, chemical and biological characteristics that influence its suitability for a specific use, in other words, the relationship between the water quality and the users needs (Ayers & Westcot, 1991)
According to Shalhevet & Kamburov (1976) irrigation water quality is mainly defined by the total quantity of dissolved salts and its ionic composition depending on the water source, location and time of water sampling
The results of the mean values, standard deviations and coefficients of variation (CV) for the characteristics evaluated and for the different water sources in the reference samples are shown in table 2
Summary
Water quality is understood as the physical, chemical and biological characteristics that influence its suitability for a specific use, in other words, the relationship between the water quality and the users needs (Ayers & Westcot, 1991). Water is becoming a growing concern, with regard to the quantity available, but mainly in relation to its quality, giving rise to losses and limitations to its multiple uses because, depending on the physical, chemical and biological characteristics, its use may be limited or unviable. Due to some difficulties in classifying irrigation water quality, some authors have used multivariate techniques (Ganfopadhyay et al, 2001; Singh et al, 2005; Stigter et al, 2006), mainly since these techniques reduce the data amount, allowing an interpretation for each variable in association with other factors, e.g, water source, climate, soil type, and anthropogenic influence
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